Preliminary Product Information
This document contains information for a new product.
Cirrus Logic reserves the right to modify this product without notice.
1
Copyright
Cirrus Logic, Inc. 2000
(All Rights Reserved)
P.O. Box 17847, Austin, Texas 78760
(512) 445 7222 FAX: (512) 445 7581
http://www.cirrus.com
CS5540
Low-Power, Low-Voltage, 24 - Bit
ADC
Features
l
Analog-to-Digital Converter
- Linearity Error: 0.0015% FS
- RMS Noise: 4
V
l
Two Channel Differential MUX
l
Buffered, Fully Differential Analog and
Voltage Reference Inputs
l
Scalable V
REF
Input: 0.1 V to Analog Supply
l
High Absolute Accuracy via Self-Calibration
l
Fixed Digital Filter
- Single Conversion Settling at 6.7 SPS
- Simultaneous 50/60 Hz Rejection
l
Simple Serial Interface
- SPI
TM
and Microwire
TM
Compatible
- Schmitt Trigger on Serial Clock (SCLK)
l
Low Power
- Single +3.0 V Supply
- 330
A Supply Current
- 10
A in Sleep Mode
Description
The CS5540 is a low-power and low-voltage
ana-
log-to-digital converter (ADC), which achieves highly
accurate conversions using a simple non-programmable
interface that is easy to understand and design-in. It is
optimized to convert analog signals in DC measurement
applications, such as temperature and pressure mea-
surement, and various portable devices where
low-power consumption is required.
To accommodate these applications, the ADC integrates
analog input and reference buffers for increased input
impedance and a two-channel multiplexer.
The CS5540 includes a digital filter, which achieves si-
multaneous rejection of 50/60 Hz signals and provides
single conversion settling at 6.7 SPS throughput. Abso-
lute accuracy is achieved via continuous internal
self-calibration. The device draws a nominal 330 A.
Low-power, low-voltage operation, and a simple serial
interface make the CS5540 an ideal device for low-cost,
power-conscious DC measurement applications.
ORDERING INFORMATION
CS5540-
AS -40 C - +85 C 16-Pin SSOP
Differential
4th Order
Modulator
Digital
Filter
Serial
Interface
Calibration
Register
Output
Register
Input
Mux
Clock
Generator
X1
X1
X1
X1
VA+
VA-
VD+
DGND
VREF+
VREF-
OSC1 OSC2
AIN1+
AIN1-
AIN2+
AIN2-
SDO
SCLK
CS
CHS
JUN `01
DS503PP1
CS5540
2
DS503PP1
TABLE OF CONTENTS
1. CHARACTERISTICS AND SPECIFICATIONS ........................................................................ 4
2. GENERAL DESCRIPTION ..................................................................................................... 10
2.1 Analog Input ..................................................................................................................... 10
2.1.1 Analog Input Model ............................................................................................. 10
2.2 Voltage Reference Input .................................................................................................. 11
2.2.1 Voltage Reference Input Model ........................................................................... 11
2.3 Power Supply Arrangements ........................................................................................... 12
2.4 Clock Generator ............................................................................................................... 12
2.5 Serial Port Interface ......................................................................................................... 12
2.6 Input Channel Selector .................................................................................................... 12
2.6.1 Switching Channels ............................................................................................. 13
2.7 Serial Port and Data Conversions .................................................................................... 13
2.7.1 Reading Conversions .......................................................................................... 14
2.7.2 Output Coding ..................................................................................................... 14
2.7.3 Digital Filter ......................................................................................................... 14
2.8 Sleep Mode ...................................................................................................................... 15
2.9 Power-Up and Initialization .............................................................................................. 16
2.10 PCB Layout .................................................................................................................... 16
3. PIN DESCRIPTIONS .............................................................................................................. 17
4. SPECIFICATION DEFINITIONS ............................................................................................. 19
5. PACKAGE DIMENSIONS ...................................................................................................... 20
Contacting Cirrus Logic Support
For a complete listing of Direct Sales, Distributor, and Sales Representative contacts, visit the Cirrus Logic web site at:
http://www.cirrus.com/corporate/contacts/
SPI is a trademark of Motorola Inc.
Microwire is a trademark of National Semiconductor Corp.
