Voltage integrator circuit

[orienteraren]

Hi!

If I´m using a voltage integrator circuit of the basic integrator circuit type (OP amp with high resistor on -input, and gnd on + and then a feedbak capacitor between the output and the -input) and then integrate and sample my values. If I then want to "reset" the integrtor circuit to zero to begin a new integration, how is this done in a good way?

In other words I want to sample continuisly for a while, then reset the integrator circuit to have a zero voltage output and the start the sampling again. I suppse there is a simple solution to this that I have missed.

Regards

Fredrik

 

[keith1200rs]

Just use an analogue switch or small MOSFET across the capacitor. You want the most pathetic analogue switch you can find because it will inject charge into the capacitor. Low resistance analogue switches will have bigger transistors and hence more charge injection.

If you go to Analog Devices web site & look for analog switches and use their selection table, you can add Qinj (pC) as one of the table parameters. Look for a low value. Something like the ADG1221 is one option. Other manufacturers will have similar tables for selection, I think.

Keith.

 

[orienteraren]

Sorry but in what way should the switch across the capacitor "reset" the integration circuit? I was thinking about a switch between gnd and the -input (grounding + and -input) giving the amplifier a zero input on both inputs. Just put zero on the negative input would just keep the output steady at the same value as it is already presenting.

Regards

Fredrik

 

[keith1200rs]

Sorry but in what way should the switch across the capacitor "reset" the integration circuit? I was thinking about a switch between gnd and the -input (grounding + and -input) giving the amplifier a zero input on both inputs. Just put zero on the negative input would just keep the output steady at the same value as it is already presenting.

Regards

Fredrik

Fredrik,

It discharges the charge built up in the capacitor. See attached simulation. Maybe I have misinterpreted your requirements?

If you short the + and - inputs all that will happen is you will stop the integration from happening by shorting the input to ground. The capacitor will still retain its charge and the output will stay where it was.

Keith.

 

[orienteraren]

Thanks for the reply! I tried to simulate it myself and found out just what you wrote. Now I just have to find a reasonable way of implementing it

Regards

Fredrik

 

[keith1200rs]

Thanks for the reply! I tried to simulate it myself and found out just what you wrote. Now I just have to find a reasonable way of implementing it

Regards

Fredrik

You just need a simple analog switch. If you are unsure of the suitability of one you choose, just post again and I will do my best to help.

Analog Devices and Maxim are two companies worth looking at.

Keith.