6 Replies Latest reply on Jun 4, 2016 2:42 AM by maku_1180581

# Opamp: 5A DC current sampling&#160; with 0.01ohm resistor

Hello everyone,

I'm trying to examine the DC current(range form -5A to 5A) on a user-circuit Vss wire using SARADC and Opamp components of "CY8C4247 /PSoC 4", the attachment is the demo circuit.
R0 equals 0.01ohm.

Since the difference voltage between Vi+&Vi- is small(50mv under 5A condition), with Opamps to amplify the voltage difference of ADC differential input ADC_in+&ADC_in- by 19.18times(gain=19.18). I wonder if this application would work properly?

signals Vin+&Vin- equals SARADC channel1 input+&input- in schematic demo2.

thx a lot

• ###### 1. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

I'm now testing the circuits with schematic below, the single-ended opamp works all right(596mv/47mv=12.68).

The common mode differential opamp circuit only has a gain of approximately 2(96mv/47mv), I wonder what might be wrong.

the opamp config circuit and resistors is positioned outside the chipboard with flying wires.

• ###### 2. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

For OPAMPs required offset.
Apply a shunt sine voltage - you will be easier to see the problem.

• ###### 3. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

Thx. I see that I configured the OPAMPs circuits worngly without applying a offset voltage circuit.

But when I tried to apply a shunt sine voltage, here's a new problem:

Since the 10ohm sampling resistor is connected to the OPAMPs upon a DC circuit, without isolating capacitors (AC circuit),the offset voltage circuit would not work properly as shown in test1.

I could directly connect R0 differential output to SARADC without OPAMPs ,this would work all right but the range is too small([-50mv, 50mv]), while ADC differential sampling range is [-1.024v, 1.024v].

• ###### 4. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

The test circuit below on the left side works all right, since R0 provide a voltage bias itself of VDD/2.

So I configured the circuits on the right side(condition 1 and condition 2) would work in my application, but since R0 only has a voltage difference of 50mv max(Ibus=5A), so the precision requirement of the 4 bias resistor value would be very strict(1% * 5v=50 mv would cover the R0 voltage). I wonder if this would work, or there's other better solution.

• ###### 5. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

Scheme assumes that the shunt circuit is isolated from the PSoC power.
Otherwise, you need to include a shunt in the feedback loop.

• ###### 6. Re: Opamp: 5A DC current sampling  with 0.01ohm resistor

Dear,

no matter whether it works or not I would always suggest using a current shunt amplifiert with a high input voltage acceptance and a desired amplification. This is the best way to be sure that you'll not destroy the CPU..