I'm trying to design an ADC protection circuit in order to measure a few voltages (a USB Bus, a wireless charger output, a battery, etc.) on some GPIO pins limited to 1.8V. There are quite a few other questions on here about ADC protection, but most surround the use of zeners, but I liked the idea of using a voltage follower instead as it also brings all the benefits of isolation they provide from the rest of the circuit.
I'm planning to use the MCP6001 for the voltage follower.
Reading through this article from Analog at the bottom there is a circuit shown below:
for protecting ADC inputs. They state:
Another possibility, as shown in Figure 5, is to use a slightly lower reference value (for example, 4.096 V when using a 5 V rail), resulting in significantly reduced voltage overdrive capability.
Why set REF to 4.096 rather than 5V? Wouldn't that lead to an overvoltage condition being impossible?
I also have a question regarding the RC filter used. In the article they state:
The series R in the RC filter between the amplifier and the ADC input can also be used to limit the current seen at the ADC input during an overvoltage condition. However, this will be a trade-off between current limiting and ADC performance. Larger series R will give better input protection but result in greater distortion in the ADC performance. This may be an acceptable trade-off, especially if the input signal bandwidth is low or the ADC is not being run at the full throughput rate, as more series R can be tolerated in this case. The size of R acceptable for the application can be determined experimentally.
I don't understand a few things about this paragraph and hoping someone can answer them:
- How would the resistor increase distortion in the ADC performance?
- Why do you even need current limiting here? If you power the voltage follower from the 5V rail, and also use it as the REF, there shouldn't be any overvoltage condition, so the internal diodes at the ADC should never be on, meaning current is limited to the reverse leakage current of the internal diodes. Am I missing something? If not, can't I just eliminate the RC filter altogether (assuming I'm ok with any HF noise on the signal)?