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OPA3S2859-EP: Input Current Noise Specification?

Collin Wells
Guru 62965 points
Part Number: OPA3S2859-EP
Other Parts Discussed in Thread: OPA3S2859, OPA859

Hello,

What is the input current-noise specification for the OPA3S2859?  We were unable to locate it in the datasheet and want to understand it's contribution to the overall system noise based on the source impedance relative to the voltage noise.

  • 0
    TI__Genius 11675 points

    Hi Collin,

    As with most CMOS input devices, we do not collect data in lab on current noise as the value is in the femtoamps. Please allow me some time to look into this and see if we can find any sim data relating to this spec.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Genius 11675 points

    Hi Collin,

    I spoke with the designer for this part, and he informed me that the reason they did not do current noise measurements was because the external resistor and switch noise would have overwhelmed the devices' current noise. However, the current noise performance of the device itself is highlighted in the OPA859 datasheet, which can be found here. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Guru 62965 points

    Thank you Ignacio.

    Is there guidance on how to include the current-noise contribution of the OPA3S2859 into a total noise-analysis calculation for the design?  Is it expected to be 2x, 3x, 50%, larger than the OPA859? Any extra guidance on how to estimate total circuit output noise including current and voltage contributions will be helpful. 

  • 0 in reply to Collin Wells
    TI__Genius 11675 points

    Hi Collin,

    I attached a couple references that highlight the noise calculations and can be found below. The current noise of the OPA3S2859 is expected to be very similar to that of the OPA859 if not exactly the same. The overall noise sources will be dominated by the chosen feedback resistor and the voltage noise since this device is a FET input device. 

    The first reference gives a more in-depth approach of how to get total noise and can be found here. Reference two highlights input referred current noise in one of its sections and can be found here.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Guru 62965 points

    Thanks - that makes more sense.

    I have a few other questions on the device.

    1.) Do you have a reference to better understand the DC error impacts of the internal switch resistances? 

    2.) Would you confirm that the internal switch resistances do not really impact the AC transfer functions? 

    3.) Do we have minimum and maximum RF values for each switch resistance? 

  • 0 in reply to Collin Wells
    TI__Genius 11675 points

    Hi Collin,

    We do not have references regarding this topic however the internal switch resistances should have minimal DC error impact to the design as the switch is not in the feedback loop of the design, therefore maintaining the appropriate gain. The designer was able to confirm that the switches do not impact the ac transfer function. As for the minimum and maximum Rf values, we did not characterize the device for each switch at various Rf values. However, the switches are optimized to work with 100k,10k, and 1k resistors highlighted in the datasheet but varying 25% percent from these values for the given switch should not have any major impact although performance degradation should be expected. 

    Best Regards,

    Ignacio