Input of unpowered OpAmp

[panfilero]

I had a voltage divider which was suppose to drop down a voltage to 3V connected to the input of an OpAmp. And it worked fine. But I noticed, when Ipowered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expecting this. What do you think would cause this? My guess is some input protection circuitry, maybe a couple diode drops or something?

 

[hamilton]

I had a voltage divider which was suppose to drop down a voltage to 3V connected to the input of an OpAmp. And it worked fine. But I noticed, when I powered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expecting this. What do you think would cause this? My guess is some input protection circuitry, maybe a couple diode drops or something?

Is the Opamp a secret ?

Yes, some OpAmps have protection diodes some do not not.

Yours, who knows.

hamilton

 

[Joerg]

Is the Opamp a secret ?

Yes, some OpAmps have protection diodes some do not not.

Yours, who knows.

There can also be internal BE junctions that are able do this. 1.7V
sound like two BE junctions plus some internal resistance. If a
substrate diode had come on it would be more like 0.6V.

 

[Lasse Langwadt Christensen]

Den torsdag den 19. december 2013 22.32.12 UTC+1 skrev Joerg:

There can also be internal BE junctions that are able do this. 1.7V

sound like two BE junctions plus some internal resistance. If a

substrate diode had come on it would be more like 0.6V.

if the diode is to Vcc trying to drive the opamp, it would depend on load and driving resistance


-Lasse

 

[Maynard A. Philbrook Jr.]

I had a voltage divider which was suppose to drop down a voltage to 3V connected to the input of an OpAmp. And it worked fine. But I noticed, when I powered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expecting this. What do you think would cause this? My guess is some input protection circuitry,

maybe a couple diode drops or something?

Yes, things like that happen..

Depending on the type of Op-AMp you use. Since you didn't post the
PN, I'll assume you are working with bipolar type inputs. These
become diodes in a sense and could leak back to the power rails when
no power is present to counter act the circuit.

Have you thought about using a bifet op-amp? I can't say it'll
fix your problem however, it's worth a try.

Jamie.

 

[panfilero]

Is the Opamp a secret ?



Yes, some OpAmps have protection diodes some do not not.



Yours, who knows.



hamilton

Sorry, it's an AD8628

http://www.analog.com/static/imported-files/data_sheets/AD8628_8629_8630.pdf

 

[Lasse Langwadt Christensen]

Den mandag den 23. december 2013 16.26.11 UTC+1 skrev panfilero:

Sorry, it's an AD8628



http://www.analog.com/static/imported-files/data_sheets/AD8628_8629_8630.pdf

INPUT OVERVOLTAGE PROTECTION
Although the AD8628/AD8629/AD8630 are rail-to-rail input
amplifiers, care should be taken to ensure that the potential
difference between the inputs does not exceed the supply voltage.
Under normal negative feedback operating conditions, the
amplifier corrects its output to ensure that the two inputs are at
the same voltage. However, if either input exceeds either supply
rail by more than 0.3 V, large currents begin to flow through the
ESD protection diodes in the amplifier.
These diodes are connected between the inputs and each supply
rail to protect the input transistors against an electrostatic discharge
event, and they are normally reverse-biased. However, if the input
voltage exceeds the supply voltage, these ESD diodes can become
forward-biased. Without current limiting, excessive amounts
of current could flow through these diodes, causing permanent
damage to the device. If inputs are subject to overvoltage,
appropriate series resistors should be inserted to limit the diode
current to less than 5 mA maximum.

-Lasse