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Can I directly connect enable pin to ground? I am not going to use it. enter image description here

datasheet

Also can I connect all unused pins of LM324 op-amp to ground through single resistor or what configuration does it need?

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Can I directly connect enable pin to ground? I am not going to use it.

Yes, generally it is good practise to tie all unused inputs of digital devices to eiter GND or positive supply voltage.

Also can I connect all unused pins of LM324 op-amp to ground through single resistor?

You can, but that would waste power; instead tie the negative input of each unused opamp to its output (which forms a voltage follower) and the positive inputs of all unused opamps to halfway between positive and negative supply voltages, using a resistor divider (or GND in the case of split supplies). E.g. like this (shown for a single opamp):

enter image description here

For more details about opamp termination, see Properly terminating an unused op amp and What shall we do with the unused opamp .

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    \$\begingroup\$ +1 because of the AD "What should we do with the unused op-amps" tune. \$\endgroup\$ – Vladimir Cravero Jun 15 '14 at 12:21
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Yes, you can connect an input directly to ground. Inputs are high impedance so they're like a big resistor anyway.

As for the op-amp, that depends on the pins you're not using. As a general rule of thumb:

  1. Inputs that are allowed to be the same potential at the same time can be connected directly together.
  2. Never ever directly connect outputs together.

So you can connect your op-amp inputs together and ground them through a resistor (though I'd be inclined to only group the same type of inputs; one resistor for all the inverting, and one for all the non-inverting), but each output must have its own resistor.

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  • \$\begingroup\$ That's interesting, why do you think that different inputs should be grounded through resistors, separately? \$\endgroup\$ – Vladimir Cravero Jun 15 '14 at 12:12
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    \$\begingroup\$ For an op-amp it has little effect, but it could be that you might want to ground some and tie some to Vcc to affect the value of an output so it's not drawing as much current. Just a good habit to get into really, no specific technical reason for this one application. \$\endgroup\$ – Majenko Jun 15 '14 at 12:18
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You can tie the enable pin directly to ground, for the op amp just leave the unused outputs not connected and connect the unused inputs to ground.

The op amp output is (ideally) a voltage generator. If you connect both inputs to ground, or just tie them together, the output should be zero and there should be no issues. The output won't be zero though because of noise, offset, production imperfections or whatever, so you have this output that wants to drive ground, a power rail, and that's bad. It can break, but if it doesn't it dissipates power.

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  • \$\begingroup\$ So I can connect all unused In+ and In- to ground through a single 4.7k? \$\endgroup\$ – a_vasilkov Jun 15 '14 at 12:13
  • \$\begingroup\$ I think you can connect them to ground and that's it. Are you using a dual power supply? \$\endgroup\$ – Vladimir Cravero Jun 15 '14 at 12:18
  • \$\begingroup\$ No. Just the same 5+ and 0 \$\endgroup\$ – a_vasilkov Jun 15 '14 at 12:22
  • \$\begingroup\$ Then fmunket answer (case a) is best. It depends also on what are your main issues, power consumption, possible oscillations... Whatever. \$\endgroup\$ – Vladimir Cravero Jun 15 '14 at 12:23
  • \$\begingroup\$ The unused op amp values are then free to hammer between the rails. This is fairly bad practice \$\endgroup\$ – Scott Seidman Jun 15 '14 at 16:25

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