multiplier: was Re: 3280 and AD533 availability, EN frequency shifter

[jhaible]

> Here's a 4200 multiplier ckt:
> http://www.omninet.co.jp/workshop/analog/RC4200.GIF

Yes. That's one more circuit that might be less precise than the
chip could be. I don't have the data sheet in front of me, but if
memory serves the impedance of the offset trimmer network (seen
from the RC4200's view) should match the impedance of the
summing resistors. Just as with any bipolar opamp, keep the
i_offset low by matching impedance, and *then* correct the
output error by compensating the v_offset on the input. In the
circuit above, you try to compensate i_offset caused by 100R
vs. some 10kOhms with the v_offset trimmer. Not good for a
precision multiplier. Soulution: Just fit in the right value of
series resistor between the 100R's and the IC pins.
Details in the data sheet.

JH.