Hello Iv��n,
An operational amplifier such as the OPA330 can be used to measure the divider voltage even thought the ground reference for the divider, and the operational amplifier, are different. There are different ways to do this, but a straightforward way is to use the OPA330 as a difference amplifier. That requires adding 4 resistors around the OPA330 as shown in the TINA schematic diagram shown below. The gain through the non-inverting input is +1 V/V, and the gain through the inverting input is -1 V/V. The voltage output VO is equal to the voltage difference between the two inputs.
The resistor inputs are connected across the divider resistor at which you wish to measure the voltage. A difference in the ground potentials of the op amp (system) and the divider circuit will be divided down by the divider on the non-inverting input, but will appear as a common mode voltage to that input. Therefore, the divided down voltage must not exceed the common-mode voltage range of the op amp.
The voltage difference between the grounds and the divider setup are arbitrary in the example circuit intended only to illustrate the concept. Since the OPA333 is powered by a 3 V battery in the example, the divider bias voltage had to be made negative so that the output would swing positive relative to the amplifier's ground. Do note the 4 resistors associated with the op amp stage would be critically matched for best performance.
There are ready made difference amplifiers and instrumentation amplifiers that could be used in this manner and wouldn't require external components: INA826, INA333, INA152, etc. They are precision and available with various fixed gains, or externally settable gains. Most often they are more precise than what can be achieved with a discretely built amplifier.
Regards, Thomas
PA - Linear Applications Engineering
