Drop-in replacement for LM208

[zigzagflux]

Looking for a suitable replacement in a 8-pin DIP package:

http://www.national.com/ds/LM/LM208.pdf

Kind of an odd duck. I should mention in circuit there are no connections to pins 1, 5, and 8. Ideas?

[Mooly]

What's it used as... and why do you want to change it ?
Can't really answer your question without knowing more as there may be some very specific reason for using this device.

[zigzagflux]

Ground fault amplifier: I want to change it because it's defective and I can't locate exact replacements. So a suitable cross would be sufficient.

Low power and amplification of small signals. All I really need is pin compatible, and where I don't need connections to those unused pins.

Low power would be a plus. Power supply is single ended, about 15V.

[Mooly]

Hmmm... well this is a problem one for sure.
Would you "know" if a replacement wasn't behaving correctly ?

This is a bipolar opamp but with very low input bias currents... more like an FET device.
Have you tried anything... maybe put a socket in test a few. I'd be tempted to do that.
Pins 1, 5 and 8 are usually used for compensation and offest null... so if your circuit has no connections to these it doesn't matter.

Without seeing a circuit it's very hard to make an informed judgement. I can't think of a bipolar device that comes near the input impedance of the LM208... and I'm guessing that's the important characteristic.

The TLO61 is FET input but very low current consumption... for what it costs nothing lost in trying it. Or a TL071.

The OP177,
http://www.datasheetcatalog.org/data...es/OP177GP.pdf

Then there are special purpose IC's although many are surface mount only.
http://www.intersil.com/data/fn/fn6631.pdf

[zigzagflux]

If I wanted to reverse engineer the board, I might be able to provide a schematic and/or determine what characteristics are the critical ones. Given the application, I don't see why very high input Z would be important; who knows, maybe they just tossed in whatever was exotic and ceramic at the time.

I suspect the OP177 should do just peachy.

Thanks!

[bichi]

If lower supply voltage and lower temp range will work in your application:

LM208 vs. LM308:
Supply Voltage: LM208 +/-20V LM308: +/-18V
Operating Temperature Range: LM208 -25C to +85C LM308: 0C to +70C

Appears Anchor Electronics, Santa Clara, Calif, have LM308 in DIP: $3.95 USD or TO-5: $0.79 USD
(see page 7, "Linear/Peripheral" section of catalog/price list)

Price List/Catalog link:
http://www.anchor-electronics.com/weblist.pdf

Site link:
Anchor-Electronics: Silicon Valley's Parts Store

If NTE replacement is acceptable, then, LM308 crosses to NTE938M.