Cirrus Logic CS5540-AS Datasheet

CS5540
Low-Power, Low-Voltage, 24 - Bit ∆Σ ADC

Features

l ∆Σ Analog-to-Digital Converter
- Linearity Error: 0.0015% FS
- RMS Noise: 4 µV
l Two Channel Differential MUX l Buffered, Fully Differential Analog and
Voltage Reference Inputs
l Scalable V 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
- SPITM and MicrowireTM Compatible
- Schmitt Trigger on Serial Clock (SCLK)
l Low Power
- Single +3.0 V Supply
- 330 µA Supply Current
- 10 µA in Sleep Mode
Input: 0.1 V to Analog Supply
REF

Description

The CS5540 is a low-power and low-voltage ∆Σ ana­log-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 mea­surement, 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 si­multaneous rejection of 50/60 Hz signals and provides single conversion settling at 6.7 SPS throughput. Abso­lute accuracy is achieved via continuous internal self-calibration. The device draws a nominal 330 µA.
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 ºC - +85 ºC 16-Pin SSOP
CHS
AIN1+
AIN1-
AIN2+
AIN2-
VA+
Input
Mux
VA-
VREF+
X1
X1
Preliminary Product Information
P.O. Box 17847, Austin, Texas 78760 (512) 445 7222 FAX: (512) 445 7581 http://www.cirrus.com
VREF-
X1
Differential 4th Order
Modulator
This document contains information for a new product. Cirrus Logic reserves the right to modify this product without notice.
X1
∆Σ
OSC1 OSC2
Clock Generator
Digital Filter
Copyright Cirrus Logic, Inc. 2000
(All Rights Reserved)
VD+
Serial Interface
Calibration Register
Output Register
DGND
CS
SDO
SCLK
JUN ‘01
DS503PP1
1
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
CS5540
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 infor­mation describes products which are in development and subject to development changes. Cirrus Logic, Inc. has made best efforts to ensure that the informa­tion 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, mechan­ical, 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 man­ufacture 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.
2 DS503PP1
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
DS503PP1 3

