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LMH6702: Opamp

Yash Ved
Prodigy 10 points
Part Number: LMH6702

How to eliminate output offset voltage ?

  • 0
    TI__Prodigy 525 points

    Hi,

    It depends on the application and circuit. There can be a few ways to reduce the contribution of an amplifier's offset at its output:

    1. In AC coupled circuits/high-pass filter circuits, the amplifier output (at a particular frequency) can be AC coupled to a desired common-mode voltage, eliminating any DC offsets to the right of the output capacitor.

    2. Using the LMH6702 in a composite loop with a slower & higher precision amplifier, so the outer amplifier controls the precision performance and the LMH6702 provides the dynamic response and settling.

    3. In DC coupled applications, the input offset voltage will appear on the output (value depends on the circuit configuration). If low offset is still needed, would recommend considering newer devices with TI's in-package trim technology like the OPA817 (max. 250 µV offset and 800 MHz bandwidth) or the OPA2863A (with max. 95 µV offset and 110 MHz bandwidth).

    -Bharat Agrawal

  • 0 in reply to Bharat Agrawal60
    Prodigy 10 points

    My application is both AC and DC. I have just made a voltage regulator for +/-5V (Meanwell N7805) powered by a 9V battery. Applied filters as suggested by regulators and LMH 6702 datasheets. The circuit is mounted on a general purpose breadboard with a gain of ~10 (Rf = 237, Ri =27) on NI pin of opamp. Initially the output without input was 0V ( No offset). But now it has developed an offset of 292mv. I want to test small dc signals of 20mv which will not be possible coz of the offset.

    What do u suggest next course of action to eliminate this offset?

  • 0 in reply to Yash Ved
    TI__Prodigy 525 points

    Hi,

    The LMH6702 is a current feedback amplifier (CFA) and CFAs aren't the best choice for lower speed, high precision applications. The input offset voltage and the bias currents at the respective inputs times the impedance at these inputs, would reflect with the set gain (1 + Rf/Rg) at the amplifier output. Is there a reason for choosing a 1.7 GHz amplifier for DC signals? You may refer to these precision labs videos which talk about offset errors and current feedback amplifier architecture:

    Offset voltage & Bias current Precision Labs video

    Current feedback amplifiers Precision Labs video

    If the 1.7 GHz bandwidth is not required, would recommend changing to lower bandwidth and higher precision amplifiers like the OPA2863A or OPA187 (max. 10 µV offset).

    -Bharat Agrawal

  • 0 in reply to Bharat Agrawal60
    Prodigy 10 points

    Dear Bharat,

    My analog application is upto 1Ghz.. Can u suggest a better opamp for the same.

  • 0 in reply to Yash Ved
    Guru 140200 points

    Hi Yash,

    there's something wrong. Is there a bigger source resistance driving the circuit?

    We need to see a schematic.

    Kai

  • 0 in reply to Yash Ved
    TI__Prodigy 525 points

    Hi Yash,

    The bias current through the resistors around the amplifier also contribute to output offset in addition to the amplifier's intrinsic offset voltage. Please advise on the circuit configuration or schematic which will help in suggesting the best amplifier for your application.

    -Bharat Agrawal

  • 0 in reply to kai klaas69
    Prodigy 10 points

    Dear Kai,

    I am using Meanwell N7805 for drawing +5V & -5V for OPAMP as per their specifications. (https://my.element14.com/mean-well/n7805-1pv/dc-dc-converter-5v-1a/dp/3819295)

    I want to use photodiode as input at non-inverting terminal for getting 10x gain with this op-amp. My signals can be as low as 20mv and 1GHz range. I am getting noise in output of 280mv in output with No or 0V input which will not resolve my low level signals from photodiode. I am using a general purpose board for soldering the stuff and testing.

    Note: When a new ic is connected the noise is 0v at output but it eventually develops.  Initially I was using a 9V dc adaptor at NO and relay with a 9V battery as NC to select between DC source and Battery for input to the switch. Which I have removed to remove suspicions.

    I am attaching my schematics for further suggestions and improvement.

  • 0 in reply to Yash Ved
    Guru 140200 points

    Hi Yash,

    first I would check whether the +/-5V supply voltages are correct. They must be stable, clean and noise-free. Eventually you should add low pass filtering at the outputs of N7805 because of the ripple noise. Remember that the N7805 is a switcher. It may be a better idea to power the OPAmp from a laboratory power supply.

    Then, the LMH6702 is an ultra fast OPAmp. The circuit should not be mounted on a general purpose board. You should use HF techniques (solid ground plane, etc.). It would be the best, if you could test the circuit on the TI's eval board.

    There's another issue: Don't directly touch any pin of OPAmp with the multimeter or scope probe. Always insert an isolation resistor first. Read section 8.2.2 of datasheet.

    Kai