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Yes Kavhra, Brad is correct, AC bridges can be tricky to balance without additional adjustments for DC leakage and other (non-ideal) capacitor characteristics.. You can always replace the cap's with equal resistors to prove your bridge is working.. Also, if you are working at 50/60 Hz you may...
From the app note FvM linked... Just an FYI, not sure if your application is "super" caps
Passive voltage balancing is only recommended for applications that
don’t regularly charge and discharge the supercapacitor and that can
tolerate the additional load current of the balancing resis-tors...
I need to make small gain adjustments to correct the DC error into a double terminated cable (75 ohms).
Normally I would place a POT across a gain resistor but how will this affect at RF frequencies.
What is the best type of potentiometer for RF freq (<30MHz) ?
Thanks.. I have .1uF (didn't make it on the schem) across now but ferrite is a great idea. Though if the diodes (actually diode connected 2N3904) are rectifying will higher capacitance cause a DC error ? I'll give it a try anyhow..
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You were right on about the...
Thank you for the response..
You may be correct about noise. The cable is fed high voltage at the center pin and the shield is fed directly to the TIA so I don't know how I would "neutralize" the capacitance in the current architecture.
Rolling off the amp really tamed a lot of the noise but...
" Not so broadband." - I guess your right, I'm usually working with precision DC so the noise was "broadband" in my world ;)
The device is battery powered and I have good bypassing on all op-amps and supplies. I will add enough C to limit to <1Hz and see what happens.
Thanks again
Thank for the response..
Yes it's partially mains noise but also a lot of broadband.
In a TIA configuration is there a maximum Cf that may cause problems ? I'm ok with limiting in the .5-1Hz if this is possible.
I'm using a classic TIA configuration to measure leakage current through coax insulation in the 200nA range.
The circuit works well using a V source and resistor to calibrate but when I connect the long, high capacitance cable the noise is terrible.
Can I bandwidth limit the op amp to say...
Thank for the response.
The resistance in series with measurement node is in the tera-ohm range, unless it breaks down of course. It's a 15KV hipot so 75G will give me full scale 200nA = 200mV in my configuration.
The offset will be adjusted at build and re-calibration if needed.
Digital...
Thank you for the response..
The simulation in LTspice seems ok but I will try it out (bias cancellation resistor).
Ib is about 1nA for this op amp and as I mentioned above it's a robust amp with built in OVP. This is for a high voltage test application and I have lost plenty of amps :(
Thank you for the reply..
I'm using this amp due to it's built in over voltage protection as it will be used to measure the leakage current of a 15KVDC hi-pot tester. The transients produced during breakdown have thwarted many OVP methods so far..
The simulated offset is only about 350uV...
I have a classic I/V setup with 1 megohm feedback using an ADA4177.
Normally the non-inverting is to ground but I would like to inject my offset bias into this input. Is this an ok approach or is there a better method for this configuration.
I'm only interested in DC current conversion, no...
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