# AD620 in shunt resistor measurement with non-symmetrical supplies

Document created by analog-archivist on Feb 23, 2016
Version 1Show Document

### Q

I'm working on the next generation of our smartcard products (micropross.com)
for 2003. I need to measure the current supplied by a variable -2V/+25V voltage
generator. We are currently using AD620 normally supplied
by +/-15V for current measurements through a shunt resistor with good results.
I thought an easy way to perform my measurement is to take the same principle
but to withstand the common mode voltage (-2/+25V), the AD620 needs to be
supplied by -5/+30V. That's why my question to know if it can work before
looking for any other solution. So, is there any problem supplying AD620 with
+30V/-5V power supplies ? Will it withstand the same common mode voltage and
give good response for shunt resistor current measurement ?

### A

What you are suggesting is plausible. Do you plan to tie the ref pin to gnd ?
For supplies of +30V..-5v, two major concerns:
First of all it must be an amplifier that is specified with a ABSOLUTE MAX
RATING Supply Voltage spec of +-18V or higher (total >36V)..(the AD620 is
spec’ed for +/-18V). Most of our older bipolar products are rated at +-18V,
+-20V, +-22V. Note that although most specs will vary only slightly if any from
the standard +-15V specs, we can't 100% guarantee each spec without testing.

Second, you must be aware that many op amps(including in-amps are single supply
amplifiers) and the older parts don't operate with input and output signals
near the supply rails. When using non symmetrical supplies (V+=30, V-=-5), the
middle of the Common Mode Range (where the amplifier is happiest) is at the
midpoint between the power supplies or +17.5V in this case. And the Inputs and
Outputs will not operate if within 1-3Volts of V+ and V-.