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Protection
LM5069
LDO
TPS71501
LDO
TPS71533
EEPROM
Isolated
Power Supply
LM5017
Protection
Protection
Filter
Filter
4 SE Voltage Inputs:
±10 VDC
0 VDC to 10 VDC
0 VDC to 5 VDC
1 VDC to 5 VDC
4 Current Inputs:
0 mA to 20 mA
4 mA to 20 mA
ADS8698
18-Bit, 8-Ch, 500- kSPS
SAR ADC
AVDD
DVDD
5-V V
ISO
5-V V
ISO
Digital Isolator
ISO7141CC
3.3 VDC
I
2
C
3.3 VDC,
15 mA
SPI
24 VDC
24 VDC_LIMIT
6-V V
ISO
5-V V
ISO
, 25 mA
9.3 VDC
50 Pin Interface
Connector
(To Base Board)
SBAS686 – JULY 2015
9.2.2 18-Bit, 8-Channel, Integrated Analog Input Module for Programmable Logic Controllers (PLCs)
Figure 109. 18-Bit, 8-Channel, Integrated Analog Input Module for PLCs
9.2.2.1 Design Requirements
This reference design provides a complete solution for a single-supply industrial control analog input module.
The design is suitable for process control end equipment, such as programmable logic controllers (PLCs),
distributed control systems (DCSs), and data acquisition systems (DAS) modules that must digitize standard
industrial current inputs, and bipolar or unipolar input voltage ranges up to ±10 V. In an industrial environment,
the analog voltage and current ranges typically include ±2.5 V, ±5 V, ±10 V, 0 V to 5 V, 0 V to 10 V, 4 mA to
20 mA, and 0 mA to 20 mA. This reference design can measure all standard industrial voltage and current
inputs. Eight channels are provided on the module, and each channel can be configured as a current or voltage
input with software configuration.
The key design requirements are given below:
•
Up to eight channels of user-programmable inputs:
–
Voltage inputs (with a typical Z
IN
of 1 M
Ω
): ±10 V, ±5 V, ±2.5 V, 0 V to 10 V, and 0 V to 5 V.
–
Current inputs (with a Z
IN
of 300
Ω
): 0 mA to 20 mA, 4 mA to 20 mA, and ±20 mA.
•
A 18-bit SAR ADC with SPI.
•
Accuracy of
≤
0.2% at 25°C over the entire input range of voltage and current inputs.
•
Onboard isolated Fly-Buck™ power supply with inrush current protection.
•
Slim-form factor 96 mm × 50.8 mm × 10 mm (L × W × H).
•
LabView-based GUI for signal-chain analysis and functional testing.
•
Designed to comply with IEC61000-4 standards for ESD, EFT, and surge.
9.2.2.2 Detailed Design Procedure
The application circuit and system diagram for this design is shown in
.
The module has eight analog input channels, and each channel can be configured as a current or voltage input
with software configuration. This design can be implemented using the ADS8698, 18-bit, 8-channel, single-supply
SAR ADC with an on-chip PGA and reference. The on-chip PGA provides a high-input impedance (typically 1
M
Ω
) and filters noise interference. The on-chip, 4.096-V, ultra-low drift voltage reference is used as the reference
for the ADC core.
Digital isolation is achieved using an
and
. The host microcontroller communicates with a
(an 8-bit, I
2
C, I/O expander over an I
2
C bus). The ISO1541D is a bidirectional, I
2
C isolator that
isolates the I
2
C lines for the TCA6408A. The TCA6408A controls the low R
ON
opto-switch (TLP3123) that is used
to switch between voltage-to-current input modes. The input channel configuration is done in microcontroller
firmware.
Copyright © 2015, Texas Instruments Incorporated
65
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