Could someone please tell me the correct way to operate this circuit from a 0V (GND) to minus 12V supply?
It works fine between 0V to positive 12V.
I suspect the DC offset of the input signal needs to be adjusted, but I am unsure of the method.
Yes, I know it is unconventional.
Many thanks,
Robert Cordell
Ass-u-ming you want the output to sit at about -6V with 0V input, try replacing R3 with 24K and R4 with 33K.
The way it's drawn, it'll rail at almost -12V out.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Don't you want to AC couple the ground referenced source? What is the DC resistance of the source? Many function generators are 50 or 600 ohms and that will throw off your bias.
Also, R3 should equal R4 to bias to the mid point of the rails. R2 will look like 2.5 K to the source through the R1 10k input R and will divide the source by 4. Do you even need the R2 pot?
Weird.. that pot on the front is changing both the gain and the DC offset? (I think, is that what you want?)
Hmm, OK then it should be "fine" if you subtract 12V for all the power rails, which in this case would mean referencing the sine wave from -12V and not from ground. (You may not be able to to that.)
George H.
Watch out for the supply rejection. "Single supply" op amps frequently have very poor rejection of junk on the negative supply rail. Since PSR is measured with respect to ground, that doesn't show up in the datasheets.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Don't worry, move it much and the output will rail one way or the other, so the gain won't usefully change much, less than five percent I'd guess (plus or minus a couple percent as a spec sheet would say).
If you let the pot rotation delta from center be a, then the resistance looking into the wiper (ignoring wiper resistance) of an R ohm pot is:
R*(0.25 - a^2).
...so it's pretty small for a > It works fine between 0V to positive 12V.
Best regards, Spehro Pefhany
I wanted it DC coupled so I have the option to input lower frequencies.
It is 50 Ohms. But the final circuit will probably use an XR2206 function generator chip.
Thank you, I will try this, and what others have mentioned too.
Robert Cordell
Let me guess, when it swings to the low output side, meaning near 0 volts, it gets somewhere in the -1.2 or there abouts, voltage?
In other words, you can't get it to drop to 0 volts or near zero?
with that op-amp, on the Vcc side, there is a darling type configuration output and it makes it kind of hard to reach the rail.
In this case, you're trying to pull the + supply close to the output, there by nullifying the - side and getting near 0 volts.
Try a rail to rail op-amp instead.. That's my beef. Jamie
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