Mixed-Signal Electronics and Data Conversion
Sampling and Quantization
Conversion Fundamentals
A sampled system must satisfy the Nyquist criterion to avoid aliasing, but practical converters also face aperture jitter, quantization noise, INL, DNL, metastability, and reference noise. The signal chain must be designed as a system, not just a device.
Resolution and Noise
For an ideal -bit converter, the quantization step is and the quantization-noise power is often approximated by under uniform-error assumptions. Effective number of bits is reduced by nonidealities such as thermal noise, distortion, and clock jitter.
System Insight
In high-speed data converters, clock quality can be as important as converter architecture because jitter directly converts input frequency into amplitude error.
What is the main purpose of anti-alias filtering before an ADC?
Anti-alias filters attenuate out-of-band content so it does not fold into the sampled band.
Correct answer: Remove frequency components above Nyquist to prevent aliasing
Write the quantization step size for an ideal -bit ADC with full-scale range .
The step size is the full-scale range divided by the number of quantization levels.
Correct answer: Delta = V_FS / 2^N
ADC and DAC Architectures
Architectural Tradeoffs
Flash converters offer extreme speed but poor resolution efficiency; successive-approximation ADCs provide an attractive balance of speed and power; pipeline ADCs support high throughput; sigma-delta converters achieve high resolution via oversampling and noise shaping. DACs similarly trade speed, linearity, and glitch energy.
Converter Architecture Tradeoffs
SAR ADC
- Excellent power efficiency for moderate speeds
- Widely used in instrumentation and embedded systems
- Resolution and speed balance are favorable
Sigma-Delta ADC
- Very high resolution via oversampling
- Best suited to lower bandwidth signals
- Uses noise shaping and digital filtering
Which ADC architecture is especially suited to very high-resolution, relatively low-bandwidth measurements?
Sigma-delta converters are ideal for high-resolution low-bandwidth applications due to oversampling and noise shaping.
Correct answer: Sigma-delta
Name one nonideality that can limit high-speed ADC performance beyond quantization noise.
Sampling uncertainty directly degrades SNR for high-frequency inputs.
Correct answer: Clock jitter