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PGA280: input bias current seems over spec limit ?

Reinhard Grottenthaler
Prodigy 20 points
Part Number: PGA280


We seem to experience increased input bias current with these devices compared to what the datasheet reports as a limit.

Compared to the reference design PGA280EVM in our implementation we have an asymetrical supply of the device:

VSN = VSON = GND

VSOP = 5V (VSCOM = 2.5V)

VSP = 24V

And the inputs INP/INN are terminated to a mid-voltage of 12V

 

Of course this Termination (1M|1M) creates some additional offset via the input bias current.

However we expect this in the range of 1-2mV maximum according to the d/s max. value of 2nA for the input bias.

This seems to be true for roughly 70% of the devices we get.

Now what we get for the remaining 30% are values of 3-8 nA up to 15nA (or mV offset respectively).

The increased bias current seems to be drawn exclusively by the INN negative input and is very much dependant on temperature (very long warm-up curve)

 

Now the question is:

Is there any reason why we should get that much parts out-of-spec. ?

Or is there a dependance imaginable with respect to our implementation ?

Another question would be whether there is a possibility to get screened parts. Maybe somebody knows about a specific distributor that provides that service ?

  • 0
    TI__Guru 68451 points

    Reinhard,

    PGA280 maximum IB of 2nA is specified only up to 105 deg C and at higher temperature IB magnitude becomes greater - see below.

    However, I suspect there is another reason for the increase in the offset voltage beyond what your DC calculation would make you to believe.  The PGA280 uses chopper technology for excellent dc stability over temperature and life of operation. It also avoids 1/f frequency (flicker) noise and therefore enables both high resolution and high repeatability for dc measurements. However, all chopper topologies also generate very short duration (few nano-second in duration) input current spikes resulting from closing/opening of the front-end switches with a magnutude in hundreds of nA.  Since chopper current spikes have opposite polarity at each input terminals (the current spikes go into one terminal and come out of the other), the offset they create will be doubled instead of being cancelled.  Thus, using very large 1M input termination resistors results in increase offset as well as the current spikes on the inverting input being coupled to the load by the feedback network - see attached presentation discussing input bias current commutation and chopper noise. The solution is to lower the value of the input resistors and/or using filtering at the output.

    3073.Chopper Noise.pdf

     

  • 0 in reply to Marek Lis
    Prodigy 20 points
    Hi Marek,
    thanks for the fast response and the information you provided. I have to go through and try to fully understand.
    As the PGA280 has a fully differential output stage i wonder why the feedthrough should not be through both legs.
    If you have some more information that you can share about the priciples/internals of these devices, would be welcome..
    Kind Regards,
    Reinhard