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A/D Board Tutorial
Slideshow Contents
Introduction
Typical A/D Board Architecture
A/D Chip Technology
Resolution and Input Range
Input Bandwidth
Input Channel Configurations
Differential Input Simulation
A/D Sampling Methods
A/D Triggers
Sampling Sequences
Data Transfer to Memory
Interrupt Timing
Source Impedance
Source Impedance - Solution
Calibration
Autocalibration
Comparison Test: Autocalibration vs. Manual Calibration
Results: Autocalibration vs. Manual Calibration
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Differential Input Simulation


Generally, it is not possible to accurately simulate a differential input by using two single-ended inputs instead.

  • Timing: The time delay between sampling the high and low inputs causes errors based on the input signal's slew rate, as shown in the illustration.

    In this case, the correct measurement at T1 is V2 - V1 (the high input minus the low input). However, due to the change in voltage of the input signal between T1 when V1 is sampled and T2 when V2 is sampled, the board measures V2 for the high input and V2 again for the low input. The resulting measurement is erroneously reported as 0V! A true differential input would capture both V2 and V1 at precisely the same time and return an accurate measurement.

    All Diamond Systems analog input boards utilize true differential input circuits to provide accurate differential input measurements.


  • Noise: Because the two inputs are not being sampled at precisely the same time, any high frequency noise appearing on the inputs will be summed together, instead of canceling out as would happen with a true differential input. Therefore the effect of noise will be far higher.