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Subject: Re: How do i measure signals without affecting them ?
Date: Tue, 19 Nov 2002 13:42:23 GMT
NNTP-Posting-Date: Tue, 19 Nov 2002 08:42:23 EST
Organization: Road Runner
> I want to measure the pulses from various devices, all of which put out some
> sort of pulse. Ie, hall effect device that has already been conditioned to
> be a square wave.
> How would i interface a PIC with the same wire the sensors main signal is on
> without affecting the signal ?
> I have been told i can use a rather large resistor (in the megaohms) in
> conjunction with a schmitt trigger to do it cleanly, but i dont quite
> understand how the voltage going through the resistor would be enough to
> activate the schmitt?
> If anyone knows of some ways to do this please post them.
> Also, what is the formal name for this type of interfacing so i can do a
> proper search for more info ?
What you want is a high impedance probe. The first thing that comes to
mind is a JFET input op-amp. Something like this:
| |\ |
,--------. 10K | \ |
| hall |----/\/\--|+ /---x---o Output
| effect | | /
This should work if the signal source is referenced to the same
potential as the op-amps -V, and the signal goes positive with respect
to that. The 10K is to protect the op-amp, in case the signal goes too
high or too low. Too high a resistance can effect the waveform if the
frequency is high enough.