OPA2277: OPA2277 Input error

Hello Álvaro,

If I understand your circuit design correctly, the OPA2277 V- pin connects to "OUT" which connects to DGND via RL. When the current through RL is zero, or very low, "OUT" will be close to 0 V. However, when high current levels flow through RL "OUT"moves positive relative to DGND. The non-inverting input of the OPA2277 moves more positive as well, because of the voltage developed across the 0.3 milliohm shunt resistor.

When the current through RL and Rshunt is close to zero the common-mode voltage VCM applied to the non-inverting input is going to be close to zero volts. That is a problem for the OPA2277. The Electrical Characteristics table in the datasheet list VCM, the Common-Mode Voltage Range as (V–) +2 V, to (V+) –2 V. Therefore, only when significantly high current flows through Rsh will VCM be 2 V above V- ("OUT"). Another issue is with "OUT" moving more positive V- is more positive and the supply voltage across the OPA2277 drops below 5 V. The OPA2277 is specified with a minimum supply voltage of 4 V so there is only 1 V of supply range remaining.

I am not really sure why you are connecting the OPA2277 V- pin to OUT, instead of DGND. However, even if you do the OPA2277 VCM limitation remains. One way to resolve the issue is connect the V- pin to a negative voltage such as -5 V, referenced to DGND. That should resolve the issue. However, if you don't have -5 V available that is a different problem.

I think the easiest solution would be to use a more modern op amp that has a VCM range that extends down to the negative supply rail. If DGND is used as V-, then that would represent 0 V, and an op amp that includes the V- rail within its VCM range would be ideal.

A couple of very low offset op amps, that have a VCM range of (V–) – 0.1 V to  (V+) + 0.1 V are the OPA388 and OPA325. They are low voltage op amps ideally suited for a single +5 V supply. You can find their datasheets here:

You can try replacing the OPA2277 with one of these op amps and see if the circuit performs to your expectations. You may have to redesign the circuit to use DGND if things aren't working to your expectations.

Do keep in mind that no op amp operating from a single supply will be able to provide an output voltage that reaches either supply voltage rail. The output is usually limited to swinging with a few tens, or hundreds of millivolts within the supply rails. Therefore, at very low current levels the output will be up against the negative swing rail until enough current flows through the RL and Rsh to move it off the rail.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hello Thomas,

Thanks in advance for your quick response.

The main purpose I am using "OUT" as virtual negative reference instead of DGND is that I did not wanted the opamp to get high level signals (IN and OUT, refered to DGND can reach levels from 0 to 350V).

Here you can see a picture of the power stage:

By the other hand, you are ccorrect. The Vcm for this opamp is very high four my purpose, and it is posible to have issues when current across Rshnst is very low, but, why I am having issues when I get a high current like 20A? Additionally to this, the output of the opamp Will never reach its rail voltaje, since for a máximum of 110A across the Rshunt, the voltaje drop will be 50mV, getting an ouput of 250mV.

Many thanks and best regards,

Álvaro.

Hello Álvaro,

Maybe I don't fully understand the situation, but with a 300 microohm Rsh resistor it will be difficult at most any current level to develop enough voltage to move the Vcm voltage away from the negative supply rail "OUT." Twenty amperes results in a voltage across Rsh of +6 mV, and 110 Amperes results in +33 mV; in both cases not even close to the (V-) + 2 V, or + 2V Vcm minimum of the OPA2277.

I really do suggest trying a precision op amp having a rail-to-rail Vcm input range, and a rail-to-rail output voltage range. The OPA388 and OPA325 that I suggested will be able to work with the low voltages developed across Rsh, at low sense current levels.

Regards, Thomas
Precision Amplifiers Applications Engineering