Question

If a sample and hold circuit is not used with an ADC, the result may be _______.

• A. more accurate
• B. completely error-free
• C. corrupted/inaccurate due to aperture error
• D. faster with high resolution

Hint:

To prevent amplitude tracking errors (aperture distortion) in high-frequency data acquisition systems, the signal must remain perfectly frozen during conversion. Hence, an ADC and a Sample-and-Hold circuit are virtually inseparable for non-DC signals.

Answer 1

The Correct Option is C

Concept: An Analog-to-Digital Converter (ADC) requires a finite conversion duration ($\tau$) to resolve an analog input voltage into a precise digital representation. If the incoming analog signal fluctuates rapidly during this conversion interval, the ADC sub-circuits will sense multiple varying voltages, leading to severe conversion ambiguity known structurally as aperture jitter or aperture error.

Step 1: Mechanism of Analog-to-Digital Conversion.
During processing, internal comparator arrays or successive approximation registers test bit-weight balances against the input voltage level. If the analog voltage $v(t)$ changes significantly such that: \[ \Delta v = \frac{dv(t)}{dt} \cdot \tau > \text{1 LSB} \] where $\tau$ is the aperture conversion time and LSB is the Least Significant Bit voltage value, the digital output code will fail to represent the signal at either the start or end of the step. This introduces severe amplitude distortion.

Step 2: Role of the Sample and Hold (S/H) circuit.
The S/H circuit addresses this by sampling the rapidly shifting continuous-time signal at a precise moment and holding that voltage constant across an internal capacitor for the entire duration of the ADC's processing phase. Omitting the S/H circuit causes the analog signal to vary during conversion, leading to highly inaccurate or corrupted digital results.