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# AD8638 weird behaviour

Hello

I'm testing a simple circuit with the AD8638 Op Amp.

The Amplifier is supplied with 12 VDC single supply and works as a kind of voltage limiter. The limiting voltage should be 9.9 V which is derived from a voltage reference.

The auto zero amplifier is there because i dont want a drift in the limit.

If I test the circuit the limiting function works nicely, but for voltages lower than 9.9 VDC at the input i get extremely bad results. If I disconnect the source (VS3) and pull the 5k1 resistor to GND i get around 8.5 VDC at the inverting input of the op amp.

I could see that this behaviour comes from the rectifier at the output.

When there is no feedback, i can also see that the input is pulled up to about 8.5 VDC.

I couldn't see this in the simulation. There everything works fine.

Question is: how is it even possible that the high impedance input can be pulled up when the feedback is not closed ?

I hope somebody has an explanation for that behaviour.

• I forget to say that the diode used in the real application is a low leakage schottky diode, not the 1n4001 in the picture. The picture is simply to show the principle.

• Hello pmatschek,

Let's look at the circuit first and do some thought experiment. R1 and R2 forms a voltage divider that is 2 to 1 ratio. So, the inverting input of the amplifier will see (2/3)*VS3. Let's say you have 12V at VS3, the max voltage you'll have at the inverting input is 8V, which is less than 9.9V. This will cause the amplifier to rail to positive supply (12V in this case) and reverse bias the diode. When VS3 is slightly larger than  9.9V*(3/2), the inverting input will have higher potential than non-inverting input and cause the amplifier to rail negative. The diode to start conducting and have the output of amplifier at (9.9V-Vdiode).

8.5V is about 2 diodes or 1 MOS threshold voltage away from 9.9V. If you read the "Absolute Maximum Rating" section on the page 5, you'll see in the footnote #2 that there is an internal protection  of 1kOhm and 1.25V vth MOSFET in case there is a large differential between inverting and non-inverting inputs. So, this is a normal behavior for this part.

For most of the input range, the amplifier is in open loop. Read the following articles. These will help you greatly

http://www.analog.com/en/analog-dialogue/articles/amplifiers-as-comparators.html

www.analog.com/MT-084

www.analog.com/AN-849

http://www.analog.com/en/technical-articles/maximum-supply-current-that-wasnt.html

Hope it helps.

Regards,

Kaung

• Hi Kaung

Thanks for the answer. I didnt see that footnote regarding the differential input voltage.

Regards,

Patrick

• What about the ADA4622 ? This one does not seem to have the differential limiting input.

• Hello Patrick,

ADA4622 is a JFET input amplifier and not a zero drift amplifier. I'm not sure if you meant ADA4522 since this is a zero drift amplifier and you mentioned you wanted an auto zero amp.

ADA4522 will have similar behavior as the AD8638 but with a different diode voltage of +/-5V. (Refer to Figure 72 of http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4522-1_4522-2_4522-4.pdf). I do not recommend using ADA4522 if you have to use this type of circuitry.

AD4622 should work but take a look at LT6015 for this type of application. See the links below for an application similar to yours. Over the top (OTT) amplifiers are better suited for this type of application if your input signal can use the input of op amp to go beyond the supply rails. Using OTT amplifiers will protect your downstream parts.

http://www.linear.com/solutions/7569

http://cds.linear.com/docs/en/datasheet/601567ff.pdf

The LT6015 is offered in a 5-lead SOT package. The LT6016 dual op amp is available in an 8-lead MSOP package.

Regards,

Kaung

• Hi Kaung

I really meant the ADA4622.

Thanks for the hint with the LT6015. For my application it is not really necessary that the op amp withstands such high voltages as the inputs are externally protected. The input is clamped to the supply rail with diodes.

But as both amps are in the same package and have the same pinout, I will test both of them and see which one gives the best results.

Thanks for your help

Regard,

Patrick