Preliminary product information describes products which are in production, but for which full characterization data is not yet available. Advance product infor-
mation describes products which are in development and subject to development changes. Cirrus Logic, Inc. has made best efforts to ensure that the informa-
tion contained in this document is accurate and reliable. However, the information is subject to change without notice and is provided "AS IS" without warranty
of any kind (express or implied). No responsibility is assumed by Cirrus Logic, Inc. for the use of this information, nor for infringements of patents or other rights
of third parties. This document is the property of Cirrus Logic, Inc. and implies no license under patents, copyrights, trademarks, or trade secrets. No part of
this publication may be copied, reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photographic, or
otherwise) without the prior written consent of Cirrus Logic, Inc. Items from any Cirrus Logic website or disk may be printed for use by the user. However, no
part of the printout or electronic files may be copied, reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechan-
ical, photographic, or otherwise) without the prior written consent of Cirrus Logic, Inc.Furthermore, no part of this publication may be used as a basis for man-
ufacture or sale of any items without the prior written consent of Cirrus Logic, Inc. The names of products of Cirrus Logic, Inc. or other vendors and suppliers
appearing in this document may be trademarks or service marks of their respective owners which may be registered in some jurisdictions. A list of Cirrus Logic,
Inc. trademarks and service marks can be found at http://www.cirrus.com.
CS5540
DS503PP1
3
LIST OF FIGURES
Figure 1. Continuous Running SCLK Timing (Not to Scale) ........................................................... 9
Figure 2. SDO Read Timing (Not to Scale)..................................................................................... 9
Figure 3. Multiplexer Configuration. .............................................................................................. 10
Figure 4. Input model for AIN+ and AIN- pins. .............................................................................. 10
Figure 5. Resolution vs. Voltage Reference.................................................................................. 11
Figure 6. Input model for VREF+ and VREF- pins. ....................................................................... 11
Figure 7. CS5540 Configured with a single +3.0 V Supply. .......................................................... 12
Figure 8. Command and Data Word Timing. ................................................................................ 13
Figure 9. Self Calibration of Offset. ............................................................................................... 13
Figure 10. Self Calibration of Gain. ............................................................................................... 14
Figure 11. Digital Filter Response................................................................................................. 15
LIST OF TABLES
Table 1. Output Conversion Data Register Description (24 bits + flags) ...................................... 15
Table 2. CS5540 24-Bit Bipolar Output Coding ............................................................................ 15
CS5540
4
DS503PP1
1. CHARACTERISTICS AND SPECIFICATIONS
ANALOG CHARACTERISTICS
(T
A
= 25 C; VA+ = +3.0 V 5%, VA- = 0 V, VD+ = 3.0 V 5%,
DGND = 0 V, VREF+ = 2.5 V, VREF- = 0 V, MCLK = 32.768 kHz, OWR (Output Word Rate) = 6.7 SPS, Input
Range = 2.5 V Differential, Vcm=1.25 V. (See Note 1.)
Notes: 1. Specifications guaranteed by design, characterization, and/or test.
2. Specification applies to the device only and does not include any effects caused by external parasitic
thermocouples.
3. Drift over specified temperature range after power-up at 25
C.
4. Wideband noise aliased into the baseband. Referred to the input. Typical values shown for 25 C.
5. For peak-to-peak noise multiply the RMS value by 6.6.
* Specifications are subject to change without notice.
Parameter
Min
Typ
Max
Units
Accuracy
Linearity Error
-
0.0015
0.003
%FS
No Missing Codes
24
-
-
Bits
Offset Error
(Note 2)
-
16
TBD
LSB
24
Offset Drift
(Notes 2 and 3)
-
20
-
nV/C
Full Scale Error
-
8
31
ppm
Full Scale Drift
(Note 3)
-
1
-
ppm/C
RMS Noise
(Notes 4 and 5)
-
4
-
V
Bandwidth (-3 dB)
-
11.96
-
Hz
CS5540
DS503PP1
5
ANALOG CHARACTERISTICS
(Continued)
Notes: 6. See Section 2.1, "Analog Input".
7. See Section 2.2, "Voltage Reference Input".
8. Absolute voltages on VREF+ and VREF- must be less than or equal to the supply voltages.
9. The CS5540 includes a digital filter. The filter which achieves simultaneous rejection of 50/60 Hz
provides single conversion settling at a 6.7 SPS throughput.
