So I have (after a brief diversion to build a soil temperature probe) breadboarded the PongSat amplifier stage.
I used pretty much exactly the circuit at http://openenergymonitor.org/emon/buildingblocks/rtd-temperature-sensing aside from, initially, the following modifications:
- No multiplexer. I can’t drive one with the uLog and there’s not enough room anyway. The RTD divider connects directly to the op amp.
- I used a TI TLV27641N op amp, chosen for its ability to go rail-to-rail and its low temperature spec.
- The 56 kΩ resistors were replaced with 330 kΩ. I figure the input voltage swing for the flight will be about 0.1 V and I want the output to go 0 to 3.3 V.
- The output resistor was 10 kΩ, not 220 kΩ.
For testing purposes I connected this up to an analog input on an Arduino Pro Mini (ATmega328, 3.3V) and looked at the digitized signal on the serial monitor. I was powering the circuit at first with an ATX power supply until it occurred to me it might be contributing to the noise I was seeing, and I switched to a 3.6V battery… sure enough, the noise went way down. (The Arduino was powered through the serial interface.)
The circuit appears to work. For now there’s a trimmer pot standing in for the RTD. For whatever reason the resistance that I thought would give me 1023 ADC channels in fact gives me only about 900, though the 0 channel resistance is right where it should be. Not sure why that is, but 900 is close enough for me.
Would it be better to reduce the 10 kΩ resistors defining the gain instead of increasing the 56 kΩ, or split the difference? I didn’t have anything on hand with a 33-to-1 ratio in that neighborhood, but I did try 56 kΩ and 2.2 kΩ; I didn’t see any significant change in the noise.
As for the output resistor, the circuit designer writes, “On an off chance while experimenting with reducing noise in the circuit, I tried adding a resistor on the output of the amplifier, interestingly the noise almost halved with resistances around the 100-220k range. Below 100k and above around 470k the noise would increase. The datasheet for the Atmel 328 states that the output impedance of circuits connected to the analog inputs should be 10k or less, so this doesn’t quite add up, at the moment I’m not quite sure why this works but it does reduce noise significantly.” My experience was different. I tried 230 kΩ (220 in series with 10), 10 kΩ, and 0 Ω. I didn’t see any dramatic difference in the noise with 230 kΩ as compared to 10 kΩ, but 0 Ω looked to be somewhat quieter. So unless I find a reason not to, I think I’ll send the op amp output directly to the uLog. Noise looks to be at about the 1 or 2 channel level, which corresponds to on the order of 0.1°C, which is just fine.