CN-0209: Fully Programmable Universal Analog Front End for Process Control...

Circuit Note
CN-0209
Circuits from the Lab™ reference
circuits are engineered and tested
for quick and easy system
integration to help solve today’s
analog, mixed-signal, and RF
design challenges. For more
information and/or support, visit
www.analog.com/CN0209.
Devices Connected/Referenced
ADP1720
High Voltage, Low Dropout Linear Regulator
ADG442
LC2MOS Quad SPST Switches
AD8275
G = 0.2, Level Translation ADC Driver
AD8676
Dual, Rail-to-Rail Output Op Amp
REF194
Precision, 4.5 V Voltage Reference
AD7193
24-Bit Σ-Δ ADC
AD8617
Low Noise, Rail-to-Rail Input/Output Amp
ADuM1400
Quad-Channel Digital Isolator
ADT7310
16-Bit Digital Temperature Sensor
ADuM1401
Quad-Channel Digital Isolator
ADG1414
Serially-Controlled Octal SPST Switches
Fully Programmable Universal Analog Front End
for Process Control Applications
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0209 Circuit Evaluation Board (EVAL-CN0209-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 provides a fully programmable
universal analog front end (AFE) for process control
applications. The following inputs are supported: 2-, 3-, and 4wire RTD configurations, thermocouple inputs with cold
junction compensation, unipolar and bipolar input voltages,
and 4 mA-to-20 mA inputs.
Today, many analog input modules use wire links (jumpers) to
configure the customer input requirements. This requires time,
knowledge, and manual intervention to configure and
reconfigure the input. This circuit provides a software
controllable switch to configure the modes along with a
constant current source to excite the RTD. The circuit is also
reconfigurable to set common-mode voltages for the
thermocouple configuration. A differential amplifier is used to
condition the analog input voltage range to the Σ-Δ ADC. The
circuit provides industry-leading performance and cost.
Because of the voltage gain provided by the AD8676 and
AD8275, the design is particularly suitable for small signal
inputs, all types of RTDs, or thermocouples.
The AD7193 is a 24-bit Σ-Δ ADC that can be configured to have
four differential inputs or eight pseudo differential inputs. The
ADuM1400 and ADuM1401 provide all the necessary signal
isolation between the microcontroller and the ADC.
The circuit also includes standard external protection and
is compliant with the IEC 61000 specifications.
CIRCUIT DESCRIPTION
This circuit provides a fully programmable universal analog
front end (AFE) for process control applications supporting
2-, 3-, and 4-wire RTD configurations, thermocouple inputs
with cold junction compensation, unipolar and bipolar input
voltages, and 4 mA-to-20 mA inputs as shown in the
configuration diagram of Figure 2.
The ADG1414, a serially controlled octal SPST switch, is used
to configure the selected measurement mode.
Voltage Measurement
This circuit supports the measurement of unipolar and bipolar
signal ranges up to ±10 V. The input signal goes through a
signal conditioning stage before conversion by the AD7193
ADC. The AD8676 amplifier buffers the inputs before the gain
stage. The AD8275 is used to level shift the input signal and
provides gain so that it matches the input range of the AD7193.
The AD8275 output is biased with a common-mode voltage
connected to its REF pin. This voltage is generated by the
REF194 precision 4.5 V reference.
RTD Measurement
As shown in the connection table, this circuit supports 2-, 3-, and
4-wire RTD configurations. In this case, the transducer is a
1000 Ω platinum (Pt) RTD (resistive temperature device). The
most accurate arrangement is a 4-lead RTD configuration. In
the application shown, an external 200 µA current source
provides the excitation current for the RTD, and the AD7193 is
operated at gain of 16 to maximize the dynamic range in the
circuit. The AD8617 amplifier is configured as a current source
when the RTD measurement mode is selected. It is reconfigured
in closed-loop to set the common-mode voltage when the
thermocouple measurement is selected. The AD8617 is a dual
low noise amplifier so that it can drive both input channels
Rev. A
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each circuit, and their function and performance have been tested and verified in a lab environment at
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©2011 Analog Devices, Inc. All rights reserved.
