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phrarod 15th December 2008 03:15 AM

Need opamp recommendation
I blew a opamp and wondered since I have to replace it if there is anything in the same package that higher quality?

Here's the spec sheet

The circuit was originally designed for a LF351N A friend had suggested the 744 series way back then. Maybe there's something even better?

lineup 15th December 2008 04:45 AM

I post your link (corrected)

Personally, I think AD744 is a very high class op-amp.
Think you should stick to that one.
Not much to improve using anything else.

Regards :)

phrarod 15th December 2008 04:48 AM

Thanks for fixing the link. Don't know what happened. If that's the best - I'm using the KN series then I'll order those. Just want to get the best part for the job. Oops. Just went to order them at Digikey and its a non stock.

Know where I can order from in the US?

lineup 15th December 2008 05:00 AM

you could see what people think. google search:
for example this topic AD744 Revisted
where is compared to some other, like AD829, AD8397

Now, what op-amp will 'be perfect' is a lot depeding on the circuit in question
What you should avoid, is to replace one JFET input op-amp
with one using higher bias current Bipolar input
It may be possible to do so. But may not work at all in other cases.
This has to do with that some circuits are supposing JFET very low input bias currents.

phrarod 15th December 2008 05:08 AM

Do you think this is a correct sub for a LF351N? I have been using the AD744 series but it could be causing issues I'm not aware of. It goes into an Audio Note board. Here's the ps schematic

Thanks for the links. I'll read through them.

lineup 15th December 2008 05:32 AM

Yes, I think AD744 is an excellent upgrade for LF351 in that DA-Converter +- 5.000 volt regulator supply.

Looking at the simplified internal diagram of LF351 and AD744, they are almost identical.
But AD744 should be the better one. LF351 is an old JFET amplifier.

AD744 also has about same supply voltage range as LF351:
+-4.5 to +-18 VDC.
For example input voltage offset (precision) is 0.5 mV vs. 10 mV max offset.
This is 20 times lower offset.

phrarod 15th December 2008 05:34 AM

Thanks again. Looks like the kn series was replaced by the KNZ. Don't know if that's a touch different or not. Good news is digikey has them.

Mooly 15th December 2008 07:26 AM

How did you blow it up ?
With the greatest respect to Lineup, I would stick to the original device type :) here.
Power supplies like this can be surprisingly fussy on the device used, and I did some work on this a while back.
Is this a commercial product ? built by a manufacturer.
The AD744 may well be unstable in this configuration.

lineup 15th December 2008 07:33 AM

I was going to ask the same question.

If AD744 did get blown in that circuit, then you have 2 options:

1. As Mooly suggest, stay with LF351.
It will probably perform very well enough. And will not oscillate & get destroed.

2. Study the datasheet and/or consult some that knows such circuits
and modify the circuit using (compensation- and bypass-) capacitors to get AD744 stable.

Mooly 15th December 2008 07:44 AM

Hello Lineup,
I don't think this is a commercial design, not that that matters. There are a few things that are odd.
The LED used as a reference, it's not stable enough as it is fed from an unreg supply via a resistor. LED's generate noise ! There is no filtering of this on the LED. The output voltage is too dependant on the exact type of LED and temperature etc.
The 100 mf cap that feeds back to the opamp. This can be a problem, some opamps will go unstable with this arrangement.
You need to measure ( scope ) the output of the opamp and the reg to see what is happening.

Sorry to be a bit negative pharod.

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