Is there a way (or some chip) to measure current at about 5 MHz when the bridge amplifier is used? The output is a sine with amplitude 15 V. I need to measure accurately both amplitude and phase (the load is not known; some complex impedance). I use a scheme similar to that on the picture (bridge connection of two op-amps).

Also, the ADC (or oscilloscope), that should measure the voltage (to measure the current) has the same ground (and power supply) as op-amps, input source, etc.

I tried to use differential/instrumentation amplifiers for the current measurements (by measuring the voltage across a shunt resistor connected in series with load). The voltage across the shunt resistor is about 200 mV. It works well at some tens-hundreds of kHz, but inaccurate at higher frequencies. Maybe common-mode characteristics of my differential/instrumentation amplifiers are not very good, I don't know.

When I use only one op-amp from the picture (half of the bridge) and connect shunt resistor between ground and load (in this case I don't need instrumentation amplifier for measuring the voltage, just a regular one), everything works well for the whole frequency range.

So, maybe there are some special techniques (or chips) for this task? Or I just should use measurement differential/instrumentation amplifiers with better characteristics?

Edit: I got better results with chip AD8421 (instrumentation amplifier). However, I ordered the chip from a Chinese internet shop, so I don't know if it is genuine. In addition to the earlier scheme (a), I also experimented with another one - (b), with two equal loads. In the figure, I.A. - instrumentation amplifier. Variant (a) gives good results up to about 500 kHz (for the higher frequencies, the measured signal gets very distorted). Variant (b) gives good results up to several Mhz. So, it may have something to do with imbalance. Unfortunately, I cannot split the load. I need to achieve acceptable results for the variant (a). enter image description here

  • \$\begingroup\$ Examine the NE602 mixer. Use two of them, one for examining the shunt voltage, one for examining the differential output voltage. And notice the NE602 will heavily distort, for inputs larger than about 0.1 volt. \$\endgroup\$ – analogsystemsrf Jan 10 at 4:22

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