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AD8137 gain

Category: Hardware
Product Number: AD8137

Hi,

I'm trying to get a gain of 10 from an AD81237 using 1k resistors for Rg and 10k resistors for Rf. Powering the amplifier with 5V single voltage, non inverting output goes about 300mV and inverting output goes 4.6V, with or without input signal. If I use all 1k resistors for Rg and Rf the gain is 1V/V and all works fine, but there is no way to have a gain of 10 V/V.
Can anyone help me? Thank you

tools.analog.com/.../



non inverting output on pin 4, and inverting output on pin 5
[edited by: Superfede76 at 11:54 AM (GMT -4) on 13 Mar 2023]
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  • Hi,

    The part should work fine with your configuration.
    Are you using an official eval board or are you using your own? Have you tried replacing the DUT if the issue still persist?

    Regards,
    Goz

  • Hi Goz,

    I'm using my own board. I've two boards with three deveces each, and all devices have the same behavior
    regards

  • Hello,

    you do not hold VCOM at a defined voltage, so it may drift to "forbidden" values. Try and set it e.g. to Vcc/2 with a simple voltage divider.

    best regards

    Achim

  • Hi Achim,

    from the AD8137 datasheet "An internal bias generator sets the V OCM level to approximately midsupply; therefore, the output common-mode voltage is set to approximately midsupply when the VOCM input is left floating." Based on this indication I left the VCOM pin floating, and with 5V of power supply the Vcom voltage mesured is 2.5V as indicated in the datasheet

    regards

    Federico

  • ups, sorry: my mistake. I didn't see this statement in the datasheet.

    Then I cannot find a reason in your schematic, why the circuit should not work. As the real cuircuit does not work, I could only assume differences between the schematic an the real circuit. So if I would be in your place I would

    - measure in the real circuit the voltages on all pins of the AD8137 (does the supply reach the IC correctly, is PD really pulled up internally,  what is seen on pin 1 and 8)

    - doublecheck, that the resistors Rf and Rg on the PCB really have the intended values

    best regards

    Achim

  • Hi Achim,
    I have already checked that the connections and the resistance values are correct... many times :( The voltage on Vcom pin is correctly 2.50V
    When Rg and Rf have the same value the pin voltage on Pin 1 and 8 is 1.25 V, while when Rf is 10 times Rg the voltage is about 30 mV on pin 1 and about 420mV on pin 8,  coherently with that of the output

    regards

    Federico

  • Hi Federico,

    thanks for the measurements.

    And your 3mOhm shunt resistor is really installed, so that you have a well defined input voltage of 0V?(or some other small voltage if a current runs over the shunt...) I still think, then the circuit should work.

    Or may the shunt resistor be missing, so that your input is open? In case of open input, the negative feedback from Pin 5 (-out) to Pin 8 (+in) can  not work as intended. Also LTSpice gives then a maximum differential output voltage (though the sign of the simulated differential outpout voltage is opposite to the one you desribed in the beginning).

    best regards

    Achim

  • well, looking at the circuit again: in the Gain=10 seupt you're violating the input common mode range specification (which requires the input voltages to be 1V above the negative supply). In your Gain=1 setup, there is no violation (as the amplifier inputs are at Vocm/2=Vcc/4=1,25V). Maybe there is the reason for your results.

    best regards

    Achim

  • Hi Achim, thank you very much for your suggestions. I think you are right, I will try another IC in order to use a gain =10. I will let you know if it works.
    Best regards,
    Federico

  • Hi Achim,

    I tried with a THS4521, with a rail-to-rail input, and it works fine.

    thank you

    best regards

    Federico

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