CHANNE L 1
CHANNE L 2
J2-3
J2-2
J2-4
J2-1
J1-3
J1-2
J1-1
J1-4
DIN
Rev. A | Page 2 of 5
P3
P2
ADG1414
SCLK
SUPPLY
+15V
SCLK
IN4
D4
IN3
D3
IN2
D2
IN1
D1
-15V
-15V
SYNC
GND2
S4
S3
S2
S1
GND2
D8
S8
ADG442
D7
S7
DIN
D6
S6
D5
D4
S5
D3
S4
D2
S2
S3
D1
S1
ADG1414
AVDD -15
AGND
AIN2_0
AIN1_0
GND2
10uF
10uF
GND2
-INB
+INB
+INA
-INA
GND2
-15V
-V
AD8676
+V
OUTB
OUTA
GND2
1/2
AD8617
+15V
+2.25V
+5V
GND2
ADP1720
V IN
VOUT
1/2
AD8617
+15V
+2.25V
-15V
0.1uF
0.1uF
11.5k
11.5k
0.1uF
GND2
REF1
REF2
+4.5V
+5V
J2-6
J2-5
J1-6
J1-5
GND2
+5V
10uF
CURRENT
CHANNE L 1&2
AVDD +15
P2
P2
4-20_2
4-20_1
AD8275
10uF
AD7193
DIN
DOUT
SCLK
DOUT/RDY
CS
DIN
SCLK
DVDD
ADT7310
+2.25V
0.1uF
+5V
GND2
1k
1k
AGND BPDSW DGND
GND2
AIN8
AIN7
AIN6
AIN5
AIN4
AIN3
AIN2
AIN1
P3
AIN2_0
GND2
0.1uF
REFIN1+ REFIN1- AVDD
+4.5V
4-20_2
4-20_1
P3
200
200
GND2
REF194
+4.5V
VOUT
AIN1_0
V IN
INT1
INT2
DOUT
AD7193
DOUT
AD7193
DIN
SCLK
SCLK
DIN
ADG1414
ADT7310
DIN
CS
DOUT
0.1uF
GND2
+5V
10k
Vdd2
Gnd2
Voa
Vob
Voc
Vid
Ve2
Gnd2
ADuM1401
Vdd1
Gnd1
Via
Vib
Vic
Vod
Ve1
Gnd1
10k
Vdd1
Gnd1
Via
Vib
Vic
Vid
Ve1
Gnd1
ADuM1400
Vdd2
Gnd2
Voa
Vob
Voc
Vod
Ve2
Gnd2
GND2
GND
INT
CT GND2
ADT7310
SCLK
VDD
+5V
GND1
+3.3V
INT2
INT1
INT1_I
INT2_I
DOUT_I
AD7193_I
SCLK_I
DIN_I
ADG1414_I
ADT7310_I
CN-0209
Circuit Note
Figure 1. Universal Programmable Analog Front End for Process Control Applications (Simplified Schematic: All Connections and Decoupling Not Shown)
09839-001
INTERFACE MICROCONTROLLER
Circuit Note
CN-0209
CONNECTION TABLE
RTD 2 WIRES
RTD 3 WIRES
RTD
JX-1 JX-2
RTD 4 WIRES
RTD
JX-3 JX-4
TC
JX-1 JX-2
RTD
JX-3 JX-4
DIFFERENTIAL INPUT VOLTAGE
TC
Va
Vb
JX-1 JX-2
JX-3 JX-4
SINGLE INPUT VOLTAGE
V
Uab = Va–Vb
JX-1
JX-2
JX-3
JX-4
JX-2
JX-3
JX-2
JX-3
CURRENT
CURRENT INPUIT
JX-6
09839-002
JX-5
Figure 2. Analog Input Configuration Table
available on the board. The resistor configuring the current
source must have a low temperature coefficient to avoid drift
errors in the measurement circuit.
The sense resistor must have a low temperature coefficient
to avoid temperature drift errors in the measurement
circuit.
