I had a voltage divider which was suppose to drop down a voltage to 3V conn ected 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 th is. 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
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.
-- Regards, Joerg http://www.analogconsultants.com/
if the diode is to Vcc trying to drive the opamp, it would depend on load and driving resistance
-Lasse
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.
Two paths:
There are usually diodes from each input pin to the V+ and V- power rails.
Some opamps have back-to-back diodes between the inputs, to prevent zenering the input transistors. There might be one or two diode drops in each direction, and sometimes some series resistance.
Measure around and try to see where the current is going: into a rail or into the other input pin.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
connected to the input of an OpAmp. And it worked fine. But I noticed, wh en I powered off the OpAmp, that 3V dropped down to 1.7V, I wasn't expectin g this. What do you think would cause this? My guess is some input protect ion circuitry, maybe a couple diode drops or something?
Sorry, it's an AD8628
V 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 expect ing this. What do you think would cause this? My guess is some input prote ction circuitry, maybe a couple diode drops or something?
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
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