Chris -- have a search around for PMA's comparison of a bunch of different opamps new and old to give you a better appreciation of what you're gaining (or not!).
Also worth minding whether multiple feedback or Sallen-Key configuration is better for your setup. (There's a good application note by Bonnie Baker about that)
Also worth minding whether multiple feedback or Sallen-Key configuration is better for your setup. (There's a good application note by Bonnie Baker about that)
Daniel - Thanks very much for the excellent Bonnie Baker tip. I'm trying to chose a filter and a search based on her name gave me what I was looking for.
Phil
Phil
Hi Daniel,
Yes, the LM318 is still useful, but it requires more care in layout as it will oscillate. You and I probably don't have a problem with this because of experience, but someone new really might have trouble with it.
Bonnie is a great author. Love her stuff. I have very likely read that app note.
-Chris
Yes, the LM318 is still useful, but it requires more care in layout as it will oscillate. You and I probably don't have a problem with this because of experience, but someone new really might have trouble with it.
Bonnie is a great author. Love her stuff. I have very likely read that app note.
-Chris
Glad I could help with the Bonnie Baker tip.
Chris/anatech, I think you merged a couple posters. 🙂
Chris/anatech, I think you merged a couple posters. 🙂
It has to be jfet input stage op amp because the filters need high input impedance / low bias current to operate correctly.
Not so, the filter is all designed around low impedance circuitry. If you were dealing with 10 or 100meg resistors in the networks then it would be a totally different story, this is definitely all low impedance stuff.
Yes, JFET devices make a good choice, but not for those reasons in an application like this.
Yes, JFET devices make a good choice, but not for those reasons in an application like this.
Hi Daniel,
You're probably right.
Hi ticknpop,
Impedance is a relative thing. BJT input op amps can represent a high input impedance. It's just that a J-FET op amp has lower noise at the higher impedances as a general rule. If memory serves, the LM308 was a very high input impedance op amp that had a BJT front end.
As Mooly pointed out, filters can be designed as a low impedance filter (think speaker crossover for an extreme).
-Chris
You're probably right.
Hi ticknpop,
Impedance is a relative thing. BJT input op amps can represent a high input impedance. It's just that a J-FET op amp has lower noise at the higher impedances as a general rule. If memory serves, the LM308 was a very high input impedance op amp that had a BJT front end.
As Mooly pointed out, filters can be designed as a low impedance filter (think speaker crossover for an extreme).
-Chris
If a bjt input stage is ok, then the njm2068 could be a candidate for low price/honnest performances.
If we're talking high/low impedance in terms of bjts, then it's primarily noise based on input bias current across the feedback resistor. So you need to look at current noise too.
In case I'm being unclear or imprecise with language: http://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdf
In case I'm being unclear or imprecise with language: http://www.analog.com/media/en/training-seminars/tutorials/MT-047.pdf
SO8 pins don't bend. They're very tiny, and you'll shear them off. It might work with some jumper wire, but it'll be a real PITA.
TI sells a nice set of adapter boards for very low cost that allow you to use a number of SMD packages in 0.1" through-hole circuits. Search for DIP-ADAPTER-EVM and you'll find it at the TI store as well as at a number of distributors. You get 6 each of 6 different SMD packages, up to 8 pins each. Here's their description: DIP-ADAPTER-EVM DIP Adapter Evaluation Module | TI.com The kit comes with header pins too, so all you need are the ICs and solder.
Hi Monte,
Ordered one from Digikey just now. Thanks for the info on that, they will save me time from making my own. Probably cheaper too.
-Chris
Ordered one from Digikey just now. Thanks for the info on that, they will save me time from making my own. Probably cheaper too.
-Chris
If soic is on the table, then the opa1642 should be as well. Especially since it's a jfet input that roundly outperforms the 2134
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Hi Daniel,
Do you actually hear a difference? I'll try couple to see. They are approx. $5 each for me in Canada.
-Chris
Do you actually hear a difference? I'll try couple to see. They are approx. $5 each for me in Canada.
-Chris
No idea audibility, I haven't given them a listening comparison. Doubt there's much if any, and most under some kind of odd input impedance or heavy loading. Just wanting to be thorough in the suggestions. 🙂
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