I was reading about current to voltage converters and simple opamp current-meters.
I found myself in confusion comparing them to passive current meters.
Active current-meter:
As you see above the Iin passes through R and the voltage drops to -Iin*R as Vout. So Vout is linearly changing and with Iin.
And below is measuring current without an opamp:
Passive current-meter:
Above the same logic, the Iin passes through R and Vout is linearly changing with Iin.
Can someone explain what is the point/benefit to use opamp here? What is the advantage of using opamp/active version over the passive one?
Best Answer
In your first circuit, so long as the current is small enough and it changes slowly enough, the negative feedback circuit keeps the inverting input of the op-amp at 0 V, so the source sees effectively 0 load impedance.
In the second circuit, the source sees a load impedance equal to the resistor value, which could change the current it produces. Furthermore, the higher the resistance value (increasing the resolution of whatever is measuring the voltage across the resistor), the greater the resistance seen by the source, so the greater the effect on its output.
simulate this circuit – Schematic created using CircuitLab
If Rsense is 0, then all of the source current goes through Rsense and the measurement does not disturb the thing being measured.
If Rsense is non-zero, then some of the source current is diverted internally through the source's internal conductance, and the measured current is lower than it would have been if measured with a 0-resistance sensor.