I am filtering out the output of
MPXV5004DP differential pressure sensor.
I played a little with 324 and currently MCP6004.
MCP is powered by a clean 5V, I have designed a few Bessel filters with Sallen-Key topology already and all work well on the same board.
The problem is surprising:
using Ti FilterPro software,
somehow the output of the filter stays as if there would be no resistors R3 and R4 (i.e. short R4, unsolder R3).
Capacitors are SMD 0805, so max 1uF (no space on board). As a result I got quite large resistors.
No noise observed, however.
Using Ti FilterPro, click:
Low-pass, Sallen-Key, Bessel, C2 set to 1uF, C1 set to 470nF, gain set to 2, 2 poles, 1Hz cutoff frequency, hoping to bring the useful input from 1...2.5V to 0...3V (have 10bit ADC, unable to get different sensors, no space on board for 12 bit MCP3208, VREF only at 3V because there are other sensors at range 0-3V).
Works excellent, but I was not able to achieve any gain (not to mention about removing the offset), the input is 1.085V as expected from MPXV5004, but the output stays the same. Dynamic characteristics as expected, indeed looks like 1Hz cutoff. The layout looked cool according to my introductory book on opamps, unless I misread something.
What went wrong, what should I learn apart from 400-page book?