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OPA2202: current of input pin of the OP-AMP in two state, chipset on and chipset off

Saeed Zahedinejad
Prodigy 10 points
Part Number: OPA2202
Other Parts Discussed in Thread: OP07, TL331, TL331B, TLV1861, TLV3011B, TLV3401, TLV7011, LM339, LM393

Hi,

I want to use an OP-amp (OPA2202) in comparator mode to compare the voltage of a lithium battery with a fixed voltage. As the circuit always is not ON, I need to know how much the input pin of the OP-AMP consume current in not only in off sate of the OP-AMP but also in ON state of that because if it is too much the battery quickly will be drained.

I have attached a schematic of the circuit. 

Regards,

Saeed

  • 0
    TI__Mastermind 18037 points

    Saeed,

    The OPA2202 cannot be used as a comparator.  Comparators require a differential input range equal to the supply voltages.  Thus, for a device that has a 0V and 5V supply the differential input range should be 0V to 5V.  Reviewing the absolute maximum ratings for this device show that the differential input range is ±0.5V (see below).  Exceeding this differential range can damage the device.  Bruce Trump wrote a nice blog on the subject of using op amps as comparators ( Op Amps used as Comparators—is it okay? ).

    I hope this is helpful to you.

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 10 points

    Hi,

    Thanks for your hint it was really helpful. If I use OP07 instead of OPA2202, dose this circuit work? and is there any difference between Input bias current of (-IN) pin when the OP-AMP Is ON or OFF? And how much?

    Regards,

    Saeed

  • 0 in reply to Saeed Zahedinejad
    TI__Mastermind 18037 points

    Saeed,

    In general it is recommended that you do not use op amps as comparators.  The only exception to this is in cases where you are using an op amp package with multiple amplifier (e.g. a quad op amp, 4 per package), and you only use a subset of the amplifiers in the package (e.g. 3 of 4 used).  Then, if the unused amplifier meets the criteria to function as a comparator you can use it as a cost / PCB area savings.  This is the only case that I agree that using an op amp as a comparator makes sense.  Is this what you are doing?  If you just want a comparator, then I strongly suggest you choose a device designed as a comparator (see Comparator selection tool ).  There are many reasons why op amps don't generally make good comparators (see E2E on op amp as comparator ).

    The OP07 is not a rail to rail device.  In the E2E post referenced above, Paul explains why this is important.  The device does allow for large differential inputs so you will not damage it with a typical comparator input (see abs max).  The device has bias current of ±1.8nA typical and bias offset of ±0.8nA typical.  The bias offset is that tells you the difference between the (-IN) current and (+IN) current.  This current is specified when the op amp is in linear operation (e.g. acting as an amplifier), the current may be higher when the amplifier output is driving to +V or -V (at the supply rail as it would be when used as a comparator).

    Here is a detailed presentation on comparators.  Please review slides 29-33 on op amps as comparators:  3583.Effective Application of Analog Comparators_TechDay.pdf.  Or alternatively, just use a comparator and you don't have to focus on if the amplifier can work in that mode.

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 10 points

    Hi,

    Thanks for the invaluable information. Would you please tell me if I use TL331 or  TL331B,is useful for this design? as well as that in both ON and OFF status of the circuit does the current consumption in (-IN pin ) of TL331 or TL331B is about 4nA?

    Regards,

    Saeed

  • 0 in reply to Saeed Zahedinejad
    Guru 259890 points

    The TL331B's input bias current is typically less than 4 nA, but up to 50 nA in the worst case. If you care, consider using a CMOS comparator instead.

    There are very few comparators that allow to be connected to an input signal when powered off. Two comparators with what the search function calls the "Fail-safe" feature are the TLV3011B and TLV1861.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    I think the both ICs need negative supply, so I have limitation in my design and I need a single supply comparator. Do you think TLV3401IDBVR is OK for this application?

  • 0 in reply to Saeed Zahedinejad
    Guru 259890 points

    All comparators can be used with a single supply.

    The TLV3401 allows up to 5 V above VCC.

  • 0 in reply to Saeed Zahedinejad
    TI__Guru 51586 points

    Hello Saeed,

    See the TLV185x/6x and TLV7011/21 family of comparators.

    These have "fail safe" inputs that are specifically designed to allow voltages to be applied to the inputs, even with the power off, and remain high impedance.

    The TL331/LM393/LM339 family will also have high impedance up to 36V with the power off. With no power, and thus no base current, the inputs are just reverse biased junctions with pA's of leakage.

    Note that most CMOS comparator and op-amp inputs will have the upper ESD clamps, which would clamp the input to the collapsed supply line.

    See the following appnote about the comparator input types:

     Comparator Input Types

    See the summary in table 6.1.