10. All outputs unloaded. All inputs at CMOS levels.
Parameter
Min
Typ
Max
Units
Analog Input
Common Mode + Signal on AIN+ or AIN-
Single Supply
Dual Supplies
0.0
VA-
-
-
VA+
VA+
V
V
CVF Current on AIN+, AIN-
(Note 6)
-
12
-
nA
Input Leakage for MUX when off
-
10
-
pA
Common Mode Rejection
dc
50, 60Hz
-
-
120
120
-
-
dB
dB
Input Capacitance
-
8
-
pF
Voltage Reference Input
Range
(VREF+) - (VREF-)
(Note 8)
0.1
2.5
(VA+) -
(VA-)
V
CVF Current on VREF+ and VREF-
(Note 7)
-
20
-
nA
Common Mode Rejection
dc
50, 60 Hz
-
-
120
120
-
-
dB
dB
Input Capacitance
-
12
-
pF
Dynamic Characteristics
Modulator Sampling Frequency
-
MCLK/2
-
Hz
Filter Settling to 1/2 LSB (Full Scale Step)
(Note 9)
6.7 SPS OWR
-
1/OWR
-
s
Power Supplies
DC Power Supply Currents
(Normal Mode)
I
A+
I
D+
-
-
225
25
280
36
A
A
Power Consumption
Normal Mode
(Note 10)
Sleep Mode
-
-
750
30
1000
-
W
W
Power Supply Rejection
dc Positive Supplies
dc Negative Supplies
-
-
80
80
-
-
dB
dB
CS5540
6
DS503PP1
3 V DIGITAL CHARACTERISTICS
(T
A
= 25 C; VA
+ = 3.0 V 5%, VA- = 0 V, VD+ = 3.0 V 5%,
DGND = 0 V.) (See Notes 1 and 11.) All voltage levels measured relative to DGND.
Notes: 11. All measurements performed under static conditions.
Parameter
Symbol
Min
Typ
Max
Units
High-Level Input Voltage:
All Pins Except
OSC1, SCLK
OSC1
SCLK
V
IH
V
IH
V
IH
0.6VD+
TBD
(VD+)-0.45
-
-
-
-
-
-
V
V
V
Low-Level Input Voltage:
All Pins Except
OSC1, SCLK
OSC1
SCLK
V
IL
V
IL
V
IL
-
-
-
-
-
-
0.16VD+
TBD
0.6
V
V
V
High-Level Output Voltage:
SDO, I
out
= -1.0 mA
V
OH
(VD+)-.25
-
-
V
Low-Level Output Voltage:
SDO, I
out
= 1.6mA
V
OL
-
-
0.4
V
Input Leakage Current
I
in
-
1
10
A
3-State Leakage Current
I
OZ
-
-
10
A
Digital Output Pin Capacitance
C
out
-
9
-
pF
CS5540
DS503PP1
7
ABSOLUTE MAXIMUM RATINGS
(DGND = 0 V; See Note 12.)
Notes: 12. All voltages are measured with respect to the digital ground pin (DGND).
13. VA+ and VA- must satisfy {(VA+) - (VA-)}
+4.0 V.
14. VD+ and VA- must satisfy {(VD+) - (VA-)}
+4.0 V.
15. Applies to all pins including continuous overvoltage conditions at the analog input (AIN) pins.
16. Transient current of up to 100mA will not cause SCR latch-up. Maximum input current for a power
supply pin is 50 mA.
17. Total power dissipation, including all input currents and output currents.
WARNING: Operation at or beyond these limits may result in permanent damage to the device.
Normal operation is not guaranteed at these extremes.
Parameter
Symbol
Min
Typ
Max
Units
DC Power Supplies
(Notes 13 and 14)
Positive Digital
Positive Analog
Negative Analog
VD+
VA+
VA-
-0.3
-0.3
-0.3
-
-
-
+4.0
+4.0
+0.3
V
V
V
Input Current, Any Pin Except Supplies
(Notes 15 and 16)
I
IN
-
-
10
mA
Output Current
I
OUT
-
-
25
mA
Power Dissipation
(Note 17)
PDN
-
-
500
mW
Analog Input Voltage
AIN and VREF pins
V
INA
(VA-) + (-0.3)
-
(VA+)+0.3
V
Digital Input Voltage
V
IND
-0.3
-
(VD+)+0.3
V
Ambient Operating Temperature
T
A
-40
-
+85
C
Storage Temperature
T
stg
-65
-
+150
C
CS5540
8
DS503PP1
SWITCHING CHARACTERISTICS
(T
A
= 25 C; VA+ = +3.0 V 5% VA- = 0 V, VD+ = 3.0 V 5%,
DGND = 0 V; Input Levels: Logic 0 = 0 V, Logic 1 = VD+; C
L
= 50pF)
Notes: 18. Device parameters are specified with 32.768 kHz clock; however, clocks up to 40 kHz can be used for
increased throughput.
19. Specified using 10% and 90% points on waveform of interest. Output loaded with 50 pF.
20. Oscillator start-up time varies with crystal parameters. This specification does not apply when using an
external clock source.
