Other Parts Discussed in Thread: PCM1861, INA190, , INA190EVM, INA132, INA240, OPA2192, INA240EVM, OPA192, THS4131, INA826, INA821, INA194, THS4551, INA149
For a loudspeaker sense application I need to amplify a small differential signal which is superposed on a large common signal.
I tried first with a current sense amplifier, but the signal to noise is very small. Which makes it impossible to use with for very small differential signal.
Then I tried with an opamp a large common mode input range (rail-to-rail at input). This works rather well for the differential signal, but from the moment the common mode signal becomes too high, my amplified differential signal starts having deformation. I still work within the specified common mode input voltage range.
The circuit drawn of what I intend to realize is attached at the bottom.
The ADC that I use is the PCM1861 (low noise ADC).
Getting the sense resistor voltage direct into the ADC differential input is not OK because there is worst case 73dB between common mode and differential mode signal. Common mode rejection ratio is 56dB for the PCM1861. So differential voltage won’t be measurable.
With the current sense amplifier, I had the issue that I could only get 58dB STN with the 4A and with 0.08A I only got about 18dB STN. I’m not sure whether it was the voltage noise density specification which was too high or the CMRR was too low (was only 100dB).
For low noise I chose not to go for another current sense amplifier but instead for a low noise opamp with rail-to-rail input. But now even with staying within voltage input specs, I have at low common mode voltage at the input pins that I get deformation at my amplified differential signal.
What opamp would you suggest?
I was thinking to try on the INA186A1 or the INA 190A1 which has higher CMRR specs but again current sense amplifier. But a current sense amplifier doesn't have good noise specifications.
For a low noise amplifier with high common mode input voltage I find it difficult on the TI website to get the amplifiers with wide Common mode capabilities or even with common mode capabilities beyond the supply voltage.