Designing Analog Electronics 11

Part 2: Signal Conditioning with Op Amps

- By Andrew Levido

Designing analog electronics hardware is less orderly and predictable than digital electronics design. In Part 2 of this article series, Andrew looks at how to design a circuit to extract a small DC signal that happens to be riding on a much larger DC offset.

In Part 1 of this article series (Circuit Cellar #424, November, 2025) [1] I introduced the importance of managing error and uncertainty in designing analog circuits. In an example, we conditioned a 12V signal for digitisation by a microcontroller's ADC, and deliberately introduced a small offset to this signal to keep it away from the supply rails to avoid errors.

In Part 2, I will look at the common problem of extracting a small DC signal riding on a much larger DC offset. One example of this might be the measurement of current flowing into or out of the positive terminal of a battery as it charges and discharges (Figure 1). We will assume that the battery voltage is a maximum of 24V and we are trying to measure a current up to ±6A.

Current will be measured by inserting a low-value series resistor and monitoring the voltage developed across it. We want to keep the value of the shunt resistor low, since we don't want to drop too much of our precious battery voltage across it, and we want to keep its power dissipation low. We could, for example, use a 25mΩ shunt, which would give us a maximum voltage drop of ±150mV (a 300mV span) and a maximum power dissipation of 900mW. We will therefore need to amplify the shunt voltage by a factor of 10 to get this signal up to a 3V span to suit the microcontroller's analog-to-digital converter (ADC).