Voltage divider for charge/discharge capacitor

Hello all!

I am doing a design to charge/discharge a capacitor to generate pulse output for calibration measuring. I started the design based on a previous circuit done by another coworker. The stage is show below:



The circuit works properly doing the discharge when the square signal activates the npn. My issue now is the voltage divider stage, I tried to implement a relay-based attenuation stage using one rotatory switch with low tolerance resistive voltage dividers in L-pad configuration, but in some configurations due to a) the 20K output resistance and b) Time constant changes due to R change, it does not work properly, and my system needs high accurate.

The coworker explained to me that the original circuit used a switched power supply with variable frequency input, so it can achieved the desired voltage, but I cant do that at this moment because the design does not implement a logic unit (and original circuit got a micro).

So I am trying some way to implement this voltage divider. I am reading about zener diode divider, T-pad and Pi-pad attenuation stage,... but dont know if it will work for low voltage values. ¿Can someone you give me some new ideas to implement this? Maybe an IC or some circuits. Thanks in advance.

Regards.

There are a lot of confusing comments in your post. What do you mean 'variable frequency input...desired voltage'? Was it a frequency to voltage converter? 'Relay based...rotary switch'. Which is it, a relay or a switch? Regardless, it looks like you don't need much current. Why not just put a voltage divider on your regulator output, and buffer that with a voltage follower?

Hello Barry,

Sorry if I explained bad.. The original circuit has a programmable switching power supply, so the "regulator stage" is not needed because the programmable power supply does the work.

For that reason I am implementing a circuit with DPDT relays, attenuation stages and some digital logic (part inside the regulator stage box that i did not show). With a rotatory switch I select relays and voltage value that I want using some IC logic.

Yes, I dont need much current. So your suggestion is to have voltage dividers with one single voltage follower at the end? Can it work with 0,01 V range for example?.

Thanks in advance,

Regards.

As long as you select your components properly you should be able to go to 10mV. You'll probably want a precision opamp, something with low offset and drift.