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OPA4H014-SEP: output issue for buffer configuration

Part Number: OPA4H014-SEP
Other Parts Discussed in Thread: INA240, INA149-EP, TINA-TI, INA149

Tool/software:

Hi expert,

My customer wants to use OPA4H014-SEP as a buffer between ADC and INA current sensor like below configuration.

And they found OPA there is a 1.26V DC at ouput without any input signal at IN+.

Here are the schematic their own.

R30, R34, R38, R42 is replace to 0 Ohm.

C49, C53, C57, C61 are remove.

Is there any suggestion why this situation happened?
How to verify the OPA function is okay?

Thanks.

  • Hi Ryan,

    The OPA4H014 as shown on the TINA schematic is in the non-inverting configuration on a gain of +2V/V, with the inverting input biased at +2.5V through the 2kΩ-2kΩ voltage divider.

    The circuit is expected to work when connected to the INA240, as long as the INA240 output is within the linear range of the circuit.

    - What is the max/min current range expected across the 10mΩ load?

    I performed a TINA open-loop simulation stability analysis and the circuit is stable.  See stability analysis below.

    TINA Simulation file:

    0317.Stability.TSC

    And they found OPA there is a 1.26V DC at ouput without any input signal at IN+.

    1) What its meant by 'without any input signal'?  Does this mean that the non-inverting input is left floating, with the INA240 disconnected from the op-amp? 

    - If you leave the buffer input floating, this will likely produce undetermined results as the amplifier input is not driven.

    - If there is no current across the current-sense shunt resistor, the expected output of the INA240 should be +2.5V, and the output of the OPA4H014 buffer should also be +2.5V.

    2) When measuring +1.26V at the op-amp output, are you measuring the output of the OPA4H014 with an external meter, or are these the ADC conversion results?

    Let me know what is the expected current range flowing through the 10mΩ shunt resistor; and we will be able to check if the circuit is inside the linear range.

    Thank you and Best Regards,

    Luis

  • Hi Luis, 

    Thanks for your reply. 

    1) Yes, the non-inverting input is left floating, with the INA240 disconnected from the op-amp.

    2) The output +1.26V is measured by an external meter.

    INA240-SEP is replace to INA149-EP currently, and it is used to sense voltage, and it do not use shunt resistors, below is its schematic.

  • HI Ryan,

    If you wish to verify/test the op-amp circuit is working, you could apply a voltage at the non-inverting input of the op-amp and check the output voltage with external meter at the ADC input. For example, applying a voltage at the non inverting input of the op-amp in the range of +1.5V to +3.5V produces a 500mV to 4.5V output.   Below is a DC sweep simulation of the OPA4H014 (stand-alone)

    TINA-TI Simulation attached:

    OPAH014-SEP_DC_Sweep.TSC

    Alternatively, if the INA149 is connected, you could ground the inverting input (IN-) of the INA149, and apply a voltage at the non-inverting (IN+) input of the INA149. For example, applying a voltage at the non inverting input of the INA149 in the range of -1V to +1V produces a +500mV to +4.5V at the output of the OPA4H014.   Below is a DC sweep simulation of the INA149+OPA4H014  

    TINA-TI simulation attached.

    INA149-OPAH014-SEP_DC_Sweep.TSC

    Thank you and Kind Regards,

    Luis