The Indirect Approach to Conversion

Unlike direct conversion methods like the Flash ADC, which instantly compare an input voltage to many reference levels, integrating ADCs use an indirect method. The core principle of this technique is to first convert the analog input voltage into a proportional time interval. This time interval is then measured by a digital counter, and the final count becomes the digital representation of the original voltage.

This approach sacrifices speed for exceptional accuracy and noise immunity, making integrating converters the technology of choice for high-precision measuring instruments. The most common architectures are the single-slope and the more advanced dual-slope converters.

The Single-Slope ADC

The single-slope ADC is the most straightforward implementation of the integrating conversion principle. Its operation is simple and intuitive.

Principle of Operation

The conversion process unfolds as follows:

1. At the start of a conversion, a control logic resets a digital counter and triggers an integrator to start generating a linear ramp voltage (a sawtooth wave).
2. The counter begins counting pulses from a stable clock source at the exact same moment.
3. A comparator continuously compares the rising ramp voltage from the integrator with the unknown input voltage $U_{IN}$.
4. When the ramp voltage becomes equal to $U_{IN}$, the comparator changes its output state. This change instantly stops the digital counter.
5. The final digital number held by the counter is the output of the ADC. Since the ramp rises linearly, the time it takes to reach $U_{IN}$ - and thus the final count - is directly proportional to the magnitude of the input voltage.

The Dual-Slope ADC: Precision Through Ratiometric Conversion

The dual-slope (or double-integration) ADC is an ingenious enhancement of the single-slope design that overcomes its critical flaw. By using two integration phases, it cleverly eliminates the dependency on the integrator's R and C components, resulting in exceptional accuracy.

Principle of Operation: A Two-Phase Process

The conversion happens in two distinct phases:

1. Phase 1 (Input Integration - "Charge Up"): The unknown input voltage $U_{I}$ is connected to the integrator for a fixed period of time, $T_1$. This fixed time is precisely controlled by allowing a counter to run for its full cycle (e.g., $N_{max}$ clock pulses). During this phase, the integrator's output ramps up to a peak voltage that is directly proportional to the average value of $U_{I}$ over that time.
2. Phase 2 (Reference De-integration - "Ramp Down"): At the end of $T_1$, the integrator's input is switched to a stable, known negative reference voltage, $-U_R$. The integrator's output, which was at a positive peak, now begins to ramp down with a constant slope. The counter is reset and starts counting again. It continues to count until the integrator's output reaches zero, which is detected by the comparator. The number of clock pulses, $N$, counted during this second phase ($T_2$) is the final digital result.

Because the ramp-up slope depends on $U_I$ and the ramp-down slope depends on the constant $-U_R$, a higher input voltage will result in a longer ramp-down time. Crucially, the exact values of R and C affect both slopes equally, and their effects cancel out in the final calculation. This "ratiometric" measurement means the accuracy is primarily dependent only on the stability of the reference voltage $U_R$.