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OPA836 pseudo latch up

Ian de Vries
Prodigy 30 points

Other Parts Discussed in Thread: OPA836

Hello

I am using two OPA836 op amps in series setup as standard differential amplifiers - 

First amp has 3.3k input resistors on pos and neg inputs and 33k feedback, positive feedback coming from Vcc/2.

Vcc =5V

Differential input for first amp is zero on start up.

The first one drives the second ones positive input - 

Second amp has 330 ohm resistors on pos and neg, and 3.3k feedback, again positive feedback from Vcc/2

I have a problem where these amps seem to go into some sort of pseudo latch up and draw 20-25mA from supply. 

In particular the second amp in the chain. I can reset power draw it by injecting noise directly onto one of the inputs to 1mA, but after a while it switches back to 20mA+

My suspicion is that the maximum differential input voltage spec of 1V may cause this, but I need some more insight - e.g. some questions

Would a differential input voltage of more than 1V cause the amp to draw this kind of current? what are other causes of this symptom? and any other effects I am not aware of?

If this is the case, bringing inputs back together does not seem to reset amp 

If this input spec is exceeded is the amp permanently damaged ?

and lastly I am not sure how the differential input can actaully be higher than 1V in this circuit, other than some transient during startup ?

thanks

Ian

  • 0
    TI__Genius 12631 points

    Hello Ian,

    Can you please send me a schematic of your circuit, along with the stimulus of your input.

    Thanks,

    Samir

  • 0 in reply to Samir Cherian
    Prodigy 30 points

    hi Samir

    please see this patent link and look at figure 4 for circuit and application. I am just testing control part of electronics, with zero voltage/zero power on mosfets/inductors. I have used LT parts in same circuit before with no problems. Was looking for a higher spec / lower cost amp to replace them with.

    http://www.freepatentsonline.com/WO2012109680A2.pdf

    Thanks

    Ian

  • 0
    TI__Genius 12631 points

    Hello Ian,

      Your 1st stage amplifier is in a gain of 10. Lets say you have a differential input of  1V, this would cause the 1st stage diff. amp to go to the positive rail (5V). Now you have a 2.5V differential voltage across 330Ohm (R3in) resistor which draws 7.5mA of current. Bottom line is with a 1 V diff input you have too much gain in the circuit which is railing out both amplifiers.

    In order to debug the circuit, I would suggest disconnecting the 2nd amplifier and also disconnect the 3 capacitors and all the passives to the left of the circuit. Connect the inputs of both R1in resistors to Vs/2. This should cause the output of the 1st amplifier to be 2.5V. First check if this is happening. Also, you can build the circuit up in TINA spice and verify the functioning of the circuit. I encourage you to do this.

    Samir.

  • 0 in reply to Samir Cherian
    Prodigy 30 points

    Hello Samir

    Thanks but tried all of that. My basic question is can you confirm that a differential input of more that 1V on these amps cause a large power supply draw of approx (20mA), and will the amp recover or not once inputs are back to normal.

    thanks

  • 0 in reply to Ian de Vries
    TI__Genius 12631 points

    Ian,

      If the 1V difference is actually on the input pins of the amplifier (as opposed to the input of the difference amplifier), then yes there will be large current draw. Generally the voltage difference between the pins of a high-speed amplifier should not exceed a few mV (few 100uV in the case of precision amplifiers).When the input pins of an opamp separate to the extent that you are seeing the opamp has lost all open loop gain and no longer behaves as an opamp.

       If the difference between the pins is sustained for large periods it could cause permanent damage to the part. Please see the Absolute Maximum Ratings table on page 2 of the datasheet. These are recommendations for Safe operation of the amplifiers. Exceeding it doesnt necessarily mean that they are going to fail catastrophically immediately, but, it will have compromised the integrity of the part for its rated specifications and life time.

    Samir