Parameter
Symbol
Min
Typ
Max
Units
Master Clock Frequency:
External Clock
Internal Oscillator (Note 18)
MCLK
5
-
-
32.768
40
-
kHz
Master Clock Duty Cycle
40
-
60
%
Rise Times
(Note 19)
Any Digital Input Except SCLK
SCLK
SDO Output
t
rise
-
-
-
-
-
50
1.0
100
-
s
s
ns
Fall Times
(Note 19)
Any Digital Input Except SCLK
SCLK
SDO Output
t
rise
-
-
-
-
-
50
1.0
100
-
s
s
ns
Start-up
Oscillator Start-up Time
XTAL = 32.768 kHz (Note 20)
t
ost
-
500
-
ms
Power-on Reset Period
t
por
-
490
-
MCLK
cycles
Serial Port Timing
Serial Clock Frequency
SCLK
0
-
2
MHz
Serial Clock
Pulse Width High
Pulse Width Low
t
1
t
2
250
250
-
-
-
-
ns
ns
CS Enabled to SCLK Rising
t
3
50
-
-
ns
SCLK falling prior to Chip Select Disabled
t
4
100
-
-
ns
SDO Read Timing
CS to Data Valid
t
5
-
-
150
ns
SCLK Falling to New Data Bit
t
6
-
-
150
ns
CS Rising to SDO Hi-Z
t
7
-
-
150
ns
CS5540
DS503PP1
9
Figure 1. Continuous Running SCLK Timing (Not to Scale)
CS
SCLK
t1
t4
t2
t3
Figure 2. SDO Read Timing (Not to Scale)
CS
SCLK
MSB
MSB-1
LSB
SDO
t5
t7
t6
t1
t2
CS5540
10
DS503PP1
2. GENERAL DESCRIPTION
The CS5540 is a low-power and low-voltage 24-bit
- analog-to-digital converter (ADC). It is opti-
mized to convert analog signals in DC measure-
ment applications such as temperature and pressure
measurement, and various portable devices where
low power consumption is required.
To accommodate these applications, the ADC inte-
grates analog input and reference buffers for in-
creased input impedance and includes a
two-channel multiplexer. Absolute accuracy is ac-
complished via self-calibration. The device also
operates with a variety of supply configurations
while drawing a nominal 330 A.
The CS5540 includes a digital filter which achieves
simultaneous rejection of 50/60 Hz and provides
single conversion settling at a 6.7 SPS throughput.
The filter's output word rate can be increased by
approximately 1.22X by using a 40 kHz master
clock, although the 50/60 Hz rejection will be sac-
rificed.
To ease communications between the ADC and a
microcontroller, the converter includes a simple
three-wire serial interface which is SPI and Mi-
crowire compatible. A Schmitt Trigger input is pro-
vided on the serial clock (SCLK) input.
2.1 Analog Input
Figure 3 illustrates a block diagram of the CS5540.
The device consists of a multiplexer, a unity gain
coarse/fine charge input buffer, a fourth order
-
modulator, and a digital filter.
2.1.1 Analog Input Model
Figure 4 illustrates the input models for the AIN
pins. The model includes a coarse/fine charge buff-
er which reduces the dynamic current demands
from the analog input signal. The buffer is designed
to accommodate rail to rail (common-mode plus
signal) input voltages. Typical CVF (sampling)
current is about 8 nA (MCLK = 32.768 kHz). Ap-
plication Note 30, "Switched-Capacitor A/D Input
Structures", details various input architectures.
VREF+
Sinc
Digital
Filter
M
U
X
AIN2+
AIN2-
AIN1+
AIN1-
X1
X1
X1
VREF-
X1
Differential
4 Order
Modulator
th
4
Serial
Port
CS
SDO
SCLK
CHS
Figure 3. Multiplexer Configuration
AIN
C = 8 pF
f = 2*MCLK = 65.536 kHz
Coarse
2
Fine
1
i
n
= CV
os
f
V
os
25mV
Figure 4. Input model for AIN+ and AIN- pins
CS5540
DS503PP1
11
2.2 Voltage Reference Input
The differential voltage between VREF+ and
VREF- sets the nominal full scale input span of the
converter. For a single-ended reference voltage, the
reference output is connected to the VREF+ pin of
the CS5540 and the ground reference is connected
to the VREF- pin. Note that the differential refer-
ence voltage can be from 0.1 V to ((VA+)- (VA-)).
The noise-free resolution of a single sample from
the ADC is directly proportional to the voltage ref-
erence as depicted in Figure 5.
