5/19/16: Fundamentals of Operational Amplifiers

What about MOSFET inputs for ultra-low voltage burden in high side current measurement?

  • MOSFETs do have very low input bias current, but they also have higher noise than bipolar or JFET transistors.    In most high-side current sense applications, a very small resistor is used to minimize the voltage drop in series with the load.   This gives a small voltage signal that we need to pick off with a low-noise input stage.   There is usually a fairly high current we are trying to sense, so a few nanoamps or even microamps of input bias current with a bipolar device will be a negligible error.  Take a look at the AD8418A current-sense amplifier.   For very common mode voltages, you could try the AD8479.