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PGA281: Input bias current at the gain setting extremes?

Chris hl93
Prodigy 160 points
Part Number: PGA281
Other Parts Discussed in Thread: PGA280

Hello,

I am doing an analysis that is suggesting that in a large sample set of PGA281's tested, some have more input bias current than others.  Bias current also varies by the gain setting of each IC.  This can result in final voltage settling times related to the R's and C's (i.e. RFI rectification filter) connected to the inputs that are sometimes very long and at other times not nearly as long.  I understand that differences from one IC to another are expected, however, I would like to get a better understanding of what might be expected for bias current beyond the G = x1 to G=x128 ranges shown on Figure18 of the DS.

Specifically, I would like to know the following:

  1. How does the bias current at the x0.344 gain compare to the bias current of of the x2 gain (~280 pA)?
  2. What should be expected for bias current at the x176 gain?
  3. Is it safe to assume that the bias current for gains between square boxes on Fig 18 (i.e. the x88 gain which lies between x64 and x128) is approximated by the linear interpolation of those points?

Thanks,

Chris

  • 0
    TI__Guru 55466 points

    HI Chris,

    Regarding the input bias questions:

    -What should be expected for bias current at the x176 gain?

    The PGA280 consists of an input stage amplifier and an output stage amplifier.  The input stage offers binary weighted gains of  x1/8, x1/4, x1/2, x1, x2, x4, x8, x16, x32, x64, x128.  The output stage offers Gain of 1V/V and 1.375 V/V.  The gain of x176 is accomplished by setting the input stage on a gain of x128 and the output stage on a gain of 1.375 V/V; resulting in an overall gain of x176.

    Since the input bias current is a function of the input stage, the input bias current of x176 is the same as the bias current of gain x128.

    - Is it safe to assume that the bias current for gains between square boxes on Fig 18 (i.e. the x88 gain which lies between x64 and x128) is approximated by the linear interpolation of those points?

    The gain of x88 is accomplished by setting the input stage to gain of x64 and output stage on gain of 1.375V/V; therefore the bias current is the same as gain of x64. 

    - How does the bias current at the x0.344 gain compare to the bias current of of the x2 gain (~280 pA)?

    The gain of x0.344 will have the same bias current as gain of x0.25.  I believe the input bias current for attenuation is about the same as the gain of 1x; but I will need to verify the bias current for this setting.

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Guru 55466 points

    HI Chris,

    The PGA281 input stage is a chopper stabilized input stage.  Confirmed in the design documentation, when setting the amplifier with all attenuation gains as well as the input stage gains x1, x2  and x4, the input stage works as a buffer with the same feedback impedance. Therefore, the input bias current for all attenuation gains and also gains 1x - 4x, the input bias current is expected to be approximately the same.  Looking at  Figure 18, the measured input bias current is approximately at the same level around ~200pA to ~250pA range for gains 1x to 4x.

    The PGA281 has internal clamp circuit that protects the precision input amplifier stage from large voltages between the inputs caused by a fast signal slew rate in the input.  

    One condition where the PGA281 input current could be high is when the slew-rate of the amplifier is exceeded.  For example, if a multiplexer is connected at the inputs of the PGA281, when channels are switched, a fast input step slew rate signal could be seen at the input of the PGA281, and this input signal could exceed the slew rate of the PGA281.  In this case, the input clamp turns on when the input signal slew is greater than the +/-1 V/uS slew rate of the amplifier.  In this overload condition, the amplifier clamp will conduct and this could result on long settling delays on the passive input filters.

    Please refer to the datasheet 'Input Clamp Conduction' section on p19 and Figure 44 of the datasheet for a detailed discussion.

    A similar device to the PGA281 is the PGA280.  The PGA280 has a very similar amplifier core and adds a multiplexer.  It also adds programmable current buffers for the inputs. On the PGA280, these current buffers are only turned on during a pre-programmed period of time, during the switch channel (or slew condition) to drive the input stage/clamp during the slew condition, and then are turned off shortly after the switching channels to perform the precision sensor measurement with the precision amplifier stage.  

    Thank you and Regards,

    Luis

         

  • 0 in reply to Luis Chioye
    Prodigy 160 points

    Ok, thanks!

    Chris