Note:
When a lower reference voltage is used, the
resulting code widths are smaller. Since the
output codes exhibit more changing codes for
a fixed amount of noise, the converter
appears noisier.
2.2.1 Voltage Reference Input Model
Figure 6 illustrates the input models for the VREF
pins. It includes a coarse/fine charge buffer which
reduces the dynamic current demand of the exter-
nal reference. The reference's buffer is designed to
accommodate rail-to-rail (common-mode plus sig-
nal) input voltages. Typical CVF (sampling) cur-
rent is about 16 nA (MCLK = 32.768 kHz).
13
14
15
16
17
18
19
0
0.5
1
1.5
2
2.5
3
VREF (V)
Noi
se-Free Resol
u
ti
on (Bi
t
s)
Figure 5. Typical Noise-Free Resolution
vs. Voltage Reference
Noise-Free Res. = log
2
(Bipolar Span/6.6*RMS Noise)
Figure 6. Input model for VREF+ and VREF- pins
VREF
C = 12 pF
f = 2*MCLK = 65.536 kHz
Coarse
2
Fine
1
i
n
= CV
os
f
V
os
25mV
CS5540
12
DS503PP1
2.3 Power Supply Arrangements
The CS5540 is designed to operate with a total sup-
ply voltage of 3.0 V 5%. For maximum flexibili-
ty, separate pins are provided for VA+, VA-, VD+,
and DGND, which is especially useful with
ground-referenced input signals.
Figure 7 illustrates the CS5540 connected with a
single +3.0 V supply for both the analog and digital
sections.
2.4 Clock Generator
The CS5540 includes an oscillator circuit which
can be connected with an external crystal to pro-
vide the master clock for the chip. The chip is de-
signed to operate using a low-cost 32.768 kHz
"tuning fork" type crystal. One lead of the crystal
should be connected to OSC1 and the other to
OSC2. A 10 megohm resistor is required in parallel
with the crystal. Lead lengths should be minimized
to reduce stray capacitance. Note that the converter
will operate with an external (CMOS compatible)
clock with frequencies up to 40kHz, applied to the
OSC1 pin.
2.5 Serial Port Interface
The CS5540's serial interface consists of three con-
trol lines: CS, SDO, and SCLK.
CS, Chip Select, is the control line which enables
access to the serial port. If the CS pin is tied to logic
0, the port can function as a three wire interface.
SDO, Serial Data Out, is the data signal used to
transfer output data from the converters. The SDO
output will be held at high impedance any time CS
is at logic 1.
SCLK, Serial Clock, is the serial bit-clock which
controls the shifting of data to or from the ADC's
serial port. The CS pin must be held at logic 0 be-
fore SCLK transitions can be recognized by the
port logic. To accommodate opto-isolators, SCLK
is designed with a Schmitt-trigger input.
2.6 Input Channel Selector
CHS, the Channel Select input, permits the user to
select between AIN1 and AIN2 for data conver-
sions. When CHS = 0, AIN1 is converted. When
CHS = 1, AIN2 is converted. Note that since the
converter continuously converts the input selected
Figure 7. CS5540 Configured with a single +3.0 V Supply
OSC2
VD+
VA+
VREF+
VREF-
DGND
AIN1+
SCLK
SDO
CS5540
OSC1
CS
+3.0 V
Analog
16
15
3
2
11
9
8
6
10
5
12
Optional Clock
Source
Serial
Data
Interface
5 kHz ~ 40 kHz
AIN1-
4
VA-
1
0.1
F
0.1
F
10
AIN2+
AIN2-
+
-
Voltage
Reference
14
13
CHS
7
Supply
Analog
Signal
Sources
Channel
Selection
10 M
CS5540
DS503PP1
13
by CHS, the channel being converted can be
switched at any time. The conversion in-process
when CHS is switched will be aborted and a new
conversion will begin on the selected channel. The
indicator bits in the SDO output will indicate which
channel was converted.
2.6.1 Switching Channels
7358 clock cycles after CHS is toggled, SDO will
fall (indicating new data present). All subsequent
data will be presented to the serial port every 4884
clock cycles, as portrayed in Figure 8.
2.7 Serial Port and Data Conversions
When power is applied to the CS5540, it is reset by
the power-on reset circuit. Then the converter en-
ters the data mode where the ADC continuously
converts the analog input channel selected by the
channel select pin (CHS). After a conversion is
complete, the SDO pin falls to logic 0 to indicate an
end of conversion and the next 32 serial clock puls-
es shift data out of the serial port. Figure 8 illus-
trates the sequence necessary to read output data
from the data conversion register.