1. CHARACTERISTICS AND SPECIFICATIONS

CS5540
ANALOG CHARACTERISTICS (T
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.)
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
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 - µV
Bandwidth (-3 dB) - 11.96 - Hz
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.
= 25 °C; VA+ = +3.0 V ±5%, VA- = 0 V, VD+ = 3.0 V ±5%,
A
24
* Specifications are subject to change without notice.
4 DS503PP1
CS5540
ANALOG CHARACTERISTICS (Continued)
Parameter Min Typ Max Units
Analog Input
Common Mode + Signal on AIN+ or AIN-
Single Supply Dual Supplies
CVF Current on AIN+, AIN- (Note 6) - 12 - nA
Input Leakage for MUX when off - 10 - pA
Common Mode Rejection dc
50, 60Hz
Input Capacitance - 8 - pF
Voltage Reference Input
Range (VREF+) - (VREF-) (Note 8) 0.1 2.5 (VA+) -
CVF Current on VREF+ and VREF- (Note 7) - 20 - nA
Common Mode Rejection dc
50, 60 Hz
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
Power Supplies
DC Power Supply Currents (Normal Mode)
I
A+
I
D+
Power Consumption Normal Mode (Note 10)
Sleep Mode
Power Supply Rejection dc Positive Supplies
dc Negative Supplies
0.0
VA-
-
-
-
-
-1/OWR - s
-
-
-
-
-
-
-
-
120 120
120 120
225
25
750
30
80 80
VA+ VA+
-
-
(VA-)
-
-
280
36
1000
-
-
-
dB dB
dB dB
µA µA
µW µW
dB dB
V V
V
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.
DS503PP1 5
CS5540
3 V DIGITAL CHARACTERISTICS (T
= 25 °C; VA+ = 3.0 V ±5%, VA- = 0 V, VD+ = 3.0 V ± 5%,
A
DGND = 0 V.) (See Notes 1 and 11.) All voltage levels measured relative to DGND.
Parameter Symbol Min Typ Max Units
High-Level Input Voltage: All Pins Except OSC1, SCLK
OSC1
SCLK
Low-Level Input Voltage: All Pins Except OSC1, SCLK
OSC1
SCLK
High-Level Output Voltage: SDO, I
Low-Level Output Voltage: SDO, I
= -1.0 mA V
out
= 1.6mA V
out
Input Leakage Current I
3-State Leakage Current I
Digital Output Pin Capacitance C
V V V
V V V
IH
IH
IH
IL
IL
IL
OH
OL
in
OZ
out
0.6VD+
(VD+)-0.45
(VD+)-.25 - - V
Notes: 11. All measurements performed under static conditions.
TBD
-
-
-
-
-
-
-
-
-
-
-
-
0.16VD+ TBD
0.6
V V V
V V V
--0.4V
1A
--±10µA
-9-pF
6 DS503PP1
CS5540
ABSOLUTE MAXIMUM RATINGS (DGND = 0 V; See Note 12.)
Parameter Symbol Min Typ Max Units
DC Power Supplies (Notes 13 and 14)
Positive Digital
Positive Analog
Negative Analog
Input Current, Any Pin Except Supplies (Notes 15 and 16) I
Output Current I
Power Dissipation (Note 17) PDN - - 500 mW
Analog Input Voltage AIN and VREF pins V
Digital Input Voltage V
Ambient Operating Temperature T
Storage Temperature T
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.
VD+ VA+
VA-
IN
OUT
INA
IND
A
stg
-0.3
-0.3
-0.3
-
-
-
+4.0 +4.0 +0.3
--±10mA
--±25mA
(VA-) + (-0.3) - (VA+)+0.3 V
-0.3 - (VD+)+0.3 V
-40 - +85 °C
-65 - +150 °C
V V V
WARNING: Operation at or beyond these limits may result in permanent damage to the device.
Normal operation is not guaranteed at these extremes.
DS503PP1 7
CS5540
SWITCHING CHARACTERISTICS (T
DGND = 0 V; Input Levels: Logic 0 = 0 V, Logic 1 = VD+; C
= 25 °C; VA+ = +3.0 V ±5% VA- = 0 V, VD+ = 3.0 V ±5%,
A
= 50pF)
L
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
Fall Times (Note 19)
Any Digital Input Except SCLK
SCLK
SDO Output
t
t
rise
rise
-
-
-
-
-
-
50
50
-
-
-
-
1.0
100
-
1.0
100
-
µs µs ns
µs µs ns
Start-up
Oscillator Start-up Time XTAL = 32.768 kHz (Note 20) t
Power-on Reset Period t
ost
por
-500-ms
-490-MCLK cycles
Serial Port Timing
Serial Clock Frequency SCLK 0 - 2 MHz
Serial Clock Pulse Width High
Pulse Width Low
Enabled to SCLK Rising t
CS
SCLK falling prior to Chip Select Disabled t
t
1
t
2
3
4
250 250
-
-
-
-
ns ns
50 - - ns
100 - - ns
SDO Read Timing
CS
to Data Valid t
SCLK Falling to New Data Bit t
CS
Rising to SDO Hi-Z t
5
6
7
--150ns
--150ns
--150ns
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.
8 DS503PP1
CS
CS5540
SCLK
CS
SDO
SCLK
t3
t1
t2
t4
Figure 1. Continuous Running SCLK Timing (Not to Scale)
t5
t7
MSB MSB-1 LSB
t6
t2
t1
Figure 2. SDO Read Timing (Not to Scale)
DS503PP1 9
CS5540

2. GENERAL DESCRIPTION

The CS5540 is a low-power and low-voltage 24-bit ∆−Σ analog-to-digital converter (ADC). It is opti­mized to convert analog signals in DC measure­ment applications such as temperature and pressure measurement, 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 includes a two-channel multiplexer. Absolute accuracy is ac­complished via self-calibration. The device also operates with a variety of supply configurations while drawing a nominal 330 µA.
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 sac­rificed.

