Confusion on the purpose of using active current-meters

Question

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?

Answer 1

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.

the "current source seeing some load impedance" [issue]... is a bit confusing since the load resistance will be in series with the current source and why would it have affect on the current. A simple illustration would help a lot if you have time,

^{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.

Answer 2

A DC coil current meter has a certain loss of winding resistance often rated at 20kΩ/V which yields 50uA full scale. Shunt R's are then added across for precise ratios to scale the full scale current.

An OpAmp due to the negative feedback will have an input impedance of Rf/Aol for the feedback R and open Loop Gain of 1e6, thus provides lower loss and higher gain and of course also can convert I to V at a wide frequency range unlike a DC coil meter and also very low currents with high gain and can be bipolar AC to DC with latter stages of precision rectifiers.

Whatever the source impedance is, the current sensor must be much lower impedance to sense all of the current.

^{simulate this circuit – Schematic created using CircuitLab}

Obviously it depends if you want a DMM output or an analog meter output for choice and AC uses a rectifier in coil meters, scaled to measure rms/peak or rms/avg so unless a pure sine may be in error and DC for rms/pk measure twice the actual in AC V or A mode in analog meters.

Answer 3

Tony has things correctly stated. I thought I'd just add another way of writing similar things without getting mired. Let's redraw the circuits:

^{simulate this circuit – Schematic created using CircuitLab}

I've drawn the regular analog meter movement on the top, and its equivalent. Here, you can see that the meter does actually inject a potentially significant resistance in series (which may upset the measurement, depending on circumstances.) The analog meter movement here presents \$500\:\Omega\$ of DC resistance.

I've drawn the opamp circuit on the bottom, and its equivalent. Here, you can see that the effective series resistance may be very significantly diminished, now. (\$A_O\$ is the open loop gain of the opamp.) The open loop gain of the opamp would have to be as bad as 100 to present the same series resistance. But that's not the usual case. Open loop gains are much, much higher in even the cheapest opamps. (I've neglected offset voltage and current and bias currents, to focus on the central point.)

By the way, you can often use much cheaper analog panel meters as a result, as well, and still get significant reductions in the impact on a circuit under measurement using an opamp. And opamps, even expensive ones, are often a lot cheaper than a very high quality analog D'Arsonval movement will be. I suspect this has largely killed wide-scale production of sensitive D'Arsonval movements, despite the somewhat more recent advent of very powerful Neodymium permanent magnets.