Hello,

I'm working on a voltage equalizer for supercapcitor stacks for my final year project, and for that goal, I need to be able to measure voltage from each supercaps.

I'm trying the differential amplifier using opamp lm358. Schematic attached. The problem I faced is the choice of resistance value. Obviously I need high input impedance, so the resistor wouldn't bleed too much of energy from supercaps. However, as I increases the resistance value, the output voltage drops significantly. What I want is for the output voltage level to be the same as the differential inputs voltage level.

Could anyone tell me how to fix this problem? The solution might be something different from my attachment as long as it fit my requirement. All help is appreciated.

 

Hello,

You could considering buffering both inputs as described here:
http://www.allaboutcircuits.com/vol_3/chpt_8/9.html

 

Thank you, patricktoday

I've tried simulate the schematic from your link. It works perfectly with ideal op-amp. But as I try to simulate using lm358 as buffers, the output goes terribly wrong.

Any suggestion?
Thx

 

For the first stage, you could use a High Impedance single supply rail to rail op amp (TLV2702)

That is just one example, but would eliminate your input impedance issues, that op amp needs only pico-amps at the input. However, it isn't low cost (~$3). There are other JFET input or low bias input op amps, more than you can shake a stick at. That is just an example datasheet I had handy. It is a very high impedance (300M Ohm) and a comparator in a package.

You will also want to look at the datasheet to make sure switching time in comparator side, and gain at desired frequency is within your needs. The TLV2702 is titled "op amp and comparator", some have open collector outputs.

 

For the first stage, you could use a High Impedance single supply rail to rail op amp (TLV2702)

You also probably need to set an initial condition for simulation purposes. If you're able to lower your capacitor's voltage to 3 or increase the power supply voltage it should work with your op amp of choice, but I don't know if that's an option.