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AD8429: CMRR worse than stated in data sheet

Question asked by Alexxx on Apr 20, 2018
Latest reply on Apr 24, 2018 by Alexxx



I want to use the AD8429 to amplify a bridge signal (with highest accuracy) because of its exeptionally low noise.
The amplified signal will be fed to differential ADC-driver => 32Bit-ADC.
The problem is now, that the bridge exication may be AC chopped at +/-10V peak @200Hz.
All the instrumentation amplifiers have limited CMRR, especially with higher Frequency.
But it seems that the AD8429 is performing much worse than stated in Data sheet!

frequency (sine)      data sheet      measured

1kHz 20Vp-p              -105dB            -75dB

10kHz 20Vp-p            -85dB              -59dB

100kHz 20Vp-p          -65dB              -39dB


With my test circuit is as follows:

- esd-protection diodes from function generator input to either supply rail.

- anti-parallel shottky diodes between IN- and IN+

- 200R 1% serial resistors on both inputs which are shorted together at the function generator output

- Rgain= 30R => V ~200

- supply voltage: +/- 17V


With Ucmm as rectangle signal the output looks really ugly. (800mVp / 2µs settlement)
As the EMI-Filter-Cs would lengthen the Ucmm ramp, it would make matters worse,
since it would take even longer to get a fully setteled output voltage.

-> Is there anything I can do to speed up settlement or to decrease the spikes?
-> I would undertake every possible effort if this would help!!!

-> How about empirically fine-tuning the balance of EMI-Cs (INx -> GND)  or the serial input resistors?
-> Is there anything like a composite INA circuit that can outperform AD8429?


Any suggestion will be very much appreciated.