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

2.4 Analog-to-Digital Converter

The ADC used in this design is the ADS8689, a 16-bit successive approximation register (SAR) converter with integrated signal-chain consisting of a programmable gain amplifier (PGA) and a 2nd-order low-pass filter (LPF). The ADS8689 accepts input voltages up ±12.288 V while only requiring a single 5-V power rail. The PGA allows a constant resolution across all voltage inputs ranges because its output always utilizes the entire ADC input range. The integrated LPF with a fCUT-OFF_-3dB = 15 kHz limits signal settling time in a multiplexed design. The SAR converter can operate at a sample rate of up to 100 kSPS. The sampling rate is set by the processing unit via the serial interface CONVST/CS signal. The precision integrated 4.096-V voltage reference reduces board space and cost. The ADS8689 employs a simple SPI for seamless connection to standard microcontrollers and microprocessors. If isolation is required, refer to Digitally-isolated ADS8689 circuit design.