I am working on a redesign and there is a negative bias generator on the ADC in the circuit. (not the circuit below) I am pretty sure that the small negative voltage instead of ground is there to ensure readings are correct when the input to the ADC is close to zero (or ground) but I am not sure I fully understand. Can someone explain the purpose of the negative bias generator more clearly? Also will the negative reference on the ground pin affect anything else in the circuit? For example do I need to compensate for the offset at the output at all?
NOTE: The circuit below is not the circuit I am working on. just an example that I found using the negative bias.
Best Answer
You are right. The negative bias is there to add some tolerance to the situation when the input touches the 0V rail, so that this input voltage is properly handled by the circuit.
Even if the op-amp is rail to rail, if the op-amp's return is referenced to 0V, then an input that touches 0 is stressing that op-amp's maximum limit. The transistors in the output stage may switch from the active region to saturation.
If 0V is a value that is expected to occur as part of normal operation, it should be within the limit, not at the limit. A device should not be on the verge of breaking when presented with a valid, expected input signal.
So, the purpose of this LM7705 is to allow rail-to-rail amplifiers operated on single-voltage supplies to cleanly touch the lower power rail voltage.