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 ∆−Σ 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 buff­er 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). Ap­plication Note 30, “Switched-Capacitor A/D Input Structures”, details various input architectures.
φ Fine
1
in = CVosf
φ
Coarse
2
AIN
To ease communications between the ADC and a microcontroller, the converter includes a simple three-wire serial interface which is SPI and Mi­crowire compatible. A Schmitt Trigger input is pro­vided on the serial clock (SCLK) input.
VREF+
X1
AIN1+
AIN1-
AIN2+
AIN2-
M U X
X1
X1
Figure 3. Multiplexer Configuration
Differential
Vos≤ 25mV
VREF-
X1
th
4 Order
∆Σ
Modulator
C = 8 pF
f = 2*MCLK = 65.536 kHz
Figure 4. Input model for AIN+ and AIN- pins
Sinc
Digital
Filter
4
Serial
Port
CS
SDO SCLK
CHS
10 DS503PP1
CS5540
13
14
15
16
17
18
19
0 0.5 1 1.5 2 2.5 3
VREF ( V)
Noise-Free Resolution (Bits)
Figure 5. Typical Noise-Free Resolution
vs. Voltage Reference
Noise-Free Res. = log
2
(Bipolar Span/6.6*RMS Noise)

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 refer­ence 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 ref­erence 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 references buffer is designed to accommodate rail-to-rail (common-mode plus sig­nal) input voltages. Typical CVF (sampling) cur­rent is about 16 nA (MCLK = 32.768 kHz).
φ Fine
1
in = CVosf
φ
Coarse
2
VREF
Vos≤ 25mV
C = 12 pF
f = 2*MCLK = 65.536 kHz
Figure 6. Input model for VREF+ and VREF- pins
DS503PP1 11
CS5540

2.3 Power Supply Arrangements

The CS5540 is designed to operate with a total sup­ply voltage of 3.0 V ± 5%. For maximum flexibili­ty, 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 pro­vide the master clock for the chip. The chip is de­signed 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 CS5540s serial interface consists of three con­trol lines: CS, SDO, and SCLK.
, Chip Select, is the control line which enables
CS 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 be­fore 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 conver­sions. When CHS = 0, AIN1 is converted. When CHS = 1, AIN2 is converted. Note that since the converter continuously converts the input selected
+3.0 V Analog
Supply
Voltage
Reference
Analog
Signal
Sources
0.1µF
+
-
14
13
16
15
1
2
VREF+
VREF-
AIN1+
AIN1-
AIN2+
AIN2-
4
VA+
CS5540
VA-
10
11
VD+
OSC2
OSC1
CS
SDO
SCLK
CHS
DGND
3
12
0.1µF
9
5 kHz ~ 40 kHz
10 M
8
5
10
6
7
Optional Clock
Source
Serial
Data
Interface
Channel
Selection
Figure 7. CS5540 Configured with a single +3.0 V Supply
12 DS503PP1
CS5540
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 en­ters 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 puls­es shift data out of the serial port. Figure 8 illus­trates the sequence necessary to read output data from the data conversion register.
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 ADCs 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.
AIN+
+
The CS5540 operates in a self-calibration mode which allows the converter to calibrate continuous­ly between each conversion (if a user requires sys­tem calibration, this can be accommodated in the system microcontroller). The sections that follow detail the conversion mode and also explain how to
SCLK
SDI set low while
CHS
SDO
= 7358 clock cycles
* t
d
= 4884 clock cycles
t
o
td *
0
reading status & data
to *
1
1
8 SCLKs to read status
Figure 8. Command and Data Word Timing
1
1
CH OD OF
AIN-
VREF-
Figure 9. Self Calibration of Offset
MSB
Data Time
24 SCLKs
-
LSB
DS503PP1 13
AIN+
Figure 10. Self Calibration of Gain
CS5540
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 channels data is ready, then fall to indicate that the conversion data is available.
Reference
+
-
AIN-
VREF+
VREF-
-

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. Never­theless, to read a conversion word, the user must is­sue 32 SCLKs. The first 8 SCLKs clear the SDO flag and read the status flags. Upon the falling edge of the 8th 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 con­version word from the port. Upon the falling edge of the 32nd 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 avail­able (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.

