I’d like to use the imp to measure low voltage signals, ~2mV with at least 16bit resolution. Where can I find the specs for the voltage input pins? The spec imp002 is a little light on details.
2mV is vyer difficult to measure. no ADC in any microcontroller can do that directly. You need a external opamp to boost the signal to 1V at least. but for 16bit resolution you would need an external ADC too, over SPI / I2C
The ADC on the imp is only 12 bits. If you want more resolution, I’d suggest you use an external SPI or I2C ADC.
As remcohn says, you will want to boost this signal up a bit - probably to at least 2v, ie 1000x - before feeding it into an ADC.
eg, here are some instrumentation op-amps: http://www.analog.com/en/specialty-amplifiers/instrumentation-amplifiers/products/index.html - you can find ones here with a gain of 1000x. You will need to be very very careful with board design, power supply and layout in order to reduce noise. If you’re looking for 16 bit resolution on a 2mV signal, this means the LSB is only 30nV of signal… you’ll be picking noise up from everything (including wifi transmissions if the imp is anywhere near your pickup wires).
…and some high resolution ADCs: http://www.ti.com/lsds/ti/data-converters/precision-adc-less-10msps-products.page
What are you trying to sample?
I have a differential type T thermocouple circuit for a custom sensor. Because the type T junctions alternate it has all copper line leads and in turn does not require temperature compensation at the PCB. The sensor elements would also be about 5ft from the Imp. Most thermocouple and low voltage loggers have 0-15mV ranges with 16bit resolution. If I can get it to work I would need to make about 100 of them this year.
I’m afraid we can’t design your product for you, but there are plenty of examples if you search around, eg:
…which is a pretty simple circuit for a type T to SPI using a 24 bit ADC with a built-in PGA.
I understand and thanks for the tips. Will come back if I have any issues with the imp.
I have a few of the 31855 boards and they are fine for low res type K TC monitoring. However for the application I’m trying to work on I need more resolution and the ability to read Type-T junctions. I am welding several type T junctions in series on one TC channel so that i get the differential voltage as an output not the absolute voltage/temperature. Also the benefit of type t materials (copper - constantan) is that for an even number of junctions in series there are only copper lines going back to the logger which removes the need for cold junction compensation. Maybe more detail than you needed…
Looks like Intersil has made an appnote very close to your application. Cold-junktion compensation in hardware, opamp, ad converter over SPI: http://www.intersil.com/content/dam/Intersil/documents/an17/an1770.pdf
actually the intersil solution has a lot of stuff i don’t need like a board temperature sensor and a reference cold junction. Plus the wireless capability of the imp means I don’t need a local PC to read the output.
It’s an appnote! You take the parts that fits your need, get inspired if they almost fit your needs and improve on them so they skip your needs - and skips all the things you don’t need!