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PGA281: PGA281 input noise due to protection R - PGA281 output driving capability

Part Number: PGA281
Other Parts Discussed in Thread: ADS9110, THS4551, OPA625

Hello Team,

  • would 15k resistors on both inputs of the PGA281 influence the noise performance?

Typically chopping input does not interface good with high impedance source but having 15K on both inputs would it help in cancelling the effect out?

  • Would the PGA281  be capable with the 6MHz BW to drive directly an 18bits 2MS/s ADC? (e.g. ADS9110)?
  • Would be a kickback filter on the input be sufficient to enable the PGA281 to drive the ADC?
  • Or would it be a must to have a buffer between the PGA281 and the ADC? (e.g. THS4551, OPA625)



  • Hi Sunset,

    Yes, in any instrumentation amplifier, the input resistors will interact with the current noise density of the instrumentation amplifier and add some level of noise, as well as the thermal noise contributed by the 15kΩ resistors.  If the application requires to add the 15kΩ protection resistors, a recommendation would be to used matched input resistors (0.1%) and add 10x differential and 1x common-mode capacitors to limit noise contribution of the resistors as well as to filter extrinsic noise.

    You are correct that matching the resistors is required to help rejecting both extrinsic and intrinsic noise sources, as well as helping with any small chopper noise residue rejection.  Adding a filter capacitance should help filter high frequency noise as well as too reduce noise contribution of the resistors.  The PGA281 small-signal bandwidth response for this instrumentation amplifier remains flat to about 10-15kHz and depending on gain and starts rolling off at frequencies above 20-kHz. This frequency response is shown on figure 16 page 9 of the PGA281 datasheet.

    The PGA281 TINA model can be used to model the effect on thermal noise and current noise density of the amplifier.  For example, at G=32V/V, adding a 15kΩ+100pF+15kΩ filter with ~53kHz BW increases the input referred noise from 5.2uVRMS @ 50-kHz (with no filter) to 6.7uVRMS @50-kHz (with 15k+100pF+15k input filter).  Below is a TINA noise simulation result and TINA file for your reference.  (Edit 11-5-21, corrected figure)

    PGA281 TINA simulation File with filter:


    The PGA281 has limited capability to drive SAR ADCs; the sampling rate supported will be below 2-MSPS (likely below100-kSPS).  The capability to charge the ADC sample-and-hold and settle within the acquisition time, is not only a function of the gain bandwidth product of the driver amplifier but also the open-loop output impedance characteristic of the device.  The open-loop output impedance of the PGA281 is not shown on the PGA281 datasheet, but we have looked at this in detail for other Customers in years past.  The TINA model on the web does not model the output impedance, however, we do have an output-impedance block model block add-on with the output impedance to estimate the ADC drive capability in TINA SPICE. If you are interested on the PGA281 SAR ADS9110 drive capability, I can look at this detail in simulation, but sampling rate supported will be slower than 2-MSPS, likely below 100kSPS.  Let me know if you would like this simulation.

    The THS4551 and OPA625 are good amplifiers to drive the ADS9110 at 2-MSPS, and these could be used to buffer the PGA281.  I will look at simulating the PGA281+THS4551 (or 2xOPA625) +ADS9110 simulations in detail and update this post hopefully by the end of today or early Monday.

    The sampling rate required is 2-MSPS with the ADS9110. What is the frequency response of the input signal or the signal bandwidth required in this application? I could look into filtering noise at around ~50-kHz configuring the THS4551 or 2x OPA625 on a filter configuration. 

    Thank you and Regards,

    Luis Chioye

  • HI Sunset,

    Below is an example of a THS4551 that can connect directly to the PGA281 differential output to drive the ADS9110 at 2-MSPS. The circuit is configured in a Second Order Low-Pass Filter (Butterworth) with a corner frequency of 50-kHz.  We can adjust the filter per the Customer requirements.

    Below is a TINA simulation result of the THS4551 Filter driving the ADS9110 at 2-MSPS, settling within 1/2-LSB of 18-B resolution (Settling error within ~3uV error).  

    TINA Simulation Result:

    TINA Simulation File:


    We can adjust the circuit per the Customer requirements.  Another option is the OPA625 buffering the PGA281.

    Thank you and Regards,


  • Hello Luis,

    thanks for the details you have shared.


  • Thank you Sunset,

    I will follow up on your query via email.

    Thank you and Regards,