# shunt voltage

Discussion in 'General Electronics Chat' started by mlkcampion, Nov 1, 2007.

1. ### mlkcampion Thread Starter Active Member

Nov 16, 2006
59
0
Hey
Trying to measure the voltage of a 100Amp-60mV Shunt. I have put together a circuit consisting of a lm358 opamp that should gain up the signal by about 54. The high current circuit is fed from a different power supply then the conditioning and monitoring circuit.
At first i appeared to be get mV losses in an about a 1 foot run of 1.5mm square cable going from the shunt to the op-amp circuit is this possible (gain of circuit appeared be at about 20) ? Also my multi-meter appeared to effect the shunt signal is this because it would be considered a very high impedance source and therefore the multi-meter was loading it too much? I moved the circuit right beside the shunt and that improved things alot however i'd like to know what is the best way to take a signal from a shunt i.e should i use ground isolation, an istrumentation amplifier, should i use certain cable, how would i determine the necessary op-amp input impedance to read the shunt?

Any sugestions appreciated!!

Mike

2. ### John Luciani Active Member

Apr 3, 2007
477
0
I have not seen your circuit or the application but have you looked at
the effect of input offset voltage from the LM358 which IIRC is in the
3mV to 7mV range?

Also what frequency is the current that you are measuring? In order to
get mV losses in a one foot cable at low frequencies you would have to
be drawing a *lot* of current. My guess is that you are picking up

Have you checked for noise at the power supply and the input and output
of your amplifier? Be careful of probe placement when you measure
noise. (Check out http://www.luciani.org/eng-notes/ee-notes/ee-notes-index.html
for an example).

The multimeter impedance is probably > 10MOhm. The shunt impedance is
600uOhm (0.06/100). The only way the multimeter could effect a change
is at very high frequencies.

An instrumentation amplifier is a good idea. If you want to use
an op-amp I would suggset a low input offset amplifier (check out
Linear Technology and Analog Devices). You could use a shielded
twisted pair cable at the output.

(* jcl *)

3. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
655
Have you checked out current sense amplifiers? Do a search for "current sense amplifier".

4. ### Distort10n Active Member

Dec 25, 2006
429
1
INA19x series of high side current shunt monitors may be of interest. Be careful of your common mode range and sense voltage with these amplifiers.