Hello,
This board is connected to a 12V wall wart (until it exploded).
A 7809 converts the 12v to 9V.
A ICL7660 creates a -9V rail. This supplies the -Vcc of all the Opamps.
-9V is filtered via a RC (1kOhm, 47µF) for the "signal rail"
This "signal rail" is feed into a pair of LM 336 create a +2.5V and a -2.4V for the sensors (the idea is to get +50mV something out of the sensor, or lower if possible).
Each sensor get it's "own" voltage set from TL084 followers.
The signal is from 0.01mV to 1mV.
The output signal is pre amplified 1M/10K (or 100x) via OP07's.
This in turn will be amplified by a TL084 running normal inverting amplifiers to a range of 0 to 3.3V
In turn fed into the ADC of an Arduino.
It works - BUT - and I love big buts. After the preamp, the noise level is still a bit high, about 10mV for a signal of 1.2V - or 1%.
It's useable, but not perfect.
If I could reduce the spike by 50%, I'd be happy.
The power rails have already been filtered:
1000µF from +12V to ground.
470µF from +9V to ground.
1000µF from -9V to ground.
270µF from -9V to +9V.
This has removed 95% of the ripple but not the most damaging "spike".
I have tried putting a RC filter on the -9V power line, but even with 100ohms, the power drop is too big for the circuit to work (negative TL04 and Op07's start acting up).
An RC filter on the outputs make the signal oscillate. Maybe I'm doing that wrong.
More µF's on the 12V, 9V and -9 does not bring any useful gain (tried up to 2500 µF, nice sparks, but no gain).
Image 1
Tools at hand; a 15MHz Philips analogue scope and a UT61B multimeter.
Image 2
The set up.
Image3
Different parts of the circuit.
Image 4
Top, negative signal out of the 7660.
Bottom: After filtering, the spike is thinner but still there.
50mV per division.
Image 5
Now at 5mV per division.
The top signal is what is feed into the sensors.
The bottom signal is the same.
Image 6
Output of the sensors.
The bottom signal is the same.
Image 7
Output of the OP07's.
The bottom signal is the same.
Output of the sensors..