Data acquisition system and tracking/hold modes

Data acquisition system in DCS

The data acquisition system collects analog signals from plant sensors, conditions them, and converts them into digital form for the computer.

 Sensor -> Amp -> LPF --+
 Sensor -> Amp -> LPF --+--> Analog -> S/H -> A/D -> CPU
 Sensor -> Amp -> LPF --+     MUX

1. Transducer: converts the physical variable (temperature, speed, level) into a voltage or current.
2. Amplifier (signal conditioning): scales the small signal to the ADC input range and gives isolation.
3. Low-pass (anti-aliasing) filter: removes noise and limits bandwidth below half the sampling frequency.
4. Analog multiplexer: connects many channels, one at a time, to a single S/H and ADC (time sharing).
5. Sample-and-hold: captures the selected signal at the sampling instant and holds it during conversion.
6. A/D converter: quantizes and codes the held voltage into an n-bit binary number.
7. Control logic: selects channels and starts conversions at each sampling instant; the computer reads the result.

All channels are scanned once in every sampling period $T$. The data are then processed by the control algorithm, and the outputs are sent to actuators through D/A converters and hold circuits (data distribution).

Tracking mode and hold mode in S/H

 Vin --[A1]--o/ o--+--[A2]--> Vout
                   |
                  === C
                   |
                  GND

Tracking (sample) mode: the electronic switch is closed. The capacitor charges through the low-impedance buffer A1, and the output follows the input, $V_{out}(t) \approx V_{in}(t)$. The time to settle to the input within the required accuracy is the acquisition time.

Hold mode: the switch is opened at the sampling instant. The capacitor keeps its charge because the switch and the high-impedance buffer A2 draw almost no current, so $V_{out}$ stays at $V_{in}(kT)$. The ADC converts this steady value. A slow fall due to leakage is called droop.