AD8224 Low Frequency Noise Issue

In one of our designs we are doing signal conditioning of a load cell (Wheatstone bridge)

We have an issue where the main instrumentation amplifier an AD8224 has an output signal that wanders by around 20mV over the course of 10's of seconds.

We have gone through troubleshooting the signal chain and tracked the issue back to here, the above plot is taken with the inputs to the amplifier shorted out and connected to 0V.

Supply to the op-amp is +-15V, and has 0.1uF bypass directly near the IC, and 10uF about 3cm away. We are using a gain resistor of 49.9ohm which should give a gain of around 991.

We are not worried about the high frequency noise, this gets tidied up latter in the chain.

I was able to find one of our old board revisions that does not show the same issue, these boards are modular, and I was able to slot in the new and old next to each other to compare the results. The old board used a TI INA2126 instead of the AD8224.

Channel 0 red is the old revision and shows what I would expect of changes due to thermal effects.

Channel 1 yellow shows the same board as per above.

In terms of ruling out other factors, this is in a room without much airflow, breathing on the board doesn't seem to have a large effect, nor does insulating it from airflow by wrapping it in rags. My reference pins are tied directly to zero volts. I have confirmed I get a nice clean (1-2 bits of noise) signal recorded if I short the output of the amplfier to 0V (as expected).

I am all out of ideas, is this expected performance of the AD8224 (from the datasheet I think I should be getting at least an order of magnitude better).



Fixed picture
[edited by: Hugoagogo at 2:28 PM (GMT 0) on 31 Jul 2019]
  • 0
    •  Analog Employees 
    on Aug 1, 2019 12:05 AM over 1 year ago

    Hi Hugoagogo,

    One question: If you lower the gain by raising RG (e.g. 499ohm for G= 100V/V), do you get a corresponding reduction (10:1) in the AD8224 apparent output "wander" over time?

    Based on everything you've said, it'd seem logical that this would happen, assuming the source of the issue is towards the input of the AD8224, but it'd be helpful if it is easy enough to try and report back.

    Also, what is your Wheatstone bridge leg resistance?

    Is the no-signal condition CM voltage at the AD8224 at very close to 0V?

    Regards,

    Hooman

  • All of the graphs in the original post are done with both inputs of the amplifier shorted directly to ground (I have removed the RFI filter caps and replaced them with wire links) for the purposes of isolating the source of the noise.

    The bridge resistance is between 100Ohms and 1K depending on where this board is used, but nothing is connected for these tests.

    The below plot is of the amplifier setup with various gain resistors (not all done at the same time I have just combined the data to plot), I should note the sample rate in all of these plots isnt that high (1kHz)

    I will also include a picture of the layout, just in case I have goofed something up there. The PCB is a 2 layer with a largely unbroken 0V plane on the reverse side. On the board in the above plot I have replaced C1-C6 With jumpers.

    P.S. Please exclude all the tiny solder boards in this photo, the photographed PCB hasn't been properly cleaned yet.

  • 0
    •  Analog Employees 
    on Aug 1, 2019 5:28 PM over 1 year ago in reply to Hugoagogo

    Hi Hugoagogo,

    Thanks for running the tests at different RG (Gain) values and it's clear that lowering the gain reduces the output "wander" or drift.

    One note: The magnitude of the drift seems a lot lower this time around. In your original post, I see that the yellow (Ch1) trace wanders around 60mV (-30mV to +30mV) over the course of 700 seconds. In your latest plot, with the same gain (RG = 49ohm), the output shift is less than 10mV (20mV to ~28mV).

    With 10mV output shift at a gain of 1,000V/V, you only need about 10uV input offset shift. With the A grade device that you're using, you have a spec of 10uV/C input offset voltage temp. coefficient. So, that's about a 1 deg. C change in die temperature. If I've done the math right, I think this much output change is not out of the ordinary assuming that there is about 1 deg. C change in temperature of the die over this time. If you use the AD8224 B grade, the Vos temp. Co. is reduced in 1/2 to 5uV/C.

    Please double check my work here but it does look like you're limited by the AD8224 A Grade performance and the B Grade (better performance grade) may be a better match.

    Regards,

    Hooman

  • Thanks Hooman, would you expect the self heating to vary the temperature in a bit of a random manor, see the second plot in my original post comparing our original amplifier with the new AD part, the original amplifier shows more what I would expect (an exponential curve) of the thermal drift.

    Do you have any tips on how one would achieve the rated spec, noting my testing was done with the board wrapped up to avoid any effects of airflow changing the temperature erratically (and the original product where the problem was first identified was inside a sealed steel enclosure)

    Earlier in the week I purchased a few B grade AD8222 amplifiers to do a comparison with, can you see any major gotchas besides the paddle on the AD8224 being connected to VS+ and on the AD8222 connected to VS-. We are getting the hidden paddle version of the AD8222 so I am hoping I will be able to use it to fix this small batch of boards without too much issue.

    In rough terms going from the AD8224A's 8224's 10uV/C to the AD8222B's 0.3uV/C should I expect a 33 times improvement in the wandering?

  • 0
    •  Analog Employees 
    on Aug 2, 2019 1:20 AM over 1 year ago in reply to Hugoagogo

    Hi Hugoagogo,

    It'd be great to try the AD8222 that you have on order. I'm hoping for the 33x improvement that you've noted due to tighter TC_Vos, but we will find out. I cannot think of other limitations of swapping out the AD8224 and replacing with AD8222.

    I don't have other recommendations related to the points you raised about keeping the die temperature constant. Sounds like you've done this before and are probably more of an expert than I could be.

    Just a note: I wanted to make sure that the issue you're seeing applies to more than one device or one board. Have you tried another AD8224 device or another board and got similar results as far as the "wander" is concerned?

    Regards,

    Hooman