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PGA280: Can I configure the front end to bring in single ended signals?

James Mrowca
Intellectual 265 points
Part Number: PGA280
Other Parts Discussed in Thread: TL7660

Hello:

 I wanted to check if I can bring in single ended analog signals into the INxa inputs of the PGA280, or do I need to bring in a differential pair to take full advantage?

If I can bring in single ended on the P line, what should I set the N line to?

I do want differential outputs.

thanks

Jim Mrowca

ex-TIer 1982-1997

  • 0
    TI__Guru 52796 points

    HI Jim,

    The PGA280 inputs are flexible and can accept fully-differential input signals or single-ended signals.

    The input voltage range is delimited by the VSP and VSN input stage supplies, where both the negative (INN) and positive (INP) inputs must remain within the range (VSN)+2.5V and (VSP)-2.5V for linear operation.  For example, if you need to feed a single-ended, unipolar signal referred to ground, the PGA280 will require a VSN negative supply of <-2.5V (or more negative).

    The device is optimized for DC precision (near-zero offset error and gain drift) and the "Small-Signal Gain vs Frequency" response is shown on Figure 17, page 10 of the datasheet.   Looking at this plot, the PGA280 can support input signals with BW to about ~10-kHz -15 -kHz while offering flat frequency response. 

    Thank you and Regards,

    Luis 

  • 0 in reply to Luis Chioye
    TI__Guru 52796 points

    HI Jim,

    Let me know what is the amplitude range of the single-ended signal,  For example, if the INP signal is single-ended ±2.5V bipolar centered around GND, you could leave the INN connected to GND and the PGA280 will produce a ± fully-differential output.  In this case, the input stage supply will need to be powered with a supply of VSN=<-5V so INP stays within the linear range.  Another example, if the INP signal is single ended, unipolar, with a range of +0V to +5V (unipolar above ground, centered at +2.5V) it may make sense to connect INN to +2.5V to use the full dynamic range of the fully-differential output.

    Thank you and Regards, 

    Luis

  • 0 in reply to Luis Chioye
    Intellectual 265 points

    Luis:

     The singe ended input will range from just above 0V to about + 2.5V.  I do have a +1.25V bias (center) signal available.

    NOW I have a question on the BGA280 value for (VSP-VSN)

    I have +5V available, and right now I am planning to use the TL7660 (switched capacitor voltage convertor) in the + & - mode to make +9V and -5V --- so that I can get (VSP-VSN) of 14V > than the minimum 10V.  Will I have problems internally in the BGA280 if I choose this +9/-5 scenario ? Or am I better off trying to boost the +5V to say +12V and feed +12V/GND instead to (VSP-VSN) ?
  • 0
    Intellectual 265 points

    Luis: additionally, if I do not use the INP2/INN2 input pair, what voltage levels should these be tied to -- or should I leave them unconnected?

    thanks

    Jim Mrowca

  • 0 in reply to James Mrowca
    TI__Guru 52796 points

    HI James,

    For the PGA280 voltage supplies, from the voltage range level perspective, using a VSP=+9V and VSN=-5V supplies will work fine, when INP is connected to a single-ended signal 0V to +2.5V, and INN=1.25V.  The PGA280 will amplify (or attenuate) the voltage difference between INP - INN, therefore, keep in mind, the +1.25V reference potential connected on INN pin needs to be low noise, low drift, stable to obtain precision results.

    For high precision applications, we recommend using linear, low noise, well regulated supplies since large ripple on the supplies could affect repeatability and noise in the measurements. The TL7660 will have some amount of ripple depending on the capacitors used, the datasheet provides some equations to estimate ripple based on the capacitor ESR, and equations to find an optimal value of C1 and C2.   Figure 70 of the PGA280 datasheet shows a suggested RC bypass filter (22-Ohm, 470nF) at the VSP/VSN supplies for decoupling as well as Schottky diodes to prevent substrate reversing.  In this case, you may consider using a similar approach, however, you may need to adjust the VSN / VSP supply bypass capacitors per your supply requirements. Another option is to cascade the -5V supply with a negative linear voltage regulator set to a voltage around say -4V to help filter or eliminate ripple on the negative supply.  If you have further questions about the TL7660 capacitor selection, ripple, or would like to explore other low noise supply circuit suggestions, it may be best to submit a new query on the power management forum:  https://e2e.ti.com/support/power-management-group/

    Using VSP=+12V and VSN=GND, while INN=1.25V and INP is in the rage 0V-5V will not work.  Both INN and INP inputs require a headroom of at least 2.5V away from the VSP and VSN supplies, and in this case the INN input is only 1.25V from the negative supply,  and INP can be also as low as 0V with no headroom to the negative supply (VSN=GND).

    A good practice would be to connect the INP2/INN2 unused pins to analog ground via a pull-down ~10k resistor.

    Thank you and Regards,

    Luis