This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

PGA281: Max BW for PGA281 and some advice

Part Number: PGA281

Dear Sir/madam,

  1. Can I know, what is the bandwidth for this IC? What is the maximum frequency that we can input to the PGA281?
  2. Need your advice if following connection is alright?

    2.1) I need a PGA281 to use for a FPGA’s ADC. Below are the schematic based on the PGA281EVM. Please, see schematic below

-          As the PGA281 at the output generate a differential signal around VOCM, we need at the output of the PGA281 max differential voltage = ±0.5V with 0.5V common mode voltage.
-          in this case,  we cannot share the supply for ADC and PGA281 because the FPGA has the reference voltage internal (on-chip reference)

2.2) In another scenario, when DVDD = 2.5V and VSON= -2.5V? what are the thresholds values for low and high logic levels,  how to control the IC in this case?

Best regards,

kpk

  • Hi KPK,

    - The PGA281 is an instrumentation amplifier optimized for high precision DC applications supporting relatively low bandwidth signals.  The small signal bandwidth vs gain is shown on page 9, Figure 16. The gain remains flat up to ~10kHz for all gains, at frequencies above around 20kHz, the small signal gain (or attenuation) is no longer constant and starts to decrease with frequency.    Please see below.

    - Your schematic with VOCM=0.5V,  with common mode voltage=0.5V powered by VSON=GND and VSOP=3.3 will work. However, please be aware that the voltage output swing of the amplifier is limited to about ~40mV (typ) from the rail supplies (100mV max with 2mA load over temperature range). Please see voltage output swing from rail on page 4 of the datasheet.  Therefore the PGA281 max output signal amplitude supported will be slightly smaller around +/- 0.450 or +/-0.4V depending on load, due to the headroom to GND (VON supply).  You could consider shifting VOCM slightly higher, to ~0.6V if you absolutely need the full ±0.5V amplitude.  Otherwise you will have to limit the output amplitude to avoid distortion or clipping.

    - The logic high level and low level thresholds are defined on the datasheet below; where "DVDD" represents the difference between DVDD-VSON; and the absolute voltage needs to referred to VSON:

    Therefore, for the case VSON=-2.5V, and DVDD=+2.5V, the max logic low threshold:  VIL(max)=-2.5 + 0.2(5V) or VIL(max) = -1.5V

    and the min logic high threshold:  VIH (min) =-2.5 + 0.8(5V) or VIH (min) = +1.5V

    If the controller is powered by +3.3V logic, you will require a  voltage level translator between the controller and PGA281 to level shift the logic, one possibility is to use a digital isolator such as ISO776x (or another similar solution)

    Thank you and Regards,

    Luis   

  • hi Luis,

    What do you mean with “VON=GND and VSON=GND”? I think you are referring just to VSON =GND because VON is output signal.

    I’d like to show you the figure below, this represents the use case for the my ADC signals. We can see that the common voltage for input to this ADC can be between 0.25V to 0.75V, at the other hand the input max amplitude on VP can be +/-0.25V and the input max amplitude on VN can be +/-0.25V. So, my differential voltage

    VP-VN = 0.25+VOCM -(-0.25 +VOCM)= 0.5V

    or

    VP-VN= -0.25+VOCM -(0.25 +VOCM) = -0.5V

    It means that the differential output (VP-VN) of the PGA281 would be +/-0.5 V but the max output on just a single line would be 0.25V. What I want to show you is that if we set   VOCM =0.5V for the PGA281 and as the peak of a single signal respect to VOCM is max +/-0.25V as shown in the below figure, it would respect the output swing. Anyway we are able to change VOCM to 0.6V; with the thing I explained is it still required to set VOCM to~0.6V?

    Best Regards,

    kpk

  • Hello KPK,

    Correct, if you require, VOP and VON swing +/- 0.250V (referred to the VOCM common-mode), producing a fully differential output signal of +/-0.5 and output supply VSON=GND, VSOP=3.3V, the minimum VOCM setting required is VOCM > 0.350V to allow 100mV swing headroom to GND. 

    Therefore, for clarification, a setting of VOCM= 0.5V will work fine for VOP/VON swing +/- 0.250V, (referred to VOCM) producing a fully-differential signal of +/-0.5 in this case with a +3.3V output supply (VSON=GND, VSOP=3.3V).

    Thank you and Regards,

    Luis