The CS5540 operates in a self-calibration mode
which allows the converter to calibrate continuous-
ly between each conversion (if a user requires sys-
tem calibration, this can be accommodated in the
system microcontroller). The sections that follow
detail the conversion mode and also explain how to
decode the conversion word into the respective flag
and data bits.
Note:
The CS5540 offers self calibration, in which
the ADC calibrates out offset and gain errors
due to the ADC itself. Calibration in the
CS5540 is used to set the zero and gain slope
of the ADC's transfer function. For the
self-calibration of offset, the converter
internally ties the inputs of the modulator
together and routes them to the VREF- pin as
shown in Figure 9. VREF- must be tied to a
fixed voltage between VA+ and VA-. For
self-calibration of gain, the differential inputs
of the modulator are connected to VREF+
and VREF- as shown in Figure10. Further
note that each calibration step (offset or gain)
is transparent to the user and is included in
the specified 6.7 SPS output rate.
SDO
SCLK
CHS
t
d
*
Data Time
24 SCLKs
MSB
LSB
0
1
CH
OD OF
1
1
8 SCLKs to read status
SDI set low while
reading status & data
1
t
o
*
* t
d
= 7358 clock cycles
t
o
= 4884 clock cycles
Figure 8. Command and Data Word Timing
AIN+
AIN-
+
-
VREF-
Figure 9. Self Calibration of Offset
CS5540
14
DS503PP1
2.7.1 Reading Conversions
At the completion of a conversion, SDO will fall to
logic 0 to indicate that the conversion is complete.
Calibration will be transparent to the user. Never-
theless, to read a conversion word, the user must is-
sue 32 SCLK's. The first 8 SCLKs clear the SDO
flag and read the status flags. Upon the falling edge
of the 8
th
SCLK, the SDO pin will present the first
bit (MSB) of the conversion word. 24 SCLKs
(high, then low) are then required to read the con-
version word from the port. Upon the falling edge
of the 32
nd
SCLK, SDO will return high, waiting
till the next conversion is complete before it falls
again.
The user is not required to read every conversion.
If the user chooses not to read a conversion after
SDO falls, SDO will rise one MCLK clock cycle
before the next conversion is completed and then
fall to signal that another conversion word is avail-
able (assuming CS is kept low).
Note:
1) If the user begins to clear the SDO flag and
read the conversion data, this action must be
finished before the conversion cycle which is
occurring in the background is complete if the
user wants to be able to read the new
conversion data.
2) If the multiplexer CHS input of the
converter is switched while it is performing a
conversion, the filter will abort the current
conversion and start a new conversion on the
new channel.
3) If the channel is switched when SDO is
low, SDO will remain low, and the previous
conversion result will remain in the serial port.
SDO will rise one MCLK cycle before the new
channel's data is ready, then fall to indicate
that the conversion data is available.
2.7.2 Output Coding
The CS5540 outputs a 24-bit two's-complement
data conversion word. To read a conversion word
the user must read the conversion data register,
which is 24 bits long and outputs the conversions
MSB first. Once a conversion is complete, SDO
falls and 32 SCLK's are required to read a conver-
sion. The first 8 SCLKs are used to clear an internal
SDO flag and clock out status flags.
The CH (CHannel indicator) bit keeps track of
which input channel was converted (0=AIN1;
1=AIN2).
The OD (Oscillation Detect) bit is set to a logic 1 any
time that an oscillatory condition is detected in the
modulator. This does not occur under normal oper-
ating conditions, but may occur whenever the input
to the converter is extremely overranged. If the OD
bit is set, the conversion data bits can be completely
erroneous. The OD flag bit will be cleared to logic 0
when the modulator becomes stable.
The OF (Overrange Flag) bit is set to a logic 1 any
time the input signal is: 1) more positive than posi-
tive full scale, or 2) more negative than negative
full scale. It is cleared back to logic 0 whenever a
conversion word occurs which is not overranged.
The last 24 SCLKs are used to clock the actual data
out of the conversion data register.
Table 1 and Table 2 illustrate the output coding for
the CS5540. Conversion data is output in two's
complement format.
2.7.3 Digital Filter
The CS5540 filter achieves simultaneous rejection
of 50/60 Hz and provides single conversion settling
at a 6.7 SPS throughput, including auto-calibration.
