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applying power to positive rail before the negative one or viceversa

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ricko_uk:
Hi, :)
assuming a the following:
- simple dual supply op-amp circuit with only a single op-amp or single instrumentation amplifier
- no input signal as initial condition (until both positive and negative supplies are switched on)
- very little output load

is there any issue in applying power to one rail first (for example the positive one) then several seconds later apply power to the other rail (the negative one)?

Also the same question for instrumentation amplifiers and specifically the attached one (in case the answer depends on the datasheet)?
If the answer depends on the part used, what do I need to check on the datasheet?

Many thanks :)

schmitt trigger:
My understanding (and please correct me if I am wrong) is that the most negative supply provides the necessary reverse bias to isolate the substrate from everything else.

If you don't have a negative bias, all sorts of parasitic junctions could form between the +supply and the I/O pins.

ricko_uk:
thank you Schmitt Trigger,
do you mean that in dual supply op-amps I should always apply the negative supply first?
Thank you

duak:
ricko, have a look at page 8 of the data sheet - i shows the absolute maximum ratings of the device.  In particular, the input voltages relative to the supply rails.  If a device has a supply sequence requirement it will be spelled out in this section or in the power supply section.  Look also at the input voltage section on page 22.

What you won't want is to allow the negative supply to ever go positive relative to circuit common (or the positive supply to go below the same).   Schmitt trigger points out that the substrate is the most negative voltage on the chip and allows the use of reversed biased diodes to isolate the devices on the chip.  Some parts required a bias voltage before, or at least no later than any of the other supplies.  Dynamic RAM chips and microprocessors with 2 or 3 supplies from the 70's needed this.  I don't know if there was damage if the negative supply came up late, but I'll bet it wouldn't work well if the supply was low.

This device probably has some input circuitry dielectrically isolated from the substrate so their voltages can go outside of the supplies, but this is the exception for analog parts.  Many have protection diodes that clamp the inputs to the supplies.  Older parts have parasitic diodes to the substrate but not usually to the positive supply.

In general, I'd say op-amps should have their supplies come up at the same time.  I could imagine if the negative supply came up first, it could apply a negative voltage to the following circuitry which, if digital, could respond in an unexpected way.

Hope this makes sense,

David Hess:
Even without signal inputs, applying the positive supply before the negative supply, or the reverse, can exceed the input common mode range of the operational amplifiers causing phase inversion.  Or there can be other problems.

In the past when this could be an issue, power supplies might use tracking regulators instead of independent regulators.  Or circuits were added to disable or control outputs until the supply voltages were in operating range; I have bench powder supplies which have exactly this to prevent output glitches when turned on or off.

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