Voltage Follower
Robert Spanton
I've just been reading the ADC docs on the MSP430. It gives a formula for the
minimum sampling time for a given source impedance. With the filter that I have
set up here, the source impedance is 500k. The formula gives a maximum sampling
frequency of 9.6 kHz. This is a bit slow.
So, a really cheap way of allowing this to be faster is to put a MOSFET voltage
follower between the filter output and the ADC. I've just built one from a
2N7000 and a 10k resistor. This works marvelously. The only thing that needs
to change though is the voltage divider in the filter. By moving this to a
higher voltage, the input signal to the FET is always above Vgs.
We're also safe from Vgs variation because the output DC level doesn't matter
that much - the ADC can handle it.
The 2N7000 costs a whole 4p from Farnell (SOT23). I think we may be able to
stretch to that. Resistors... well they're like dust.
Rob