AIN+
AIN-
+
-
VREF+
VREF-
+
-
Reference
Figure 10. Self Calibration of Gain
CS5540
DS503PP1
15
The filter is optimized to yield better than 80 dB re-
jection between 47 Hz to 63 Hz (i.e. 80 dB mini-
mum rejection for both 50 Hz and 60 Hz) when the
master clock is 32.768 kHz. The filter's output
word rate can be increased by approximately 1.22X
by using a 40 kHz master clock, although the 50/60
Hz rejection will be sacrificed. The filter has a re-
sponse as shown in Figure 11.
Note:
The converter's digital filter linearly scales
with MCLK.
D31
D30
D29
D28
D27
D26
D25
D24
0
1
1
1
1
CH
OD
OF
D23
D22
D21
D20
D19
D18
D17
D16
D15
D14
D13
D12
MSB
22
21
20
19
18
17
16
15
14
13
12
D11
D10
D9
D8
D7
D6
D5
D4
D3
D2
D1
D0
11
10
9
8
7
6
5
4
3
2
1
LSB
Table 1. Output Conversion Data Register Description (24 bits + flags)
Note:
Plus or minus VFS is defined as a differential input signal equal in magnitude to the voltage between the
VREF+ and VREF- pins. See the text about error flags for overrange and underrange conditions.
Bipolar Input Voltage
Two's
Complement
>(VFS-1.5 LSB)
7FFFFF
VFS-1.5 LSB
7FFFFF
-----
7FFFFE
-0.5 LSB
000000
-----
FFFFFF
-VFS+0.5 LSB
800001
-----
800000
<(-VFS+0.5 LSB)
800000
Table 2. CS5540 24-Bit Bipolar Output Coding
-140
-120
-100
-80
-60
-40
-20
0
0
20
40
60
80
100
120
Frequency (Hz)
Ma
gni
tude
(
d
B
)
47 Hz
63 Hz
Figure 11. Filter Response (MCLK = 32.768 kHz)
CS5540
16
DS503PP1
2.8 Sleep Mode
The CS5540 features two power consumption
modes: normal and sleep. The normal mode, the
default mode, is entered after a power-on-reset. In
normal mode, the CS5540 performs conversions
and typically consumes 750 W.
To enter sleep, transmit 8 SCLK's (low-high tran-
sitions) after SDO falls, indicating that a conver-
sion is complete (32 SCLK's would be needed to
read the conversion). One conversion cycle later,
when SDO falls again, the part enters sleep, reduc-
ing the consumed power to around 30 W. During
sleep, most of the analog portion of the chip is pow-
ered down and filter convolutions are halted. While
the part is in the sleep mode, the SDO pin will re-
main in a logic high state, as long as the Chip Select
is enabled.
To exit the sleep mode, transmit 8 more SCLK's.
Since the sleep mode disables the oscillator, ap-
proximately a 500 ms crystal oscillator start-up de-
lay period is required before the ADC returns to the
normal mode. If an external clock is used, the ADC
will return to normal power mode within 3 milli-
seconds. In either case, the SDO pin will go low to
indicate when new data is available and can be
read.
2.9 Power-Up and Initialization
Care must be exercised to insure that no pins are
ever taken below the negative analog supply (VA-)
potential. The analog and digital supplies should be
applied simultaneously to assure that the power-on
reset circuit will automatically reset the ADC when
both supplies are at acceptable levels.
Conversions will begin once a stable clock is avail-
able to the ADC. If a 32.768 kHz crystal is being
used, it will take approximately 500 ms for the os-
cillator to stabilize and to begin conversions after
power has been applied to the converter. If a
CMOS compatible clock source with no start-up
delay is being used to drive the ADC, then conver-
sions will begin immediately.
Note:
The CS5540 includes an on-chip power on
reset circuit to automatically reset the ADC
shortly after power-up. When power to the
CS5540 is applied, the ADC is held in a reset
condition until the master clock has started
and a counter-timer elapses (i.e. the
counter-timer counts 490 clock cycles to
make sure the oscillator is fully stable).
After a valid reset, the ADC is initialized into the
data state where it begins to continuously calibrate
the ADC and convert the analog input. Once a valid
conversion is complete, monitor the SDO pin for a
falling edge to indicate that the data is ready to be
read.
2.10 PCB Layout
The CS5540 should be placed entirely over an ana-
log ground plane with the DGND pin of the device
connected to the analog ground plane. Place the an-
alog-digital plane split immediately adjacent to the
digital portion of the chip
See the CDB5540/41 data sheet for suggested lay-
out details and refer to Applications Note 18 for
more detailed layout guidelines.
Applications engineering provides a Free and Con-
fidential Schematic Review Service.
