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Measure voltage drop-age across the switch using difference amplifier

NIKHIL VALIYA VALAPPIL
Intellectual 520 points
Other Parts Discussed in Thread: LM358, TINA-TI

Hi

I am trying to difference two voltages to measure the voltage drop across the device by using differece amplifier with help of LM358.

The switch is powerd with +24VDC and operating load of 20mA. The dropage while it gets ON is vaies from 3 to 8 mVDC.the gain of the amplifier is 10 and am expecting output voltage 30 to 80 mVdc. But unfortunately the voltage at output of the amplifier is 21VDC.

why its happening like this?
does the op- amp is capable to operate to differ the low voltage?

Please help to solve the issue.


Below shows the circuit which am testing.

I referring the below link.

e2e.ti.com/.../271917

Regards,

Nikhil

  • TI__Expert 6275 points

    You need to change R3 and R8 to 10k ohms for the difference amplifier to work correctly.  Consider simulating in our free SPICE simulator (TINA-TI) before ever building: http://www.ti.com/tool/tina-ti

  • TI__Guru**** 152006 points
    Nikhil,

    In addition to Tim's comments.

    The switch shorts out the voltage difference of closed contact so output should be near 0.
    The open contact should have over 20V input for a railed output.

    Is op amp VCC really 26V (2V greater than input)?
  • in reply to Ron Michallick
    Intellectual 520 points

    Dear Tim and Ron Sir,

    Thank you for your support.

    I did some changes in circuit and tried it.

    In that condition am getting  op amp out put around 795 in both close conditions. The actual dropage is 6 MV and 10 mv.

    Tim Sir,

    Your right sir.. in open condition of both contact it will gives around  24v (supply voltage ) as the difference.

    About the power supply:- i am using SMPS of 22 to 28 VDC adjustable power supply of 2A.

    It will be more clear while you seeing the following diagram. Please refer the diagram.

    Hope your clear now about the circuit.

    If you still need clarify please rise your queries.

    once again thanks to all

    Regards,

    Nikhil

  • in reply to NIKHIL VALIYA VALAPPIL
    TI__Expert 6275 points
    Something does not seem right. For a 10mV differential voltage in I would expect 100mV out plus worst case Vos * 10. Vos max is 7mV so I would expect about 170mV worst case for a 10mV input differential voltage. Did you measure differential input voltage with a precision meter? Did you look at power supply of LM358 with an oscilloscope? Did you look at output of LM358 with an oscilloscope? Above I assume your 795 is 795mV? When you probe with an oscilloscope move time base down to look for 50/60Hz oscillations. If you are striving for accuracy LM358 is not the answer as you can see you will get at least 70mV of error out for a differential in of 10mV and x10 gain.
  • in reply to NIKHIL VALIYA VALAPPIL
    TI__Guru**** 152006 points
    Nikhil,

    What is the purpose for the bridge rectifier?
    Your input voltage is current through switch contact resistance?

    The VIO error and resistor matching is a big unknown when compared to such a small input voltage.

    Lastly and most importantly, the LM358 has two sink drivers in the output stage. There is a weak sink that can be as low as 12uA, typically 30uA. This sink can get down to 0 V. The other sink driver is the PNP on the output schematic (see data sheet); this driver can pass much more current, but it can only go down to 1 diode forward voltage (about 0.7 V at room temperature).

    The feedback provides more current than the weak sink can provide. Try setting pin 4 of the op amp to -1V and measure the output. I believe you will see it being much closer.