In the above image there are two figures, Fig(a) consists of force sensor connected to a resistor in a simple voltage divider configuration, and its output is fed to an ADC. Now coming to Fig(b), its the same circuit as Fig(a), however in this case the voltage divider circuit is followed by an op-amp in voltage follower configuration.
The circuit in Fig(a) does not have any output offset voltage, whereas the circuit in Fig(b) has an output offset voltage of 650 mV. I know that the circuit in Fig(b) is mostly preferred over the one in Fig(a) due to the op-amp having high input impedance, but I don't understand what difference it makes in the output fed to the ADC. Now my questions are as follows:
- What is the difference between the two circuits with respect to the output being fed to the ADC i.e. in Fig(a) and Fig(b). And why is the circuit in Fig(b) more preferred than in the Fig(a)?
2.The OP177 op-amp has pins 1 and 8 for trimming the offset i.e. 650 mV using a POT, however this isn't working. So why isn't the offset nullification working?. The POT has 3 terminals among which, 1st and 3rd terminals are connected to pin 1 and pin 8. And the middle terminal is connected to +Vss(pin7) for offset trimming.