Rich, you say;
>>....... My experience with 723s (backed up by a fair
>>number of engineers who've had similar problems) is that they are very
>>prone to RF pickup.
>This is a non-issue. The fix is 2, 100k, 1/4w resistors at the 2 input
>pins.
Don't agree. For RF fields picked up on the chip ( and we've done some
measurements on that) the 100K resistors don't do anything. In fact,
they increase the impedance that is seen looking out. Don't have the 723
data sheet to hand: what's the input bias current and the input bias
current offset? 100K could be a bit high to offer best performance .
> The other aspect
>that makes me a bit dubious about using 723s for a new design is their
>age - they could very rapidly disappear - semiconductor companies have
>a habit of killing older products on older processes.
>723s are still being manufactured, as are 3-500Zs, as are 811As. Buy
>some spares. I'm guessing that the 723 used in the amplifier will
>probably last longer than the amplifier. I am still using the original
>723 in my 1000v regulated 8169 screen supply.
A year ago, you could have said that about SL521 log amp ic's. After
all, they'd been around since 1964, and were used in all sorts of kit.
Now they're almost unobtainable. Don't rely on ICs being around for
ever. For the IC manufacturer, the ideal cycle is 5 years from start
design to obsolescence. Agreed, designs like the 723 are more amenable
to process transfer - including from 4 inch to 6inch slices. If it
hasn't been transferred, then it's definitely sunset technology.
Fortunately, the reliability is high, but I'd still lay in some spares
if I was you - and keep them in a dry place in anti static packaging.
73
Peter G3RZP
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