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op-amp integrator basic's

Hi, I am using a classic opamp integrator, my input is a 500us pulse at a freq. of 80hz. The signal can vary in amplitude from several mv to about 8volts.

The integration time is 10ms, which is about 20x longer than the expected input. I noticed, if the input voltage exceeds ~4 volts, a dc offset appears at the output and increases as a function of input voltage. To mitigate the problem, I placed a 75k resistor across the .1uf, to discharge the cap before the next pulse. The integrator's C=.1uf and the r=100k.

I have several questions: Would it be better to increase the time constant, rather than placing a

resistor across the integrator cap?

As far as integrating cap's is there a specific type used, is ceramic ok?

I can provide a 5volt logic signal at the proper time for to control a switch which would discharge my cap, can a device be recommended?

thanks boats_ranger

Reply to
boats_ranger
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There is no such thing as an "integration time" that can be considered a property of the integrator itself. If you are refering to the R*C product, that affects the gain of the integrator, which has units of (volts out) per (volt second product in).

Perhaps your integrator is reaching the limit set by the supply. That would call for decreasing its gain.

Yes, assuming that you mean decreasing the gain of the integrator and it is important to have an integrator rather than a low-pass filter.

That depends on what kind of linearity, accuracy, stability, and insensitivity to acoustic noise your application requires. Not knowing any of that, I am in no position to gainsay your choice of cap.

I like the 4066 for this sort of thing. But your supply levels may preclude its use and require something like a DG211 or its ilk. If the input has a known polarity, a single MOSFET of the right polarity can do the job, or, if offset at reset is not critical, a BJT can even be used.

Anytime.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield

Hi, I have several more questions:

In this situation, I'm integrating over a single pulse. I was under the assumption that a long integration time improved linearity (as a by product reduced the output voltage). Is this correct?

If linearity then accuracy are my primary concern can a capacitor type be recommended? Is there a type to avoid?

Which value should be used, 50hz or pulse width (500us) of the input signal to calculate the corner freq? ( I hope I stated this correctly)

Can the 4066 conduct voltage in both directions?

If I used a mosfet, would it be connected across the integrator cap? If so, the source would in my case be at ground.

thanks, boats_ranger

Reply to
boats_ranger

Yes, in general.

Avoid the high-K dielectrics in that case. If high linearity is important, you may want to use a film capacitor, such as polystyrene or polypropylene. However, since you were willing to put a resistor across the integration capacitor, I suspect that your linearity requirement is modest and just about any capacitor would do.

I do not know how to usefully assign a corner frequency to an integrator. Its dominant pole is so close to zero that the difference is not worth talking about here.

I would start with the output that you wish to see for the largest input (expressed in volt-Seconds). This sets the gain your integrator will have. It is easy to convert volt-Seconds in, divided by the input R, to a charge. It is equally easy to convert that charge and the desired integrator output, to a C value. Or set 1/(R*C) == integratorOut / voltSecondsIn.

Yes.

Yes, and that is why it works out, often.

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Reply to
Larry Brasfield

for example: the input to the integrator is 1v r = 1000ohms c= .1uf

output voltage =(1/(r*c)) * input voltage

output voltage = (1/(1000 *.1uf) * 1volt output voltage = 10000v/sec

My first impression is to use a N channel device, with the drain connected to the opamp " -" and the source connected to the opamp output pin. To turn the device on, the gate potential needs to be greater than source. To ensure proper operation, 5v may not be enough.

If this is the case, it seem a 4066 maybe a better fit solution.?

Your thoughts.

thank's boats_ranger

Reply to
boats_ranger

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