ADC types — which converter outputs a 1-bit stream whose density corresponds to the analog level? Identify the ADC architecture that converts the analog signal into a high-rate, 1-bit bitstream, where the proportion of 1s over time (bit density) represents the input amplitude.

Introduction / Context:
Sigma-delta (ΔΣ) conversion is widely used in audio and precision measurement due to its excellent noise-shaping and high resolution after digital filtering. Unlike stepwise search or comparator arrays, it produces a 1-bit, high-frequency stream whose density encodes the analog input. Recognizing this signature behavior distinguishes ΔΣ ADCs from SAR, flash, and dual-slope approaches.

Given Data / Assumptions:

• Bitstream output is 1-bit at a much higher sampling rate than the target Nyquist rate.
• Average (or filtered) value of the bitstream corresponds to the input voltage.
• Digital decimation filters reconstruct the multi-bit code at the output rate.

Concept / Approach:
A sigma-delta modulator consists of an integrator, a 1-bit quantizer (comparator), and feedback (the “delta”) path. The modulator forces the average of the 1-bit stream to track the input level, while pushing quantization noise to higher frequencies (noise shaping). A subsequent digital filter/decimator extracts a high-resolution multi-bit word. In contrast, SAR produces an N-bit code directly after N trials, and dual-slope integrates and measures times; neither emits a 1-bit density-coded stream.

Step-by-Step Solution:

Verification / Alternative check:
Audio ADCs (e.g., 24-bit ΔΣ) list oversampling ratios and decimation filters. Their raw modulator outputs are 1-bit streams whose moving average equals the input fraction of full scale.

Why Other Options Are Wrong:

• Successive approximation: outputs an N-bit word after a fixed number of comparisons.
• Dual-slope: measures time intervals from an integrator; not a density bitstream.
• Pipeline: uses stages with residue amplification, not 1-bit density coding.
• “None of the above”: incorrect because sigma-delta fits perfectly.

Common Pitfalls:

• Confusing PWM with ΔΣ; both are density-modulated, but ΔΣ specifically shapes noise and uses feedback integration.

Final Answer:
Sigma-delta