Small capacitance sensing board for pressure sensors

[frasdoge]

Hello,

Day to day I work on testing capacitive pressure sensors for our lab in the range of 5 - 200 pF. One application of our sensors is to measure pulse rate. We can do this by strapping a sensor to the wrist like a watch, though the wires still go off to a bench LCR meter. We can then find pulse rate by performing an FFT. What I'd like to do is make this field portable by having some capacitance sensing electronics in the casing of the "watch" to record to an SD card.

There are few issues with going about this that I need some advice on.

Firstly, the choice of capacitance sensing method. My idea at the moment is to have a micro generate a 1 Vpp 1 kHz sine wave from either a built-in or external DAC, buffer / amplifier if needed, current sense resistor, followed by our sensor. Then use an ADC over the resistor for current, over the capacitor for voltage, and use V = I/jwC to get the capacitance. If the series resistance needs to be accounted for I could instead measure the peaks and phase difference between the current and voltage and work backwards from impedance to capacitance. Is this a good way of going about it?

Secondly, I've not implemented a micro myself before. I've only programmed dev boards where everything has been plug and play. To fit everything into the size constraints (~ size of a watch) I think I would need to make my own but it's intimidating. Especially for connecting my own programmer (never used JTAG, and it looks large profile) or an external crystal. If there are any great tutorials I'd appreciate being directed to one.

Cheers,
Fraser

[frasdoge]

Thanks for the depth of the reply!

I'll have to spend quite a bit of time digging through this to decide what will work best for me, but the FDC2x1x option is looking very appealing. I'll just need to fit in my numbers to see what sort of resolution and range I'll get. Any MCU with I2C should be able to handle it. If it doesn't do the job I'll stick with the sine generation and possibly the AD5940/1 impedance measurement.

On the front of MCUs, is there much added complexity or power draw going with a 32-bit like the PSOC5LP you suggested? It seems very over spec for what I need but if it fits in with size, complexity, and power draw requirements I don't see a reason to not use it. Benefit being I'll have options if I change my mind. I figure I'll be powering this off a 2032 cell and boost converter.