EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Marmotta on May 17, 2022, 11:08:04 pm
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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 way
https://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 (https://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)
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TI's advice:
https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do (https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do)
Also:
https://www.ti.com/lit/pdf/sboa204 (https://www.ti.com/lit/pdf/sboa204)
Basic advice is to use a voltage follower configuration with the non-inverting input tied to an intermediate voltage within the input common-mode range.
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OPA1678 (https://www.ti.com/lit/ds/symlink/opa1678.pdf) common-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.
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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 ! )
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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 way
https://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 (https://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)
The 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.
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- no opamp in unity gain . some opamps do not like this and they will ... you guessed it : oscillate
Very few of them and see above.
- no hard tying to vcc / ground. that saturates the output drivers and increases power consumption
Some 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.
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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.
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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.
That's good advice, with the proviso that ground is within the common mode range of the op-amp.
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IF you are using it for audio, then parallel them up, and the performance of it will be bettererer.
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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...
(https://i.imgur.com/dahD92Hh.jpg)
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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.
(https://www.eevblog.com/forum/beginners/tl072-microphone-preamp-(differential)/?action=dlattach;attach=388679;image)
Thanks for the replies, should have mentioned to circuit is running on 5V.
Connect the unused op-amps as unity gain buffers, with the inputs going to half the power supply voltage.
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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:
(https://i.imgur.com/4dOCQqkh.jpg)
Following the information in this link (https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do), 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.
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Most of it is the setup I've used with the 2-channel OPA1678, but moved over to the OPA1679.
Following the information in this link (https://e2e.ti.com/blogs_/archives/b/thesignal/posts/the-unused-op-amp-what-to-do), 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.
The 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.