Thermocouple Measurement
In a thermocouple application, the voltage generated by the
thermocouple is measured with respect to an absolute
reference, provided externally to the ADC. The cold junction
compensation is implemented using the ADT7310 16-bit
temperature sensor. Because the signal from the thermocouple
is small, and to maximize the dynamic range in the circuit, the
AD7193 is operated at its highest gain range of 128. Because the
input channel is buffered, large decoupling capacitors can be
placed on the front end, if required, to eliminate any noise
pickup that may be present in the thermocouple leads. The
common-mode voltage for the thermocouple measurement is
provided by the AD8617 amplifier.
Regulator and Reference Selection
4 mA-to-20 mA Current Measurement
This circuit also supports 4 mA-to-20 mA current
measurement. The current is converted to a voltage using an
on-board sense resistor. To use the full dynamic range of the ADC
in the current measurement mode, a 200 Ω resistor is used.
The ADP1720 was chosen as the 5 V regulator for this circuit.
The ADP1720 is a high voltage micropower linear regulator
particularly suitable for industrial application.
The 4.5 V REF194 was chosen as the reference for this circuit,
and the E-grade device has an initial accuracy ±2 mV at 25°C
and a drift of 5 ppm/°C maximum. It is a low dropout device
and consumes less than 45 μA, with performance specifications
over −40°C to +125°C.
Isolation
The ADuM1400 and the ADuM1401 are quad-channel digital
isolators based on Analog Devices’ iCoupler® technology. These
are used to provide isolation between the field side and the
system microcontroller, with an isolation rating of 2.5 kV rms.
Four wires are used through the ADuM1400, all for transmit
(SCLK, DIN, ADG1414, ADT7310). Four wires are used
through the ADuM1401: one for transmit (AD7193) and three
for receive (INT1, INT2, DOUT). The DIN, DOUT, and SCLK
lines are connected to the SPORT interface.
Rev. A | Page 3 of 5
CN-0209
Circuit Note
Table 1. Measured Performance Based on 1000 Samples
Input
±10V Input Range
4 mA to 20 mA Range
RTD
Thermocouple
AD7193 Configuration
Gain = 1; 50 Hz and 60 Hz rejection; output data rate = 50 Hz
Gain = 1; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz
Gain = 16; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz
Gain = 128; 50 Hz and 60 Hz rejection; output data rate = 2.63 Hz
Figure 3 shows a histogram plot of the AD7193 output
performance when configured in bipolar input mode with the
inputs connected to ground. This histogram shows the effects of
input-referred noise. The effective resolution achieved in this
mode is 19.2 bits.
Table 1 shows the performance for other operating modes based
on 1000 data samples from the ADC.
This design also includes external protection such as standard
protection diodes and transient voltage suppressors (TVS
devices) to enhance the robustness of the circuit. Refer to the
schematics and other resources in the CN0209 Design Support
Package: www.analog.com/CN0209-DesignSupport.
200
NUMBER OF OCCURRENCES
180
160
140
RMS Noise (nV)
7940
931
243
220
Effective Resolution (Bits)
19.15
22.24
20.29
19.23
Equipment Needed
• PC with a USB port and Windows® XP or Windows Vista®
(32-bit) or Windows® 7 (32-bit)
• EVAL-CN0209-SDPZ Circuit Evaluation Board
• EVAL-SDP-CB1Z SDP Evaluation Board
• CN0209 Evaluation Software
• Power supply: +15 V and –15 V
• RTD temperature sensor
• Thermocouple
Getting Started
Load the evaluation software by placing the CN0209 Evaluation
Software disc in the CD drive of the PC. Using "My Computer,"
locate the drive that contains the evaluation software disc and
open the Readme file. Follow the instructions contained in the
Readme file for installing and using the evaluation software.
Functional Block Diagram
See Figure 1 of this circuit note for the circuit block diagram,
and the file “EVAL-CN0209-SDPZ-SCH-Rev0.pdf” for the
circuit schematics. This file is contained in the CN0209 Design
Support Package: www.analog.com/CN0209-DesignSupport.
