AD8253 excessive bias current

Using AD8253 in classic scope-type DAQ front end. Both inputs have 1 Mohm to ground. In an AC-coupled mode, the amp Ib develops offset voltage across the input resistor. Data sheet specs indicate 5 nA typical, 50 nA max over temp, further supported by Figures 8, 9, 18 and 19. I am seeing consistent 30-50 nA = 30-50 mV on tens of samples with all my other circuitry isolated. Any hints?

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    •  Analog Employees 
    on Mar 16, 2017 6:58 PM

    Hi Gary,

    For the AD8253, I would expect the distribution for Input Bias current to be the same for all customers.  Taking a look at your circuit, other than leaking from the diodes, or potentially leaving both AD8253s switched in, I suspect you are up against the input bias current for the part, with perhaps, another couple of nA due to input impedance.  One additional factor is that  the part is specified at +/-15V supplies, so by using +/-7V supplies, the inputs at 0V common mode are effectively closer to the rails giving another potential several nA as shown in figure 18 in the datasheet.

    Despite the parts being close to the edge of the distribution at room temperature, I would still expect the parts to maintain specifications of 60nA over the specified temperature range under datasheet conditions.

    It seems that when your are observing this effect, it is during an AC coupled measurement.  What is the target DC offset accuracy during this measurement?  

    Regards,

    Paul

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  • 0
    •  Analog Employees 
    on Mar 16, 2017 6:58 PM

    Hi Gary,

    For the AD8253, I would expect the distribution for Input Bias current to be the same for all customers.  Taking a look at your circuit, other than leaking from the diodes, or potentially leaving both AD8253s switched in, I suspect you are up against the input bias current for the part, with perhaps, another couple of nA due to input impedance.  One additional factor is that  the part is specified at +/-15V supplies, so by using +/-7V supplies, the inputs at 0V common mode are effectively closer to the rails giving another potential several nA as shown in figure 18 in the datasheet.

    Despite the parts being close to the edge of the distribution at room temperature, I would still expect the parts to maintain specifications of 60nA over the specified temperature range under datasheet conditions.

    It seems that when your are observing this effect, it is during an AC coupled measurement.  What is the target DC offset accuracy during this measurement?  

    Regards,

    Paul

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