This is a fairly simple question, but I'm afraid the description may be long winded...
I'm using two OAs from an LM324 as a VFO. The circuit has two separate ground paths, decoupled with an RC network. The VFO is grounded to the "dirty" path. My 1/2 V+ supply is grounded to the "clean" path.
Normally I would terminate unused OAs as voltage followers, with the + inputs tied to 1/2 V+. In this case, that would put the return though the "clean" ground path.
Soooo, my question is this: would it be wise to construct another voltage divider (across the "dirty" ground path) just to terminate the unused OAs? That would be my first instinct, but I would like to hear the pros and cons of both solutions. Granted, two resistors and a cap are pretty cheap insurance, but limited board space has forced me to consider a compromise...
I don't have an answer, but have two questions:
Why not use an LM358 or another chip without excess op-amps?
Why would unconnected OAs cause a problem? I suspect people duals as singles and leave the extra OA "hanging".
cheers
Quote from: brett on March 06, 2007, 08:29:30 PM
I don't have an answer, but have two questions:
Why not use an LM358 or another chip without excess op-amps?
I tried a TL072, and it didn't work well in this circuit. It may be fine if I were using a bipolar supply, or it may be BJT vs. FET inputs. I don't know. I do know that the LM324 works. An LM358 would probably be just as good, but I don't have one. I _do_ have about 150 LM324s...
Quote from: brett on March 06, 2007, 08:29:30 PM
Why would unconnected OAs cause a problem? I suspect people duals as singles and leave the extra OA "hanging".
cheers
Maybe I'm just being anal, maybe it's a product of old CMOS design habits. I've heard rumors that not terminating unused OAs in a multiple device package can lead to induced noise in the operational OAs, as well as the potential for increased current consumption. If it wasn't for the fact that when I placed the components for the PCB layout the LM324 was squeezed down into a corner of the board, I would just add the divider (as I drew up the schematic). Unfortunately, adding these three components and their traces will require me to significantly rip and reroute...
Ah well, I guess I was just looking for an experienced opinion that mixing the grounds would be OK for the unused OAs, even though I know that "crossing the beams" of clean and dirty grounds is a no-no in mixed signal circuits.
Quotenot terminating unused OAs in a multiple device package can lead to induced noise in the operational OAs, as well as the potential for increased current consumption.
Cross-talk between the OAs should be very low.
As far as I know, if you leave the output unconnected, the power consumption will be low.
Alternatively, what about +ve to Vbias and -ve to out ?