120
100
80
60
40
0
–0.01699 –0.01698 –0.01697 –0.01696 –0.01695 –0.01694 –0.01693
AMPLITUDE (V)
09839-003
20
Figure 3. Noise Distribution Histogram,1000 Samples
AD7193 50 Hz Data Rate, Gain = 1, Input = 4.5 V Reference
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0209-SDPZ circuit board and
the EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance. The EVAL-CN0209-SDPZ board
contains the circuit to be evaluated, as described in this note,
and the SDP evaluation board is used with the CN0209
evaluation software to capture the data from the
EVAL-CN0209-SDPZ circuit board.
Setup
Connect the 120-pin connector on the EVAL-CN0209-SDPZ
circuit board to the connector marked “CON A” on the
EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware
should be used to firmly secure the two boards, using the holes
provided at the ends of the 120-pin connectors. With power to
the supply off, connect a +15 V power supply to the pin marked
“+15 V,” a −15V power supply to the pin marked “−15 V”and
“GND” on the board. Connect the USB cable supplied with the
SDP board to the USB port on the PC. Note: Do not connect
the USB cable to the mini USB connector on the SDP board at
this time.
Test
Apply power to the ±15 V supply connected to the
EVAL-CN0209-SDPZ circuit board. Launch the evaluation
software and connect the USB cable from the PC to the USB
mini-connector on the SDP board.
Rev. A | Page 4 of 5
Circuit Note
CN-0209
Once USB communications are established, the SDP board can
now be used to send, receive, and capture serial data from the
EVAL-CN0209-SDPZ board.
Voltage Measurement
If you want to measure the noise of the voltage measurement
circuit, connect both inputs J3 and J4 to the ground. Then, click
on the button of the matching channel of the software: either
V1 (if you are using channel 1) or V2 (if you are using channel 2).
If you want to measure a voltage, connect both inputs J3 and J4
as shown in Figure 2, the analog input configuration table.
Then, click on the matching button of the software as
previously explained.
Results are displayed as a waveform and a histogram. You have
the option to select the scale of the voltage result between µV,
mV, and V using the switching button.
RTD Measurement
If you want to measure the temperature through an RTD
temperature sensor, connect inputs J1, J2, J3, and J4 as shown
in Figure 2. There are three different configurations of
connection as you are using RTD 2-, 3-, or 4-wire. Then, click
on the matching button of the software (RTD1 for channel 1,
RTD2 for channel 2).
The switching button above the waveform allows you to display
the result in Fahrenheit, Celsius, or Kelvin.
LEARN MORE
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise? Analog Devices.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics, Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
Sensor Signal Conditioning, Analog Devices, 1999, Section 7.
Data Sheets and Evaluation Boards
ADuM1400 Data Sheet
ADuM1401 Data Sheet
AD7193 Data Sheet
AD7193 Evaluation Board
AD8275 Data Sheet
REF194 Data Sheet
AD8676 Data Sheet
AD8617 Data Sheet
ADG442 Data Sheet
Thermocouple Measurement
ADG1414 Data Sheet
If you want to measure the temperature through a
thermocouple, connect inputs J1, J2, J3, and J4 as shown in
Figure 2. Select the type of thermocouple you are using (B, E, J,
K, R, S, T, N). Then, click on the TC button of the software
(TC1 for channel 1, TC 2 for channel 2).
ADT7310 Data Sheet
REVISION HISTORY
The switching button above the waveform allows you to display
the result in Fahrenheit, Celsius, or Kelvin.
4/11—Revision 0: Initial Version
11/11—Rev. 0 to Rev. A
Changes to Figure 2 .......................................................................... 3
Current Measurement
If you want to measure a current, connect both inputs J5 and J6,
as shown in Figure 2. Then, click on the matching button
(I1 for channel 1, I2 for channel 2).
You can select the scale of the current result between µA, mA,
and A using the switching button.
Information regarding the SDP board can be found in the
SDP User Guide.
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CN09839-0-11/11(A)
Rev. A | Page 5 of 5
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