CS5540
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17
3. PIN DESCRIPTIONS
Clock Generator
OSC1; OSC2 - Master Clock.
An inverting amplifier inside the chip is connected between these pins and can be used with a
crystal or resonator to provide the master clock for the device. Alternatively, an external
(CMOS compatible) clock (powered relative to VD+) can be supplied into the OSC1 pin to
provide the master clock for the device and the OSC2 pin can be left unconnected.
Control Pins and Serial Data I/O
CS - Chip Select.
When active low, the port will recognize SCLK. When high the SDO pin will output a high
impedance state. CS should only be changed when SCLK = 0.
SDO - Serial Data Output.
SDO is the serial data output. It will output a high impedance state if CS = 1.
SCLK - Serial Clock Input.
A clock signal on this pin determines the output rate of the data for the SDO pins respectively.
This input is a Schmitt trigger to allow for slow rise-time signals. The SCLK pin will recognize
clocks only when CS is low.
CHS - Channel Select Input.
CHS permits the user to select between AIN1 and AIN2 for data conversions. When CHS = 0,
AIN1 is converted. When CHS = 1, AIN2 is converted. Note that since the converter
continuously converts the input selected by CHS, the channel being converted can be switched
at any time. The current conversion will be aborted and a new one started on the newly
selected channel. The serial data status flags will indicate which channel was converted.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
VREF+
VREF-
AIN2+
AIN2-
DGND
VD+
SDO
SCLK
CS
AIN1-
AIN1+
VA+
VA-
OSC2
OSC1
CHS
C
S
5540
CS5540
18
DS503PP1
Measurement and Reference Inputs
AIN1+, AIN1-, AIN2+, AIN2- - Differential Analog Inputs.
Differential input pins into the device.
VREF+, VREF- - Voltage Reference Inputs.
Fully differential inputs which establish the voltage reference for the on-chip modulator.
Power Supply Connections
VA+ - Positive Analog Power.
Positive analog supply voltage.
VA- - Negative Analog Power.
Negative analog supply voltage.
VD+ - Positive Digital Power.
Positive digital supply voltage.
DGND - Digital Ground.
Ground return for the digital supply.
CS5540
DS503PP1
19
4. SPECIFICATION DEFINITIONS
Linearity Error
The deviation of a code from a straight line which connects the two end points of the A/D
Converter transfer function. One end point is located 1/2 LSB below the first code transition
and the other end point is located 1/2 LSB beyond the code transition to all ones. Units in
percent of full-scale.
Differential Nonlinearity
The deviation of a code's width from the ideal width. Units in LSBs.
Full Scale Error
The deviation of the last code transition from the ideal [{(VREF+) - (VREF-)} - 3/2 LSB].
Units are in LSBs.
Offset Error
The deviation of the mid-scale transition (111...111 to 000...000) from the ideal (1/2 LSB below
zero volts differential between the AIN+ and AIN- pins). Units are in LSBs.
CS5540
20
DS503PP1
5. PACKAGE DIMENSIONS
Notes: 1. "D" and "E1" are reference datums and do not included mold flash or protrusions, but do include mold
mismatch and are measured at the parting line, mold flash or protrusions shall not exceed 0.20 mm per
side.
2. Dimension "b" does not include dambar protrusion/intrusion. Allowable dambar protrusion shall be
0.13 mm total in excess of "b" dimension at maximum material condition. Dambar intrusion shall not
reduce dimension "b" by more than 0.07 mm at least material condition.
3. These dimensions apply to the flat section of the lead between 0.10 and 0.25 mm from lead tips.
INCHES
MILLIMETERS
NOTE
DIM
MIN
NOM
MAX
MIN
NOM
MAX
A
--
--
0.084
--
--
2.13
A1
0.002
0.005
0.010
0.05
0.13
0.25
A2
0.064
0.069
0.074
1.68
1.75
1.88
b
0.009
0.012
0.015
0.22
--
0.38
2,3
D
0.232
0.244
0.256
5.90
6.20
6.50
1
E
0.291
0.307
0.323
7.40
7.80
8.20
E1
0.197
0.209
0.220
5.00
5.30
5.60
1
e
0.022
0.026
0.030
0.55
0.65
0.75
L
0.025
0.0295
0.041
0.63
0.75
1.03
0
4
8
0
4
8
JEDEC #: MO-150
16L SSOP PACKAGE DRAWING
E
N
1 2 3
e
b
2
A1
A2
A
D
SEATING
PLANE
E1
1
L
SIDE VIEW
END VIEW
TOP VIEW
Notes
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