Op-amp output versus function generator output? Please HELP!

[slotbot]

Hi.

I am having a problem with a circuit i am designing. Basically here is my issue:

Im using one half of an 556 (dual 555) to generate an almost triangle wave (taking the signal from charging cap) and then running it though a pair of op-amps (tl062 at the moment) to get the amplitude and gain right to make the signal jsut how i want it. All of that works great.

The signal is being used as the analog input to a sample and hold circuit.

The problem is that when i put the output  of the second op amp into my s/h circuit the output is ALWAYS the max of the input voltage. IE if the signal swings from 4-6 v peak to peak the sampled voltage is always 6.

So ok maybe theres something wrong with the s/h circuit right? BUT when i make a similar wave (same voltage swing , frequency and dc offset etc) with my function generator and put it into the analog input of the s/h everything works fine. you get a  nice array of sampled values.

SO i guess my question is what is the difference between the output of a function generator and the output of a tl062? also i have tried different pin for pin replacements of the dual opamp  (tl072, tl082, rc4558 etc) and im getting the same result.

The output impedance of my function generator is aparently 50 ohms if that helps at all.

If anyone has ANY suggestion please let me know. HAPPY VALENTINES DAY!

scott.

[niftydog]

can you draw up the schematic?

I would suggest that there's something strange going on with the impedances (op amp output Z too high). Try hanging a low value resistor to ground from the output of the 2nd op amp. Maybe start high, say 1k and try smaller resistors down to 50 ohms. Be aware that this may affect the performance of the op amp and the gain structure may need adjusting afterwards. But, it may help you match impedances, or even provide a current path that was not there before.

Otherwise, any capacitors in the circuit? What frequency range are we talking here?

[toneman]

U wrote:
SO i guess my question is what is the difference between the output of a function generator and the output of a tl062? also i have tried different pin for pin replacements of the dual opamp (tl072, tl082, rc4558 etc) and im getting the same result.

ND has the right idea, i think.
A function gen is a low Z(impeadance) out.
It usually can drive anything(almost...not a speaker)
An opamp (function)generator, the triangle out, needs 2 B buffered
B4 using it to drive another circuit, especially the S&H cap.
As cap charges, it draws charging current from the tri wave.
Buffer first, then compensate 4 amplitude, *then* S&H.
stayjunctioned
@ the function
tone

[slotbot]

yes im pretty sure it is an impedance problem aswell. But how do i calculate the output Z of my opamp circuit? and arent op amps designed to automatically have crazy low output impedance? (let me know if im wrong)

Also is it possible that it could be something like the function generator can source more current than the dual op amp chip?

also the analog frequency is around 6.5 kHz. looks like a sawtooth wave.

Also i will try buffering the tri wave before altering it and let you knwo what the results are like. but i dont think it will change anything becasue i think my first op amp is doing that no?



Thanks for the quick help guys.

[puretube]

tried to lower the triwave to 0V-2V (instead of 4-6V)?
got a cap at your output?

[niftydog]

But how do i calculate the output Z of my opamp circuit?

You don't really, but you can use practical methods to estimate it, but I wouldn't bother as it's all relative - you can simply lower it by bunging in a resistor to ground and then it's virtually "set" at the value of the resistor.

and arent op amps designed to automatically have crazy low output impedance? (let me know if im wrong)

Yes they are, but there are other factors. This is partly why I asked if you had a schematic drawn up.

Also is it possible that it could be something like the function generator can source more current than the dual op amp chip?

Yes, that's possible.