---
title: "Data acquisition system and tracking/hold modes"
subject: "Digital Control System"
chapter: 1
key: "semester_6/digital_control_system:data-acquisition-tracking-hold"
source: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-tracking-hold
site: BCT Engineering
---

# Data acquisition system and tracking/hold modes

Digital Control System · Chapter 1: Introduction to Discrete Time Control System

- Asked in: 2074 Bhadra, 8 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/digital_control_system/data-acquisition-tracking-hold
- Whole chapter: https://www.bctengineer.com/md/semester_6/digital_control_system/chapter-1

## Question

Explain the data acquisition system in digital control system. Define tracking mode and hold mode in Sample-and-hold circuits.

## Short answer

A data acquisition system collects sensor signals, amplifies, filters, multiplexes, samples/holds, and converts to digital via ADC. In tracking mode, the S/H switch is closed and output follows input; in hold mode, switch opens and capacitor holds the voltage constant during ADC conversion.

## Answer

#### 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.

```text
 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

```text
 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.

| Point | Tracking mode | Hold mode |
|---|---|---|
| Switch | Closed | Open |
| Output | Follows input | Constant |
| Duration | Acquisition time | Conversion time |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the order of blocks in the data acquisition system?
   A. Sensor, A/D, MUX, S/H, amplifier
   B. Sensor, amplifier, LPF, analog MUX, S/H, A/D
   C. Sensor, S/H, LPF, MUX, A/D
   D. Sensor, MUX, ADC, amplifier
   Answer: B. Signals are conditioned and filtered per channel, then multiplexed, held and converted. (Key point: Order in DAS)

2. How often are all channels scanned?
   A. Once in every sampling period T
   B. Once per hour
   C. Only when the computer is idle
   D. Each channel has its own period only
   Answer: A. All channels are scanned once in every sampling period T. (Key point: Channel scanning)

3. Why does the capacitor keep its charge in hold mode?
   A. The input keeps feeding it
   B. The switch and high-impedance buffer A2 draw almost no current
   C. It is connected to ground
   D. A1 is turned off
   Answer: B. With almost no current path, the voltage stays at Vin(kT). (Key point: Hold mode retention)

4. In the comparison, what duration corresponds to hold mode?
   A. Acquisition time
   B. Conversion time
   C. Aperture time
   D. Sampling period only
   Answer: B. Tracking lasts the acquisition time and hold lasts the conversion time. (Key point: Duration of modes)

## Related in Digital Control System

- [Sample and hold: tracking and hold modes](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Asample-hold-tracking-hold) — Digital Control System, chapter 1, asked 3×
- [DCS block diagram and S/H tracking/hold modes](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adcs-block-diagram-tracking-hold) — Digital Control System, chapter 1
- [Define tracking and hold mode](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adefine-tracking-hold-mode) — Digital Control System, chapter 1, asked 2×
- [Blocks of a data acquisition system](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-system-blocks) — Digital Control System, chapter 1, asked 2×
- [Importance and operation of sample and hold](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Aimportance-of-sample-hold) — Digital Control System, chapter 1
- [Sample-and-hold capacitor voltage and advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Asample-hold-capacitor-voltage) — Digital Control System, chapter 1

## Related in other subjects

- [Digital data acquisition system block diagram](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Adigital-data-acquisition-system) — Instrumentation I, chapter 6
- [Training and tracking modes of adaptive equalizer](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aadaptive-equalizer-training-tracking) — Wireless Communication, chapter 5
- [Sample and hold circuit](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Asample-and-hold-circuit) — Instrumentation I, chapter 6
- [Real-time versus ordinary software and OS; data acquisition](https://www.bctengineer.com/question.md?key=semester_5%2Fsoftware_engineering%3Areal-time-vs-non-real-time) — Software Engineering, chapter 4