At 09:05 AM 12/2/2005 +0000, you wrote:


>Hi Guys
>I am building an integrator circuit using an OPA627 and I got the
>integrator to generate the ramp I require, but what I need also is to
>when the integrator reaches a certain point on the ramp, which will be
>variable, I need it to stop integrating. I don't mean reset as I do that
>after, but I need it to stop at that value for something like 300nS. I
>have -10V DC on the input. Values for C is 2.2n and for R 470ohms. In
>theory if I remove the input signal the integrator should just stop
>integrating but in practice do to switching delays, decay of signal and
>OpAmp offset voltage it will not work.
>Any ideas how to achieve this?
>Thanks
>         Luis

300ns is a pretty short time, even for a fast op-amp like the OPA627, and
you'd also have to worry about charge injection in the analog switch(s)
(though not so much with such a high integrator current).  How are you
'removing the signal'? If you ground the input, the drift due to offset
should be no more than 0.5mV/(470 * 2.2nF) =  484V/sec ~= 0.15mV in 300ns.
If you open the resistor up, it should be less again. What spec are you
looking for?

Is this to get an ADC measurement? Could you simply follow the
integrator with a S&H circuit?

You can also consider using a current source and demoting the op-amp to
buffer status.

Are you building something like a LIDAR that you need sub-nanosecond
resolution?

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


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