. Gain switching the feedback dividers eliminates effects . __ from the CMOS switch's series resistance. . ------|+ \\ . | >-----+----+----+--- . .--|-_/ R1 | | . | | R3 | non-inverting gain . '---o-->o----+ | R5 . o--- | ---+ | . o--- | -- | ---+ . | | | . R2 R4 R6 . | | | . GND --+----+----'
And if Gmin = 2, i.e., R1 => R2, etc., then a +/-5V switch can be used with +/-10V output signals.
-- Thanks, - Win
***END QUOTE***Hi, if you recall from an old thread I had to design a voltage gain switch using solid state parts. I finally made the circuit, and chose the above configuration for the switch which is a 4051BP(stage 2). The first stage is through a discrete in-amp(3 op-amp config, 10k resistors, 200ohm gain select resistor) with a gain of 100. Stage 2 has gains of 1, 5, 100 and 500, and Stage 3 is a follower. All Op-Amps are OP-07.
Before assembly I first tested the ckt in a protoboard with DC as input signal. It worked fine. When I finished the PCB I tested it and the output won`t swing past +-6V. The DC power supply is +-12V (OP-07s), and the 4051BP is powered with 5V1 zenners from positive and negative supply respectibly. Supply power to ICs is checked. Output at Stage 2 Input is aprox. +-10V. Output without 4051BP and manual feedback (ie short the pins) is +-10V. Always DC input signal. I measured with a DVM the drop across the switch when any gain is selected and it equals the output voltage (+-6V). When the output doesn`t saturate the circuit works fine.
I don`t know what's the deal with this, I`m sure I did it on protoboard and it worked fine. Tomorrow I`m gonna assemble it again and see what happens, but I`m posting here because I assume the problem is something else. I`m not sure why the output saturates at 6V, if anything I would expect 5V.
Thanks in advance.