trimpot guts

[Joerg]

On Tue, 22 Jan 2008 18:24:37 -0800, Joerg

John Larkin wrote:
[...]

Isn't that kind of non-linear?
The signal path should be pretty good to at least +-200 mV swing into
the second (G=10) amp. Gain control is an s-curve on gate voltage, but
not too radical. The active gate control range is 0 to about -0.8
volts, but we'd need less that that to do a +-25% gain trim, probably.

You could servo out the gate curve with a 2nd FET plus opamp. OTOH uC
horsepower and DAC bits are cheap. Although we had to budget over four
bucks for a 12bit octal in a similar application :-(

FIFTY CENTS per dac? Poor baby!

Well, around a dime per channel:
http://www.rohm.com/products/databook/general/pdf/bh2223fv-e.pdf

Call me Uncle Scrooge but my first one had a servo and 8 bits. It cost a
lot less because this was a multi-channel system.

The cheapest dac is a uP or fpga pin, pwm or delta-sigma. That could
cost 2 cents, for the r-c output filter.

Yes. But only if there already is a uC or FPGA. I try to stay away from
FPGA because they rarely have the production lifetime we need plus they
cost a lot. Sometimes a little serial-load DAC just hits the spot.