AD8628/29/30, thermopile application, power supply decoupling?

We are attempting to use an AD8630 to amplify the signals from four thermopiles.  This amplifier was recommended by Dexter Research, a thermopile manufacturer.  One thermopile is equipped with broad-band optical window which admits a lot of light, and the others have narrow-band windows.  Our stronger signals are showing oscillating behavior with a long time constant, around 2 seconds.

I set up an experiment in which I exposed our sensor to ambient conditions, then placed it in front of a blackbody radiator, and finally returned it to its original position.  This is what I consider a good result.  It comes from one of our weak, narrow-band detectors.  The gain is very high, 22000 X in a single stage, but it appears to be clean.  

The horizontal time scale in all graphs is 2.0 seconds / division.  The vertical voltage scale in this graph is 50 mV / division.

Our broadband detector admits more light.  The gain is 1200 X but the raw signal is stronger.  When I repeated the experiment above, I obtained this.  Notice the strong peaks on the leading edge of the pulse, and the long settling times on both the rising and falling edge.  Vertical scale = 200 mV / division.

I was concerned that my movement of the sensor was not very exact, so I repeated the experiment with the sensor fixed in place.  I blocked the black body with an insulating Styrofoam sheet, and raised and lowered the sheet to record the response and the recovery.  I observed the small signal and the large simultaneously.  This setup improved the large-signal response, but there is still ringing on the rising and falling edges, and ripple on the top.  Vertical scale = mixed, 50 mV / division for the lower signal and 200 mV / division for the upper signal.

I was looking through the AD8628/29/30 data sheet and noticed that a thermopile amplification circuit is shown in Figure 62.  Surprisingly, a decoupling capacitor is shown connected to the 5V supply.  This is not something I normally expect to see in a linear circuit, but after seeing it I realized that the switching circuitry in the zero-drift circuitry may demand variable amounts of current. 

My circuit does not have a decoupling capacitor.  Adding one will be a chore, but I will figure something out if it will help.  Because I am testing thermopiles, I had to manufacture a PCB with an EMI shield from the start, a breadboard was too noisy to test.  Could the absence of a decoupling capacitor be compromising my signals?  What value should I use for this capacitor?  It is not described anywhere in the data sheet.

Thanks for your assistance.

  • I just rechecked the board for shorts with an ohmmeter and found none.  In any case, I know that all four channels of the amplifier are functioning. 

    However, your suggestion also prompted me to re-examine the board under a microscope.  I have a few tin whiskers growing on the terminals of U1.  Ugh.  When I make the next board, I will trim the leads on U1 as flush to the board as possible before soldering.

  • John,

      A couple more comments.

    Op amps have a finite output impedance, so any C load will result in phase shift, degrading

    loop stability.  The AD8630 only has about 45 degrees phase margin on 2.7V, so a cable

    a foot or so long can be problematic.  I would put a series resistor right at the output of

    the op amp, after the feedback resistor.  Say 100 ohms or so.  See:

    https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html

    Also, I'm not a big fan of quads.  When you spin the board, I would switch to two duals;

    AD8629.  Makes the layout easier, less coupling between sections.  See:

    https://www.planetanalog.com/author.asp?section_id=3370&doc_id=528114

    https://www.planetanalog.com/author.asp?section_id=3370&doc_id=528116

    Harry

  • I didn't catch what you meant at first.  You meant that there was an extra junction on the schematic.  I don't know how that got there.  It appears that a PCB design software update triggered a rewrite of my schematic file?  It wasn't present when I prepared my Gerber file.  As I discussed in an earlier message, there is no short between the pins on the actual PCB.  I manually corrected the schematic to match.

  • I would put a series resistor right at the output of

    the op amp, after the feedback resistor.  Say 100 ohms or so.

    Once I switch to a production board with an integrated ADC < 1 cm from the amplifier, do you think the series resistors would still be necessary?

  • I have started to read through ADI's Designer's Guide to Instrumentation Amplifiers (even though the AD8630 is not an instrumentation amp) and Application Note AN-202.  I am seeing conflicting information regarding the proper power decoupling strategy for op-amps in these two documents.

    In the Designer's Guide, in Figure 5-2, I see a recommendation to use two decoupling caps on each power rail, 0.01 μF and 0.33μF.  To me, this looks like it is intended to deal with high-frequency transient power demands.  There are no high-frequency transients in our thermopile signals (although I'm still not sure whether the high-frequency switching network inside the op-amp affects the power supply requirements).  Right above Figure 5-2, I can read a warning that power supply decoupling needs to be designed to fit the situation, and that improper decoupling could make the situation worse.

    I have also spoken with a colleague of mine who is a more experienced analog electronics engineer.  We have come up with a few questions which we hope you can answer.

    • Do you think that we have coupling from the input to the output?
    • If so, where? Through the power supply?
    • If that is the case, could adding decoupling capacitors make the issue worse?
    • What decoupling capacitor value(s) do your recommend? As I stated earlier, our thermopile input currents and voltages are tiny. Our signals are low-frequency.

    I found the SPICE model for the AD8628.  Maybe we can figure out how to simulate our problem? I'm not sure whether I can find or construct a SPICE mode of the thermopile.

    Thanks again for any assistance you can provide.