SBAA551 June   2022 ADS8661 , ADS8665 , ADS8671 , ADS8675 , ADS8681 , ADS8685 , ADS8689 , ADS8691 , ADS8695 , ADS8699 , INA823 , INA826 , LM317L , LM337L , MUX509 , TPS2661 , TPS560430

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction
  4. 2Circuit Description
    1. 2.1 Input Stage
    2. 2.2 Multiplexer Stage
    3. 2.3 Instrumentation Amplifier
    4. 2.4 Analog-to-Digital Converter
    5. 2.5 Power Supply
  5. 3Results
    1. 3.1 Accuracy and Noise Performance
    2. 3.2 Common-Mode Rejection of Input Signal
    3. 3.3 Input Impedance
    4. 3.4 Settling Time
    5. 3.5 Input Protection
  6. 4Summary
  7. 5References

1 Introduction

This analog input module front-end design features 8 channels with each channel that can be configured to measure bipolar, universal voltage, and current inputs with a resolution of 16 bits. The module supports a scan rate of 1 ms for all 8 channels. Multiple input voltage ranges are supported with a programmable gain amplifier (PGA). Input ranges supporting up to 20% overrange. Any common-mode voltage (VCM) is effectively rejected. An open-wire detection feature identifies a broken connection to the sensor field transmitter. The design supports the extended industrial operating temperature range of –40°C to 125°C. Table 1-1 summarizes the key parameters of this design.

Table 1-1 Design Parameter
ParameterValue
Input channel8
Input typeUniversal (Current, voltage), bipolar
Input modeDifferential or single-ended
Differential input signal (maximum)±12.288 V
Common-mode input signal (maximum)±12 V
Abs input voltage (each pin)±12 V (referred to module ground)
Resolution16 bit (15+1bit)
Sample rate8 kSPS

Voltage input ranges (including 20% overrange)

Voltage input ranges (no overrange)

Current input ranges

±10 V

±5 V

+10 V

+5 V

±2.5 V

0/4...20 mA

±20 mA

Input impedance

Voltage mode: > 1 MΩ

Current mode: 250 Ω

Open-wire detectionYes
50 or 60 Hz rejectionNo
Operating temperature–40°C to 125°C

All terminals are fully protected against overcurrent up to ±30 mA and overvoltage up to ±33 VDC (permanent). Surges according to IEC61000-4-5 (±1 kV at 42 Ω) are effectively suppressed. Measurements on this reference design board demonstrate 16 bits of effective resolution (or ENOB) and 13–14 noise-free bits (NFB), depending on the selected voltage input range. The accuracy at room temperature is < 0.1%. Table 1-2 summarizes the performance specifications for this design.

Table 1-2 Design Performance
ParameterPerformance
Overvoltage protection±33 V permanent
Overcurrent protection±30 mA
Surge protectionIEC61000-4-5 (±1 kV/42 Ω)
Mis-wiring protectionYes, voltage and current input mode
Effective number of bits (ENOB)≥ 16.3 bit
Noise-free bits≥ 13.7 bit
Accuracy error< ±0.1% at room temp
Common-mode voltage rejection> 80 dB