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PGA280 Power Supplies

Charles Ewing
Prodigy 160 points
Other Parts Discussed in Thread: PGA280, ADS1259

Question: Can the PGA280 be powered with +/- 5v to the high voltage supply pins (VSP = +5v, VSN = -5v), and the same +5v to the output supply pins (VSOP = +5v, VSON = 0v)?

The application is digitizing negative-going signals in the 0 to -2.5 volt range; the PGA280 would be followed by the ADS1259, whose analog section would be powered with the same +5v, 0v as the output stage of the PGA280.  VOCM on the PGA280 would be taken from the 2.5v reference output of the ADS1259.

It would seem that the part should work this way but the condition listed in the "power supply: output stage" section on p. 5 of the specifications ("VSP - 1.5v >= VSOP") appears to contradict it.

Thanks for any assistance.

  • 0
    TI__Mastermind 27715 points

    Hello Charles,

    You are correct in that the specs table requires VSP to be at least 1.5V above VSOP, but I'll test this to be sure. I should have a confirmation either way for you by tomorrow.

    Best regards,

    Ian Williams
    Linear Applications Engineer
    Precision Analog 

  • 0 in reply to Ian Williams
    TI__Mastermind 27715 points

    Hello Charles,

    I just tested this condition (VSP = VSOP = VDD = 5V, VSN = -5V), and the PGA280 worked well. However, this is an informal test done only on one device so I can't guarantee the behavior of all devices. I suspect that the condition of (VSP - 1.5V)  >= VSOP in the data sheet is there for a reason, so I will check with the designer to find out more information.

    Also, keep in mind that the input voltage range of the PGA280 is from (VSN + 2.5V) to (VSP - 2.5V). Therefore, in your power supply condition the input voltage range is ±2.5V. If your input signal goes below -2.5V, the input stage will saturate and you will see distortion at the output.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 160 points

    Thank you, Ian.  I appreciate your checking with the designer; I don't want a marginal design.

    For what it's worth, I reproduce the text from the spec sheet ("Power Supply" section on p. 25) which appears to support the power supply scheme we'd like to use:

    The low-voltage analog output supply, VSOP and VSON, can operate within the high-voltage supply boundaries with two minimal limitations:

    1.  The usable range for VSON is from a minimum 5v below VSP to as low as VSN.  This 5v provides the headroom for the output supply voltage of +2.7v to +5v.  Even with less than 5v supply, this voltage difference is required for proper operation.

    2.  The common-mode control input, VOCM, requires a voltage at least 2v from VSP, in order to support internal rail-to-rail performance.

    Using  +/- 5v for VSP and VSN, and +5v and ground for VSOP and VSON, and 2.5v for VOCM seems to conform to these guidelines.

    Regards,

    Charles Ewing

  • 0 in reply to Charles Ewing
    TI__Mastermind 27715 points

    Hello Charles,

    Thank you for your patience. I have discussed this question with the design team over the past few days.

    The short answer is that the 1.5V headroom between VSP and VSOP is required. The reason is that the input stage of the output amplifier is fed with current sources powered by VSP, and the chopping circuitry also derives part of its power from VSP. Because of the way these subcircuits are designed, setting VSP = VSOP would cause performance to suffer, especially over temperature. This would manifest in the form of additional switching noise at the output as well as instability issues.

    I’m sorry that a simpler power scheme is not possible. For the most robust design, you must use a third analog power supply such that VSP >= VSOP + 1.5V.

    Best regards,

    Ian Williams

  • 0 in reply to Ian Williams
    Prodigy 160 points

    Hello Ian,

    That's not what I was hoping to hear - but better to know so we can design the circuit properly.

    Thank you for looking into this for us.

    Regards,

    Charles Ewing