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

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:

https://electronicsknowhowblog.wordpress.com/2015/02/24/cursns2-current-sense-resistors/

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 cscltd@btconnect.com 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.

http://www.eetimes.com/document.asp?doc_id=1279404

http://uk.farnell.com/avago-technologies/acpl-c790-000e/ic-amp-isolation-8soic/dp/1854250

http://uk.farnell.com/texas-instruments/ina159aidgkt/differential-amplifier-1-5mhz/dp/1690665

http://uk.farnell.com/texas-instruments/ina149evm/eval-module-ina149-diff-amplifier/dp/2081122

Author
PeterCoxSmith
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