# Sensing high differential voltages using operational amplifiers

Is there any solution to sense high differential voltages using operational amplifiers (without using resistive voltage divider) ?

The input voltage range is [-30, 80] Volts.

I've searched on all operational amplifiers and no one fill this range.

I can't use a normal attenuator (Differential amplifier with gain < 1 ) since the common mode input voltage of the op amp is limited, and the input votlage has an independant ground ( I will not be sure if the CMIV will not be exceeded )

Thank you.

• Why is a prescaling not an option? What precision do you need on this? And what do you mean with sense, as in, where is the output of your circuit going? Commented May 8, 2017 at 14:55
• Are you saying that the independent ground could be hundreds or thousands of volts different to the ground for a circuit that might be useful in sensing? Commented May 8, 2017 at 14:56
• Show us some schematic of source of voltage signal - there is always a possibility to measure any voltage, even when DM is in millivolts range and CM in kilovolts (I've designed such circuits which are now in production). Commented May 8, 2017 at 15:13
• Use a gold-leaf-foil sensor, with laser interferometer readout of the tilt-angle of the two leaf's separation angle. The Interferometer needs OpAmps to process and recover the laser optical beat notes. Commented May 8, 2017 at 15:36
• The precision i need in the global sensing is 0,5%. For the independant ground, i don't have any information about it so i can not assume that the common mode voltage at V+ and V- will not be exceeded using a differential amplifier. Commented May 9, 2017 at 15:37

## 1 Answer

You know the inputs are in the (+80V, -30V) range. So, you could apply a battery-powered voltmeter and read the difference directly.

Yes, of course it can be also be done with low-voltage op amps. The high voltage can be turned into a modest current, and with a few (-30, -35, -40V) power rails to power an op amp, one can generate a ground-referenced (negative) voltage from each input. Do it twice, and you can apply a normal difference amplifier on the translated/scaled voltages.

simulate this circuit – Schematic created using CircuitLab

The scheme (1) reduces the voltages to something less, (2) applies an offset to near-ground levels. $$A_{scaled} = -(A + 30) *( R8*R4)/(R1*R7)$$

so (80V, -30V) inputs are changed to (-0.550V, 0V) outputs

(There's some linearity errors near the (-30V) value, if that end of the range is important, moving the power supplies down by ten volts will clean it up).

Actually, though, voltage dividers aren't so bad; the proliferation of power supplies is rather a nuisance, doing it this way.

• Thank you very much for your quick answer. I'm trying to understand this schematic. I will get back to you once i will have a fixed idea about it. Commented May 9, 2017 at 15:48