Need help with op amp selection 1

[eeprom]

I'm working with a signal with frequencies up to 3000Hz, amplitude of 100mV. I'd like to scale this to 6V. I'm using an LM324, which should be able to handle this, but I am getting some odd results, which make me think the op amp is slewing. I need an input impedance of >1Mohm and <100pF. Can someone please recommend a better op amp than the 324.

thanks,
EE

 

[Skogsgurra]

What supply voltage(s) are available?

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[eeprom]

I'm using two 9V batteries in series to make +/- 9V.

 

[Skogsgurra]

Three kH at 6 V amplitude doesn't require more than a little more than .1 V/us slew rate. And the 324 typical slew rate is around .25 V/us. But that number is given at +/-15 V supply, so it is probable that the +/-9 V supply and 6 V amplitude can be a problem. There are also devices from other sources than NS that have deviating specs.

The TL 084 has served me very well in medium speed applications. It has a little better than 10 V/us and is uncritical in all respects.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[eeprom]

Thank you. My datasheet shows a slew of 0.5V/uS for the 324, which should be good enough. But if the TL 084 can do 10V/uS, that is much better.

Thanks for your help.

EE

 

[jimkirk]

When you say "signal with frequencies up to 3000 Hz", are you talking about sine waves, or some signal with a fundamental at 3 kHz?

I onced worked with a guy who asked me to design a peak detector. I asked him what amplitude & frequency range it would see. "Couple of volts, 60 Hertz," he replied. Seemed like a simple design with an average opamp.

Turned our he was looking at signals from a hard drive with a platter rotating at 3600 RPM. Yep, a fundamental at 60 Hz, but 99.99% of the energy between 10 MHz & 20 MHz.

 

[eeprom]

No, I'm looking at a 60 Hz fundamental with a bunch of harmonics. The composite wave is not a sine wave, but a composite of many sine waves and some noise. I know that most of the energy exists between 60 and 800 Hz. I need a pretty accurate, scaled version of the waveform so that I can decompose the signal into it's composite parts.

 

[Skogsgurra]

800 Hz? That wouldn't work with the 324. TL084 is OK, though.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Sorry. I will work. I was somewhere else...

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

"It", of course.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[MiketheEngineer]

Dumb structural here - but two 9V batteries in series would provide 18V+/- No??

 

[eeprom]

Two 9V in series would provide a total of 18V, which could be split to +9V and -9V, using the common point as ground (common).