How it works
A DAS consists of: (1) Transducer — converts physical quantity to electrical signal (RTD gives resistance change, strain gauge gives ΔR, thermocouple gives millivolts); (2) Signal Conditioning — amplification using INA128 instrumentation amplifier, filtering, bridge completion; (3) Multiplexer (MUX) — selects one channel at a time, e.g., CD4051 8-channel analog MUX controlled by digital address lines; (4) Sample and Hold (S/H) circuit — freezes the signal during ADC conversion using a hold capacitor (typically 100 pF–10 nF) and an op-amp; (5) ADC — converts the held voltage to digital code; (6) Digital Interface — parallel or serial (SPI, I²C, USB) output to a microcontroller or PC. The anti-aliasing filter at the MUX input must have cutoff below fs/(2·N_channels).
Key points to remember
Aperture time of the S/H circuit determines the maximum slew rate of input signal that can be accurately held; for a 12-bit system with 5 V range and 1 kHz input, maximum allowable aperture time is 1/(2^13·π·1000) ≈ 39 ns. Throughput rate of a multiplexed DAS = ADC conversion rate / number of channels; an ADC converting at 100 kS/s with 8 channels gives only 12.5 kS/s per channel. The INA128 instrumentation amplifier has CMRR > 90 dB, essential for rejecting 50 Hz common-mode noise on long sensor cables. Quantisation error of an N-bit ADC is ±½ LSB, equivalent to ±Vref/(2^(N+1)); for a 12-bit, 5 V ADC, this is ±0.61 mV. IEEE-488 (GPIB) and USB interfaces connect laboratory DAS modules to PCs; modern industrial DAS use 4–20 mA current loop for noise-immune long-distance signal transmission.
Exam tip
Every Anna University instrumentation paper asks you to draw the complete block diagram of a DAS and explain the purpose of the sample-and-hold circuit — state that it freezes the analog input during ADC conversion to prevent errors from signal change during the conversion time, then give one example aperture time value.