---
title: "Define tracking and hold mode"
subject: "Digital Control System"
chapter: 1
key: "semester_6/digital_control_system:define-tracking-hold-mode"
source: https://www.bctengineer.com/q/semester_6/digital_control_system/define-tracking-hold-mode
site: BCT Engineering
---

# Define tracking and hold mode

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

- Asked in: 2073 Magh, 4 marks; 2072 Asoj, 4 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/digital_control_system/define-tracking-hold-mode
- Whole chapter: https://www.bctengineer.com/md/semester_6/digital_control_system/chapter-1

## Question

Define the term tracking mode and hold mode in sample and hold circuit.

## Short answer

In tracking mode the switch is closed, the output follow the input, and the capacitor charge. In hold mode the switch is open, the capacitor retain the voltage at that instant, output stays constant, allowing A/D conversion.

## Answer

A sample-and-hold (S/H) circuit has an electronic switch, a hold capacitor $C$ and buffer amplifiers. It works in two modes set by a logic control signal.

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

#### Tracking (sample) mode

- The switch is **closed**.
- The capacitor charges through the low-impedance input buffer and the output **follows the input**: $V_{out}(t) \approx V_{in}(t)$.
- The time taken for the output to settle to the input value is the **acquisition time**.

#### Hold mode

- The switch is **opened** at the sampling instant.
- The capacitor keeps the voltage it had at that instant, and the high-impedance output buffer prevents discharge, so $V_{out}$ stays constant at $V_{in}(kT)$.
- The A/D converter converts this constant voltage during the hold time.
- Small errors in this mode: **droop** (slow fall of voltage due to leakage) and **feedthrough**.

| Point | Tracking mode | Hold mode |
|---|---|---|
| Switch | Closed | Open |
| Output | Follows input | Constant |
| Purpose | Acquire new sample | Allow A/D conversion |

## Check yourself

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

1. In tracking mode, what is the switch state and what happens to the output?
   A. Open; output stays constant
   B. Closed; capacitor charges and the output follows the input
   C. Closed; output stays at zero
   D. Open; output follows the input
   Answer: B. The closed switch lets C charge through the input buffer so Vout follows Vin. (Key point: Tracking mode definition)

2. What happens to the output in hold mode?
   A. It follows the input closely
   B. It becomes zero
   C. It stays constant at the value Vin(kT) present when the switch opened
   D. It doubles
   Answer: C. The capacitor keeps the voltage at the sampling instant. (Key point: Hold mode definition)

3. What is the acquisition time?
   A. Time for the output to settle to the input value in tracking mode
   B. Time the A/D takes to convert
   C. Time the capacitor takes to discharge in hold
   D. Delay between hold command and switch opening
   Answer: A. It is the time taken in tracking mode for the output to reach the input within the stated accuracy. (Key point: Acquisition time)

4. What is the purpose of hold mode?
   A. To acquire a new sample
   B. To allow the A/D converter to convert a steady voltage
   C. To amplify the input
   D. To remove noise
   Answer: B. Tracking acquires a new sample; hold keeps it steady for the A/D conversion. (Key point: Purpose of each mode)

## 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
- [Short note: Sample and hold circuit](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Ashort-note-sample-hold) — Digital Control System, chapter 1
- [Data acquisition system and tracking/hold modes](https://www.bctengineer.com/question.md?key=semester_6%2Fdigital_control_system%3Adata-acquisition-tracking-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
- [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

## Related in other subjects

- [Sample and hold circuit](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Asample-and-hold-circuit) — Instrumentation I, chapter 6