OP-AMP Negative Feedback Valid Config?

Thread Starter

imzack

Joined Nov 3, 2010
71
Is this a valid Op Amp config?

All my spice simulations seem to work as I intend, but I am wondering if there might be a startup issue; since the input at the Inverting pin, might for a short time period, be outside of the Op Amps operating range.

Another way to say this, is:
When I turn on everything, I don't know what the INV pin will be, until the device reaches steady state (If it ever does)


Can anyone put my mind at ease, or tell me what would happen? I have yet to breadboad this, but will soon.

Thanks


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Thread Starter

imzack

Joined Nov 3, 2010
71
More Details, if needing clarification.


My device is supposed to take a -25V, and use a AD8030/31 Op amp to scale this, and do a voltage offset.

My concern, is since my rails, and the op amp, only support, GND to +5v, if the -25 volts will affect this circuit in any negative way...

Or will the negative feedback kick in fast enough, and drive both input nodes of the op amp to +1VDC (well within the devices operating range)


I hope this might clear up any confusion.
 

crutschow

Joined Mar 14, 2008
24,935
If the -25V is available before the +5V supply then you should add a small Schottky diode from the (-) input to ground to keep the voltage from going significantly below ground.
 
Generally, there is substrate diodes that can blow up if the OP amp is unpowered and the signals are present. Vcc is essentially zero and Vcc+Vd and GND-Vd is still valid. vd being a small diode drop. You can add resistors (e.g. 10K) in series with the inverting and non-inverting inputs, but that does increase noise. Ideally, No current flows into the OP-amp, so a resistor doesn't matter. Well it does in the details.
 

MisterBill2

Joined Jan 23, 2018
6,031
The comments are right! Op amps can be fragile under many conditions. The standard procedure is to have clamp diodes connected so that the inputs do not do more than a diode drop below or above the supply voltages.. That is cheap and easy and usually adequate unless your application is either high impedance of high frequency. If your V3 source is a very low impedance the a series resistor to limit the current should be added. Enough resistance to prevent any damage but low enough to not change the output. 1K ohms is usually OK.
 
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