Hi,
I've build a prototype of a device in which I can plug two analog audio interlinks and the device outputs the difference between the two. The purpose of which should be clear.
I've equipped it with on-board jumpers which bypass the cable so that I can calibrate it. Calibration is done with a multi turn pot.
This is (the right channel of) the circuit:
J2 and J3 are RCA jacks. JP3 and JP4 are the jumpers.
At 1 kHz, I get a rejection ratio of 48 dB (7 Vrms in becomes 26 mVrms out, measured with digital scope). I of course want to improve upon that.
There is a region in the trimpot R26 where adjustment doesn't change the output voltage, i.e. it won't go lower than 26 mV for a few turns to goes back up again. DC offset remains the same in that zone.
I thought improving the virtual earth impedance of IC5B with a higher open-loop gain opamp would help, so I replaced it with an Burr Brown OPA2134, but that did not make one dB of difference.
I can't read any signal between the several ground points on the PCB, so ground appears to have low enough impedance.
Can anybody give advice how to improve rejection ratio?
I've build a prototype of a device in which I can plug two analog audio interlinks and the device outputs the difference between the two. The purpose of which should be clear.
I've equipped it with on-board jumpers which bypass the cable so that I can calibrate it. Calibration is done with a multi turn pot.
This is (the right channel of) the circuit:
J2 and J3 are RCA jacks. JP3 and JP4 are the jumpers.
At 1 kHz, I get a rejection ratio of 48 dB (7 Vrms in becomes 26 mVrms out, measured with digital scope). I of course want to improve upon that.
There is a region in the trimpot R26 where adjustment doesn't change the output voltage, i.e. it won't go lower than 26 mV for a few turns to goes back up again. DC offset remains the same in that zone.
I thought improving the virtual earth impedance of IC5B with a higher open-loop gain opamp would help, so I replaced it with an Burr Brown OPA2134, but that did not make one dB of difference.
I can't read any signal between the several ground points on the PCB, so ground appears to have low enough impedance.
Can anybody give advice how to improve rejection ratio?
look up instrumentation amp - a popular cirucit, lots of details when rolling your own
I would reduce (scale) R values by ~5-10x
the inverter and unity buffer have different feedback ratio, hence differing frequency response, phase shift - "noise gain" compensation may help
but really the simplest solution is to just buy a chip from Analog, Linear, TI, or THAT
I would reduce (scale) R values by ~5-10x
the inverter and unity buffer have different feedback ratio, hence differing frequency response, phase shift - "noise gain" compensation may help
but really the simplest solution is to just buy a chip from Analog, Linear, TI, or THAT
I actually already ordered some INA128 chips from TI for a related project. It didn't occur to me to use them here too. I will look into that.
In the mean time, I will try fiddling with the resistor values a bit to see what happens.
In the mean time, I will try fiddling with the resistor values a bit to see what happens.
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