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.