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 SCLKs are required to read a conver­sion. 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 oper­ating 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 posi­tive 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.
14 DS503PP1
CS5540
The filter is optimized to yield better than 80 dB re­jection between 47 Hz to 63 Hz (i.e. 80 dB mini­mum rejection for both 50 Hz and 60 Hz) when the master clock is 32.768 kHz. The filters 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 re­sponse as shown in Figure 11.
Note: The converters digital filter linearly scales
with MCLK.
0
-20
-40
-60
-80
-100
Magnitude (dB)
-120
-140 0 20 40 60 80 100 120
47 Hz
63 Hz
Frequency (Hz)
Figure 11. Filter Response (MCLK = 32.768 kHz)
D31 D30 D29 D28 D27 D26 D25 D24
01 1 1 1 CHODOF
D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12
MSB 2221201918 17 16 15 14 13 12
D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
11 109876 5 4 3 2 1LSB
Table 1. Output Conversion Data Register Description (24 bits + flags)
Bipolar Input Voltage
>(VFS-1.5 LSB) 7FFFFF
VFS-1.5 LSB
-0.5 LSB
-VFS+0.5 LSB
<(-VFS+0.5 LSB) 800000
Two’s
Complement
7FFFFF
-----
7FFFFE
000000
-----
FFFFFF
800001
-----
800000
Table 2. CS5540 24-Bit Bipolar Output Coding
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.
DS503PP1 15
CS5540

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 µW.
To enter sleep, transmit 8 SCLKs (low-high tran­sitions) after SDO falls, indicating that a conver­sion is complete (32 SCLKs would be needed to read the conversion). One conversion cycle later, when SDO falls again, the part enters sleep, reduc­ing the consumed power to around 30 µW. During sleep, most of the analog portion of the chip is pow­ered down and filter convolutions are halted. While the part is in the sleep mode, the SDO pin will re­main 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, ap­proximately a 500 ms crystal oscillator start-up de­lay 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 milli­seconds. 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

delay is being used to drive the ADC, then conver­sions 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 ana­log ground plane with the DGND pin of the device connected to the analog ground plane. Place the an­alog-digital plane split immediately adjacent to the digital portion of the chip
See the CDB5540/41 data sheet for suggested lay­out details and refer to Applications Note 18 for more detailed layout guidelines.
Applications engineering provides a Free and Con­fidential Schematic Review Service.
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 avail­able to the ADC. If a 32.768 kHz crystal is being used, it will take approximately 500 ms for the os­cillator to stabilize and to begin conversions after power has been applied to the converter. If a CMOS compatible clock source with no start-up
16 DS503PP1
3. PIN DESCRIPTIONS
CS5540
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.
AIN1+
AIN1-
VA-
VA+
CS
SCLK
CHS
OSC1
1
2
3
4
5
6
7
89
CS5540
16
15
14
13
12
11
10
AIN2+
AIN2-
VREF+
VREF-
DGND
VD+
SDO
OSC2
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.
DS503PP1 17
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.
CS5540
Positive digital supply voltage.
DGND - Digital Ground.
Ground return for the digital supply.
18 DS503PP1

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
DS503PP1 19

5. PACKAGE DIMENSIONS

16L SSOP PACKAGE DRAWING
N
CS5540
D
E
A2
A
E1
1
2
b
SIDE VIEW
1
23
e
TOP VIEW
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
A1
SEATING
PLANE
L
END VIEW
0° 4° 8° 0° 4° 8°
JEDEC #: MO-150
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.
20 DS503PP1
• Notes •
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