Right now I have an LMP7721 with the ion collector electrode connected to the (-) input through a 10M resistor and a 10M feedback resistor. The (+) input goes to earth/ground. This is followed by an inverting buffer opamp with an offset voltage (small) applied to the (+) input and unity gain (10K feedback and input resistors). The output goes to the AD through a 10K resistor and there's a 1MFD ceramic capacitor to ground to give 10mS smoothing. The setup is a little haunted by the fact that the lamp is powered by 100MHz RF and I've done a lot to try to exclude this - it isn't appearing in the noise level at present however.
I'm planning to systematically go through the analog to tie down the sources of noise - not done it yet.
Currently, of course, 1 picoamp ion current will appear as 10uV at the AD1230 input - 640 uV at the actual AD and utterly invisible at present but an eventual goal.
I am contemplating inputting the signal on the (+) input of the LMP7721 and simply connecting the (-) to the output; thereby reducing resistor noise but maybe seeing charge buildup on the input as the device has such a huge impedance.
The ultimate goal will be a sensitivity (benzene) of better that 0.1 PPB in air for an injected sample of 1 mL - I have done this routinely in the past but with older components and non digital.
The above stuff isn't relevant to the theme of this group, though - I apologize for that.
I'm planning to systematically go through the analog to tie down the sources of noise - not done it yet.
Currently, of course, 1 picoamp ion current will appear as 10uV at the AD1230 input - 640 uV at the actual AD and utterly invisible at present but an eventual goal.
I am contemplating inputting the signal on the (+) input of the LMP7721 and simply connecting the (-) to the output; thereby reducing resistor noise but maybe seeing charge buildup on the input as the device has such a huge impedance.
The ultimate goal will be a sensitivity (benzene) of better that 0.1 PPB in air for an injected sample of 1 mL - I have done this routinely in the past but with older components and non digital.
The above stuff isn't relevant to the theme of this group, though - I apologize for that.