GigaOhm measurement
GigaOhm measurement
(OP)
Hello,
i'm building a GigaOhm measuring device.
I'm making a voltage divider with the GigaOhm resistance and a 68MOhm resistance and i apply that signal to a non-inverting amplifier with a gain of 40.
My problem is that my signal takes too long to settle. How can i make it faster??
i'm building a GigaOhm measuring device.
I'm making a voltage divider with the GigaOhm resistance and a 68MOhm resistance and i apply that signal to a non-inverting amplifier with a gain of 40.
My problem is that my signal takes too long to settle. How can i make it faster??
Best regards, Carlos Cardoso





RE: GigaOhm measurement
If you want lower than that, you'll need fF capacitance and VERY careful board and wiring layout. AND you might need to precharge, although that'll add capacitance as well
TTFN
RE: GigaOhm measurement
I'm doing my prototype in Vero Board, i've already lowered my voltage divider resistance to 10 MOhm's but it is still too slow.
What did you ment by fF capacitance?? Does it mean add a capacitor in my circuit??
Precharge is out of the question because my microcontroller will only work for 3 maybe 4 seconds and then will go to sleep. So i cannot precharge it from time to time.
Best regards, Carlos Cardoso
RE: GigaOhm measurement
But, there still seems to be a disconnect somewhere. The implied load capacitance of the divider is something like 3 nF, which is absurdly high. What are you trying to drive with your amplifier? Some amplifier configurations will reflect their load capacitance into their inputs, so if your amp is driving 120 nF, that might account for it.
Your voltage divider needs to see not more than maybe 10pF. This would require minimizing the node capacitance using a JFET input op amp, short traces on the board, etc. You might look at reconfiguring your amplifier configuration to isolate its load capacitance from the input.
TTFN
RE: GigaOhm measurement
Can't remember what the node capacitance was in the tester.
rgds
Zeit.
RE: GigaOhm measurement
Best regards, Carlos Cardoso
RE: GigaOhm measurement
RE: GigaOhm measurement
Even if you waited 5 or 10 time constants to settle, this is pretty short.
Does the 68 mOhm terminate on ground or on some floating point. Especially if this is battery operated, you might be seeing a power supply feedback component.
Are switching the power to the op-amp on and off? There may be some other compensation as simple as bypass caps that don't settle. And if the 68 mOhm terminates on anything other than ground then switching the power would be a problem for more than one reason.
What is the reference to the A/D. Are you measuring this drift with the A/D or with a scope?
Does the circuit have a single r/c approach to its final value, or does it simply take a long time to get to within the required tolerance? The 0.47 cap on the output, if electrolytic, could have enough memory to keep the output from settling for quite a while.
Consider using an inverting amplifier referenced to ground. This eliminates voltage changes at the input and so the input capacitance can be ignored. If you need a reference voltage, use a voltage reference chip that settles in milliseconds after power up.
These are just some ideas. Usually you will find what the real problem is in the process of eliminating potential problems.
DspDad
RE: GigaOhm measurement
Regarding the long stab time, you might get some useful ideas by looking at the technique of 'shield driving' used on hi-Z instrumentation amplifiers to effectively cancel shield capacitance.
RE: GigaOhm measurement
Try connecting the measurement node to the op-amp input in the air, with none of the connections touching the vector board.