An opamp Integrator 1

[milindengg]

Hello,
I have a simple question regarding op amp integrators. An integrator normally has a capacitor connected between the output and the inverting terminal and this integrates the current coming in to the voltage out. In some circuits I have seen there is a resistor placed in parallel with this capacitor.
What is the purpose of this resistor. Solving for the circuit creates a complicated expression for the output voltage. Please give me some idea.

 

[Skogsgurra]

It is about real world. An ideal opamp has nu offset voltage and no bias current. Real ones have. So if you leave an integrator with zero input, it should stay with a constant output forever. But the bias current and the offset voltage makes it drift away until it eventually hits the output limit, which today often is one of the supply rails.

By adding a high valued resistor in parallel to the capacitor you make the DC gain less than infinite and reduces the drift. But you also get some leakage so your integrator will not stay constant any more. But if you need to integrate for a few seconds and if you do read the result within another few seconds then the resistor usually doesn't hurt. For minutes and hours of integration time, you will need good opamps and good capacitors. You also have to pay special attention to leakage currents in the PC board and probably use guard rings around the input pins of your opamps, sometimes even driven by the output via a resistor. Read Jim Williams and Bob Pease. They are quite good at these things.

 

[milindengg]

Thanks for the helpful reply. One question could u tell me the book title for Jim Williams and Bob Pease? Also can I get any online links?

 

[IRstuff]

Bob Pease's old columns can be found on the National Semiconductor website; look for "Pease Porridge"

TTFN

 

[nbucska]

for books search for author's name at <

http://www.amazon.com>

<nbucska@pcperipherals.com>

 

[Comcokid]

It is a common practice in op-amp gain circuits to put a small capacitor across the feedback resistor. This gives higher frequency signal components lower gain - a simple way to filter noise (i.e. low pass filter). Don't confuse this type of gain stage as a pure integrator.

The books you are asking about were listed in the following thread. thread240-75845