op-amp integrator basic's

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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

 

[Larry Brasfield]

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.

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).

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.

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

I have several questions:
Would it be better to increase the time constant, rather than placing a
resistor across the integrator cap?

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.

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

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 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?

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.

thanks boats_ranger

Anytime.

 

[[email protected]]

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

 

[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?

Yes, in general.

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

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.

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)

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.

Can the 4066 conduct voltage in both directions?
Yes.

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

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

 

[[email protected]]

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.

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

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

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

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