Hello all,
I am a complete electronic newbie. So, sorry if this question is lame or trivial.
I am looking at a bridge circuit and it has 4 leads... 2 for power and
2 for output. It is used as a pressure measurement sensor.Now, the output is taken as a difference of these two signals. My question is why do we take the difference? Why not take the absolute value of one signal?
Thanks, Luca
If all 4 resistors in the bridge are pressure sensitive, then taking the difference doubles the signal voltage (one goes up when the other goes down).
The signal is a very small part of the total voltage across the bridge. If you measure the absolute voltage of one half, you are measuring a large DC voltage with a tiny variation. You have to ignore almost all of the signal and amplify only the tiny variation. This is especially difficult if the supply voltage is not perfectly quiet, stable and ripple free. That large DC with a tiny signal fraction will also include some small hum component picked up from any nearby power wiring fields.
By amplifying the difference of two nearly equal DC voltages, all that they have in common (including almost all the supply noise, variation, ripple, and if arranged symmetrically, the hum component) cancels out and almost all that remains is the sum of the two tiny pressure signals. Once you have this small, clean difference signal separated from all that other stuff you want to ignore, it is much more practical to amplify the heck out of it, to get a reasonable clean and robust output.
-- Regards, John Popelish
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