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.

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  • John,

      Sometimes for the sake of clarity and simplicity, schematics are shown without bypass caps.  But everybody "knows" they are needed

    on each analog and digital part.  In fact, there are some schematics that don't show the power pins on an op amp at all.   Again, it's

    understood the op amp needs to be powered.

      WRT to your problem, if the thermopile is ten feet away, you have a nice antenna that will pick up every thing in the spectrum.

    You can add some R-C filtering, both common mode and normal mode, similar to fig 5-24 in the InAmp handbook, 3rd edition.

    Can you post your schematic?

    Harry

  • Hello Harry,

    Thank you very much for your reply.  I have limited experience with specialized op-amps, but I have seen Class A amplifiers on production boards with no decoupling capacitors.

    Here is my schematic.  U1 is the four-channel thermopile sensor.  The feedback loop on each op-amp is identical, and implements low-pass filtering with a -3 dB point of 5 Hz.  I want the DC signal, so I do not plan to use AC coupling anywhere in my system.  My shunt resistors (R5-R8) can be quite small.

    Here are some pictures of the actual PCB, which is 35 mm in diameter. The TO metal can containing the thermopiles is grounded.  The connections between the sensor and the op-amp are all < 14 mm in length, and are completely EMI shielded. 

    If I try to operate the sensor without the EMI shield in place, the outputs can be nearly meaningless.  I observe the expected 60 Hz oscillations from the environment, but I can also get strong signals in the 500 - 1000 Hz range. The weaker signals actually look better.  The stronger input signals have larger oscillations.  I think I might be seeing intermodulation distortion between the ambient 60 Hz signal and the chopper / zeroing circuitry. 

    I will try to hack in a decoupling capacitor connecting V+ to GND, to see whether I can improve the system performance.  At some point, V- and GND may also become distinct voltages.  That isn't possible with this PCB.  I am redesigning it in any case.  I may need to add a decoupling capacitor for the V- rail.

    Thanks for any further suggestions and advice.

  • Extra solder dot near U1 shorting out pins 3 and 4.  Did you build your PC from the schematic netlist?

    H

  • To take the oscilloscope pictures I showed, I attached 10 cm unshielded jumper wires to J3.  These wires are in turn clipped to the oscilloscope probes.  These are the amplifier outputs, not the inputs.  As I showed above, the signals are in the 100 - 800 mV range on J3, and the weaker signal was actually the cleaner of the two.

    This is version 1 of our PCB.  Version 2 will also include an analog-to-digital converter, but we lack the ability to solder a QFN package in-house.  When this PCB is not connected to an oscilloscope, J2 and J3 connect to a female pin receptacle mounted on one of these boards: https://www.adafruit.com/product/1083

  • 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.

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  • 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.

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