EEVblog #476 – Opamp Offset Voltage Measurement

Dave investigates an issue with the input offset voltage on the Analog Devices AD8628 zero-drift chopper opamp.
The answer will be presented in the next video.

Chopper Amplifier video HERE

Forum Topic HERE

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  1. From what I can remember of the video, you tried using a single cap from + to – and that didn’t help. Then you tried a single cap from – to ground and that made things quite a lot worse. That makes me think the issue is primarily on the the – supply input.
    What you didn’t try (as far as I can remember) is using a single cap from + to ground and NO cap at all on the – input.

    Also, there’s a possibility that you’re encountering SOME degree of issue due to the inherent resistance and capacitance of the PCB itself (and / or solder mask?). While this is a VERY high resistance and VERY small capaitance, it’s still not ZERO.
    From what I can see, you used the 8-pin version of the AD8628. The pinout of this puts the V- input right next to the IN+ input. If there was any ‘leakage’ (PCB-induced) from the V- pin into the IN+ pin, I would _expect_ there to be variation in output as the supply voltage changes. Since it’s configured as an inerting amp with IN+ ‘grounded’, I would expect the output to increase as the supply voltage increases.

    But what would _I_ know anyway…
    I’m just a ‘hobbyist beginner’….

  2. Probably an effect of the chopping switching inside the chip, for example charge injection of the mosfet switches, having an effect due to the wildly different input impedances at the + and – inputs. if you put a 1k on the + input, it will probably disappear….

  3. Differnetial input bias current through unequally terminated inputs could explain the power supply sensitivity. There are potentially 200-300 of picoamps of input bias current on this device flowing through a 1K resistor in the negative input while the positive input is clearly at ground.

  4. My guess (and I had to skip through the video, because it’s way too long and tedious – I have actual work to do) – input offset & bias current.

    Add series 1kOhm from in+ to ground to equalise Ib source impedance.

  5. Dave

    For a Fundamentals Friday – an explanation of what is ‘Common Mode’ imho would be a great introductory topic.

    I know it took me ages to get my head around it – and I still stop and think about it.

    Keep up the excellent work!

  6. Dave
    On the trackwork, do you have a guard ring around the + input ?
    The guard ring is usually tied to the – input to stop leakage currents running around the board.

  7. Great discussion. Helped a lot with a current OpAmp I am toying with from texas instruments.

    Have you explored the OPA2277 chip yet?

  8. Excellent post. I was checking continuously this blog and I’m impressed!Very helpful info specially the last part 🙂 I care for such information much.I was seeking this certain info for a long time.Thank you and best of luck.

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