Hi, I want to build a electrical impedance measurement device using AD4003/AD4001, where a small current (around ~1mA, at 1MHz) is inject into the body and the difference voltage at two electrodes on the forehead is measured. The signal to be sampled is a small signal (Vpp<1V) with a relative large common mode voltage (from -10V to 10V). I draw a simplified schematic below.
I found some solution using AD629, however this opamp can not be used when the frequency (ac) is 1MHz. I also checked some instrumental amplifiers, however there is non with a relative large slew rate while maintain reasonable CMRR at 1MHz. So, how could I design a ADC front end for this specific application?
Have you considered using the AD8429, which has very low noise of 1nV/√Hz and min CMRR of 90dB (G=1) for your design?
I checkout the datasheet of AD8429, however the ac specification of CMRR at 1MHz is rather low (approximately 40dB).
Is it OK to use it where the signal frequency to be sampled is around 1MHz ?
It may be ok, but keep in mind your overall signal chain performance will be limited by the choice of an instrumentation amplifier.
I build diff-to-single conversion using AD8429: but the output seems to be negative shifted.
The outputs are great!
Thanks for your reply!!
BTW, I checked AD8251, AD8429, these two instrumental amplifier have only 20V/us slew rate, which may limit there application with frequency <100KHz with a 10V Vpp signal. It seems like that in order to sample a 10V Vpp, 1MHz signal with a large CMRR, I need to build an instrumental amplifier from op-amps, right ?