Voltage measurement attenuator amp -
LF356 opamp FET input is in a inverting amplifier
configuration here. R4 + R5 make up Ri each 1/4 W
MFR withstands 250V so both will take upto 500V. Use
more in series for higher voltage withstand with
care in PCB layout and cabinet insulation.
Rf is selected by a digital value at A-B inputs of
4052, that way R1, R2, R3, R6 are selected for four
ranges giving various attenuation levels. The
important thing in this circuit is the on resistance
of 4052 of 100 ohms comes in series with the output
resistance of opamp, so the output is taken before
the cmos switch . The output of this digital
attenuator should not be loaded and should be
buffered before use.
Gain = Av = Rf/Ri and Vout = -(Rf/Ri) * Vin
To view circuit below, Click the Link of PNG or PDF
and view the Circuit, PNG can be Scrolled with Mouse
measurement attenuator amp - del20013
Rf and Ri are 0.1% MFR, if costly, use bourns 10T
trimpot or a difficult way use series parallel
scratch 10% part of a network R to increase value.
(first remove paint on resistor with a blade then
with ohmmeter connected scrape black film to
increment value till done, seal in dust free unit).
I have also made a OpAmp Calculator which you can
use to calculate Values.