spottymaldoon
Newbie level 5
I'm using a TI LMP7721 3-femtoamp bias current op amp to measure an extremely small current in an ion cell. I have it in inverting mode with a guard around the input, driven to follow the non-inverting input voltage. This works extremely well and gives me an output between +/-5V.
After some discussion in an earlier post relating to choice of a suitable AD converter I have fixed upon an Analog Devices AD7171 which uses the sigma-delta routine; has 16 bit resolution and a conversion rate of 125 S/s. It also has a differential input of +/-5v (with Vdd=Vref=5v).
I am proposing to pass the -5v rail voltage to the negative AD input and the amplifier output to the positive. Now, relative to ground, this may mean that there will sometimes be a negative voltage applied to the positive input (unless I apply bias which would be a pity since it then cuts my dynamic range).
Looking at the spec and absolute limits however I see this:
Does this kill my idea? If so my fall-back will be to ground the negative input and ensure the signal is suitably biassed. Your thoughts would be appreciated.
After some discussion in an earlier post relating to choice of a suitable AD converter I have fixed upon an Analog Devices AD7171 which uses the sigma-delta routine; has 16 bit resolution and a conversion rate of 125 S/s. It also has a differential input of +/-5v (with Vdd=Vref=5v).
I am proposing to pass the -5v rail voltage to the negative AD input and the amplifier output to the positive. Now, relative to ground, this may mean that there will sometimes be a negative voltage applied to the positive input (unless I apply bias which would be a pity since it then cuts my dynamic range).
Looking at the spec and absolute limits however I see this:
PHP:
Absolute AINx Voltage Limits GND − 0.03 VDD + 0.03
Does this kill my idea? If so my fall-back will be to ground the negative input and ensure the signal is suitably biassed. Your thoughts would be appreciated.