CurSns4. Current Shunts need an Amplifier: Difference or Isolation.

Blog Index.

This blog, CurSns4, will look into:

· Current Shunt Amplifiers

· Differential amplifier, common mode limit

· Isolation amplifier

Current sensor resistors are used to convert current to an easily measured voltage that is proportional to current as discussed in the blog:

Differential Amplifier, Common Mode Limit.

So let’s define a few terms:

R2 = R4 and R3 = R5.

RSense << R2 and R3.

Vout = (V1 – V2) x (R3 / R2) + Vref.

G = R3 / R2; Gain of Difference Amplifier

VDM = V1 – V2; Difference Mode Input Voltage to Different Amplifier.

VCM = (V1 + V2)/ 2; Common Mode Input Voltage to Difference Amplifier.

VOH = Maximum High Level Vout of OpAmp. (From datasheet)

VOL = Minimum Low Level Vout of OpAmp. (From datasheet)

VCMRH = Maximum High Level Common Mode Input Voltage of OpAmp. (From datasheet)

VCML = Minimum Low Level Common Mode Input Voltage of OpAmp. (From datasheet)

So the VDM range will be:

VDM > (VOL-Vref)/G and VDM < (VOH – Vref)/G

And the maximum applied voltage will correspond to the OpAmp maximum common mode input VCMRH:

V1 < VCMRH (1 + 1/G) + Vref.

So for an OpAmp with 5V rail, Gain of 50, VOH 5V, Vref 2.5V and VCMRH 3V:

The output will saturate when VDM = (5-2.5)/50 = 50mV or full scale for a 50mV shunt…

…and the maximum input supply voltage will be 3 (1 + 0.02) + 2.5 = 6.52V.

If we want to operate a high side current shunt in a high voltage motor drive, say ±400V then the common mode range of a differential amplifier does not help.

Isolation Amplifier.

The Agilent ACPLC790 is an isolation amplifier designed for use with current sense resistors in high-side applications. The circuit above forms the basis of the isolation amplifiers (item 5) in this test stand:


The input/output barrier has a specification of 1230 Vpeak working insulation voltage.

The test stand instrumentation operates at 5V referenced to 0V and the isolated side of the amplifier is powered from a DC-DC converter U2 with an insulation working voltage of 1000 Vpeak, which sets the common mode limit of the module in the test stand.

Contact for more information on this isolation amplifier. It is 2 channel, current and voltage, suitable for phase parameter measurements. Or put a query in the blog.

Here are a few useful references.

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