Counter-ramp (digital-ramp) ADC principle: In a counter-ramp analog-to-digital converter, the comparator continuously compares the unknown analog input voltage against which reference signal within the conversion loop?

Introduction / Context:
A counter-ramp (also called digital-ramp) ADC is one of the simplest converter architectures. It builds a staircase reference using a DAC and a counter, then uses a comparator to determine when that staircase reaches the unknown input level. Knowing exactly what the comparator is comparing clarifies how code, time, and voltage relate in this ADC type.

Given Data / Assumptions:

• The converter contains a binary counter feeding a DAC.
• The DAC produces a staircase (increasing) voltage as the counter increments.
• A comparator senses the relationship between the analog input and the DAC output.
• Clock and control logic stop the count when the staircase just meets the input.

Concept / Approach:
The counter drives the DAC with successively larger digital codes. The DAC converts each code to an analog level, forming a staircase. The comparator continuously evaluates V_in − V_DAC. When V_DAC becomes equal to or just exceeds V_in (depending on design), the control logic stops the counter. The digital count then represents the measured value of the input.

Step-by-Step Solution:

Verification / Alternative check:
An oscilloscope view shows a staircase (DAC output) rising until it meets the static input. The instant the two match, the counter freezes—confirming that the comparator compares V_in to the DAC output, not to the raw counter value or a passive divider.

Why Other Options Are Wrong:

• a binary number: The comparator operates on voltages, not abstract numbers.
• the output of a counter: The counter is digital; the comparator needs analog levels.
• a voltage divider network: A passive divider is not how the reference staircase is generated here; the DAC provides the reference.

Common Pitfalls:
Confusing the counter (digital) with the DAC (analog); assuming the comparator directly “reads” the counter value; overlooking that resolution and conversion time depend on clock rate and full-scale steps.

Final Answer:
the output of a DAC