Should spare op-amp connections be left floating or tied to ground? - Page 1
Forum section: electronics / beginners

author: Marmotta
author_rank: Contributor
timestamp: May 17, 2022, 11:08:04 pm
content: I'm working on a circuit that uses a 2 channel op-amp, but as the part is out of stock at the moment, I was going to amend it using a 4 channel op-amp, with two channels going unused, but am unsure what to do with the unused pins. The part is a TI OPA1678 and I can't see any specific mentions either wayhttps://www.ti.com/lit/ds/symlink/opa1678.pdf?HQS=dis-dk-null-digikeymode-dsf-pf-null-wwe&ts=1652355647548&ref_url=https%253A%252F%252Fwww.ti.com%252Fgeneral%252Fdocs%252Fsuppproductinfo.tsp%253FdistId%253D10%2526gotoUrl%253Dhttps%253A%252F%252Fwww.ti.com%252Flit%252Fgpn%252Fopa1678

author: ledtester
author_rank: Super Contributor
timestamp: May 17, 2022, 11:10:40 pm
content: TI's advice:https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-doAlso:https://www.ti.com/lit/pdf/sboa204Basic advice is to use a voltage follower configuration with the non-inverting input tied to an intermediate voltage within the input common-mode range.

author: floobydust
author_rank: Super Contributor
timestamp: May 18, 2022, 12:13:58 am
content: OPA1678common-mode voltage is spec'd at +0.5V above V- to -2V below V+.So that looks like in the middleish and I would use a resistive divider or shared Vref connection.

author: free_electron
author_rank: Super Contributor
timestamp: May 18, 2022, 03:25:45 am
content: tie the inputs in such a way that the opamp output is set mid-point.- no floating , that's asking for an oscillator- no tying both inputs to same level , that is asking for an oscillator. the input offset voltage will create misery.- no opamp in unity gain . some opamps do not like this and they will ... you guessed it : oscillate- beware of current feedback opamps !. never tie output to inputs ! there needs to be a resistor.- no hard tying to vcc / ground. that saturates the output drivers and increases power consumption. you may even exceed differential input levels ( not all opamps can have their inputs go against the rails ! )

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2022, 08:31:30 am
content: Quote from: Marmotta on May 17, 2022, 11:08:04 pmThe OPA167x is unity gain stable, so it can be configured for unity gain, with the input set to a fixed voltage, within the common mode range.Please post a schematic.

author: magic
author_rank: Super Contributor
timestamp: May 18, 2022, 08:32:43 am
content: Quote from: free_electron on May 18, 2022, 03:25:45 amVery few of them and see above.Quote from: free_electron on May 18, 2022, 03:25:45 amSome jellybeans can actually be "shut down", with reduced power consumption, by reverse biasing both input transistors. The output stage goes to one rail (VCC likely) and doesn't consume much power unless it's loaded.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 18, 2022, 10:11:06 am
content: Connect spare opams as buffers and tie the input to ground. I usually tie the input to ground with a reistor. A spare buffer is always handy.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2022, 12:00:32 pm
content: Quote from: Terry Bites on May 18, 2022, 10:11:06 amThat's good advice, with the proviso that ground is within the common mode range of the op-amp.

author: tszaboo
author_rank: Super Contributor
timestamp: May 18, 2022, 12:03:23 pm
content: IF you are using it for audio, then parallel them up, and the performance of it will be bettererer.

author: Marmotta
author_rank: Contributor
timestamp: May 18, 2022, 12:43:05 pm
content: Thanks for the replies, should have mentioned to circuit is running on 5V. This is the schematic that I have so far based on the information given...

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2022, 12:52:21 pm
content: Oh dear, I can't make sense of that badly drawn schematic. It would make more sense if you used the proper op-amp symbol, rather than putting them all in a box.Here's are some examples showing the correct way to draw op-amp circuits.Quote from: Marmotta on May 18, 2022, 12:43:05 pmConnect the unused op-amps as unity gain buffers, with the inputs going to half the power supply voltage.

author: Marmotta
author_rank: Contributor
timestamp: May 18, 2022, 03:25:10 pm
content: Most of it is the setup I've used with the 2-channel OPA1678, but moved over to the OPA1679.The only part that's really relevant is the marked area here:Following the informationin this link, I tied the positive inputs of the spare channels (C and D) to the reference voltage (5V) and connected the negative inputs to their respective outputs.

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2022, 04:49:28 pm
content: Quote from: Marmotta on May 18, 2022, 03:25:10 pmThe input voltage range for the OPA167x is 0.5V above the negative rail and 2V below the positive rail, so it's a bad idea to connect the input to 0V, or 5V. I recommend connecting it to half the power supply voltage, which can be found on R18.And I repeat: please learn how to use that CAD package to use the standard op-amp symbol, I showed in m previous schematic. Any decent program allows op-amps to be drawn as a triangle, with the appropriate pins for the IC you're using. You might be able to find a specific library, or modify an existing one.