LMV358: Standby Output voltage remains at 100mV

Ishan Panchal

Part Number: LMV358
Other Parts Discussed in Thread: OPA2333P, INA185, MC33078, OPA2333

Hi There,

I am using LMV358 in differential amplifier configuration with a gain of 50 to measure current using shunt resistor. The value of shunt resistor is 250uohm (8 x 2mohm resistors in parallel). Please find attach schematic for your kind reference. Please note that we have used both opams for bi-directional current measurement with inputs of second OPAM inverted.

Issue : In standby condition, we are observing the output of 100mV. The voltage across BAT-_P and BAT-_N is 0V but voltage across pins 2 and 3 is 2.3mV. This 2.3mV with a gain of 50 is giving output of 100mV in standby condition. Due to this we are unable to measure current less than 8A. Request you to provide your inputs to remove this offset of 100mV somehow.

Note : We isolated inputs to 2nd opam and measure the output and it still remains 100mV for the 1st opam output, so 2nd opam with inputs is not creating any issues.

Thanks..!!!

over 2 years ago

0 Clemens Ladisch over 2 years ago

Guru 260980 points

The schematic did not attach.

Anyway, the LMV358 is specified as having a worst-case input offset voltage of 9 mV, so the worst-case output error from this alone is 450 mV. How much of an error can you afford?

0 Ishan Panchal over 2 years ago in reply to Clemens Ladisch

Intellectual 640 points

Hi Clemens,

Sorry, please find attached.

We must be able to measure at least 500mA, that is 0.5mV.

0 Clemens Ladisch over 2 years ago in reply to Ishan Panchal

Guru 260980 points

If you want to measure 0.5 mV, the offset voltage must be much smaller than 500 µV.

Use a precision opamp like the OPA2333P. Or better an integrated, bidirectional current-sense amplifier like the INA185. What is the maximum current/voltage you need to measure?

0 Ishan Panchal over 2 years ago in reply to Clemens Ladisch

Intellectual 640 points

Hi Clemens,

Thank you for the kind inputs and solutions.

In case if I want to keep the present circuit as it is, Can I use "MC33078DR" which is having an offset of 150µV ? I will decrease the shunt resistor so minimum measurement value comes around 15-20mV per Ampere.

I am bit concerned about the current supply spec of 2.05mA as we will be using it for a BMS application.

I went through the datasheet of INA185 and it seems suitable but our ADC has input limit of 2.5VDC. With bi-directional configuration the effective range for 100A measurement would be 0-1.25VDC (12mV for 1A) and our ADC has an input offset(scale error) of 30mV.

I am looking for a solution where discharging current is measured as 0-2.5VDC while charging current is measured as 2.5-0VDC just as in present configuration. I am fine using two opams as we have 2 ADC's available. Please let me know your thoughts on this one.

Thanks..!!!

0 Clemens Ladisch over 2 years ago in reply to Ishan Panchal

Guru 260980 points

The MC33078 has a worst-case offset of 2 mV.

If you want to keep the SOIC package, consider the OPA2333. Here is a search for low-offset, 3.3 V, SOIC dual opamps: ti.com/amplifier-circuit/op-amps/precision/products.html#p480=2;2&p1261min=1.6;3.3&p2max=0.002;0.05&~p2954=SOIC

0 Ishan Panchal over 2 years ago in reply to Clemens Ladisch

Intellectual 640 points

Hi Clemens,

Based on the inputs suggested, I would like to move forward with the "INA213BIDCKT" in a bi-directional configuration with a precision Vref = 1.25VDC.

Just need your thoughts on the INA213BIDCKT application for minimum measurement of 1A @ 12.5mV.

Thanks..!!!

+1 Clemens Ladisch over 2 years ago in reply to Ishan Panchal

Guru 260980 points

With an offset of 100 µA max., this looks OK.

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LMV358: Standby Output voltage remains at 100mV