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

3.3 Input Impedance

Input impedance is an important design parameter in voltage-input mode. Analog input modules commonly require > 1-MΩ input impedance to minimize resistive loading when a signal is applied to the analog front end. The majority of the input current leakage shown in the following table is caused by the TPS26611 loop protector and is dependent on the absolute voltage at the inputs. With –5.2 µA, worst case at –10 V, the input impedance is 1.9 MΩ. This measured value is well above the required 1 MΩ.

Table 3-3 Input Leakage Current at Terminals (Voltage Mode)
VINx+ VINx– IINx+ IINx–
+10 V GND –2.5 µA –4.7 µA
–10 V GND –5.1 µA –3.5 µA
GND +10 V –2.5 µA –4.7 µA
GND –10 V –5.2 µA –3.5 µA