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When choosing a split supply for an op-amp circuit, how do I determine my power supply requirements? For a single supply, this is easily determined by finding the required current - but how is power distributed in split supplies? For example, if the op-amp in question states it draws 100mA - does this mean thatt both the negative and positive power supplies should be capable of sourcing 100mA?

Thanks!

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  • \$\begingroup\$ Even more than adequate power supply is adequate (and dampened) -VDD and +VDD bypassing capacitors. Assume 10nH inductors everywhere; use formula sqrt(L/C) to compute the dampening resistor. \$\endgroup\$ – analogsystemsrf Aug 24 '17 at 16:59
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There is current that flows from the positive supply to the minus supply. Most op-amps do not have a ground pin. Whatever current the op-amp itself requires will thus be supplied by both supplies. Usually this is only part of the whole picture though since the op-amp by itself won't do anything useful.

The load resistors (and resistors associated with the feedback network) are often to ground rather than to a supply, so the current will flow from either the positive supply or the negative supply to ground. If you only drive the op-amp output (say) positive and there is a heavy load to ground, you may not need as much capacity on the negative supply. You won't find the numbers on the op-amp datasheet- you will have to analyze the circuit.

Generally though you will be safer with the same rating on both supplies. Note that SPICE models of op-amps may not bother to model the op-amp supply currents accurately.

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    \$\begingroup\$ It's worth saying that op-amp quiescent currents are usually much less than 100 mA, and even the ability to source/sink 100 mA from the output is taking you into the realm of specialized "high power" op-amps. \$\endgroup\$ – The Photon Aug 24 '17 at 16:18
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    \$\begingroup\$ +1 It has always bothered me, this dual supply/single supply thing. Other than a few exceptions all op-amps are really single supply, the only difference being most work best if said supply is offset around ground. I'd even argue a single supply op-amp is really just a "rail-to-rail" regular op-amp. \$\endgroup\$ – Trevor_G Aug 24 '17 at 16:35
  • \$\begingroup\$ @ThePhoton Right, and/or high speed op-amps which need to work into 50 ohm loads. Even the AD8000 1.5GHz amplifier which can drive 100mA only draws 10 or 15mA quiescent current. \$\endgroup\$ – Spehro Pefhany Aug 24 '17 at 16:52
  • \$\begingroup\$ @Trevor: the 'dual power supply' is a simplified variant for IC op amps. Discrete transistor amps used +24, +12, GND, -12, -24, and tubes had A (filament), B (grid) and C (plate) batteries or power supplies. IC op amps use, instead, internal bias regulators. \$\endgroup\$ – Whit3rd Aug 25 '17 at 8:45
  • \$\begingroup\$ @Whit3rd yup but it is still just a naming convention. As a black box, a 'dual power supply' op-amp it is still only single supply. \$\endgroup\$ – Trevor_G Aug 25 '17 at 15:01

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