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PGA870: Does the input voltage is limited to ±0.2V around VCM when DC-coupled ?

Shinichi Inoue
Guru 21791 points
Part Number: PGA870


Tool/software:

Dear Specialists,

My customer is considering PGA870 and has questions.

I would be grateful if you could advise.

---

(1) I am considering using the DVGA PGA870 as a front-stage amplifier for an AD converter (ADC3223IRGZR). Can I use it?

There is no reference circuit for DC input, but can I use it without any problems with DC?

Could your please provide a circuit example.

(2) On page 18 of the data sheet, Single-Ended to Differential Operation, it says:

" If a single-ended signal source is to be dc-coupled to the device, its voltage swing should be centered about the midsupply reference, VMID1. If the input dc voltage is greater than 0.2 V from midsupply, then increased distortion and reduced performance can result. The non-driven input pin of the PGA870 should be ac-coupled to ground through a capacitor. In this configuration, the PGA870 amplifies the difference between the dc-coupled input signal and the midsupply reference. "

Does this mean that the input voltage is limited to ±0.2V around VCM when DC-coupled?

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I appreciate your great help in advance.

Best regards,

Shinichi

  • +1
    TI__Mastermind 23669 points

    Hello Inoue-san,

    Please find my answers to your questions here:

    1) The PGA870 can be used as a differential ADC driver; the device is designed for low distortion when driving an ADC input stage.  Please refer to this image below and the discussion starting on page 20 of the PGA870 datasheet for guidance on ADC driving with PGA870:

    2) This statement is referring to the DC voltage of the input signal being within +/- 0.2V of the midsupply reference.  This is only a limitation on the DC signal reference, not the AC components of the signal.  When using 5V single supply operation, the midsupply reference would be 2.5V.  A valid input signal would be a 2Vpk-pk sine wave with a DC offset (or reference at 2.5V; the AC sine signal swings between 1.5V and 3.5V, but the DC component of the signal is within +/-0.2V from the midsupply (2.5V).

    Unless you are driving the PGA870 with a DC (steady-state) signal, the AC component of your input signal is not limited to +/-0.2V from midsupply.  This limitation exists to minimize distortion and keep the DC component of the input signal close to the midsupply reference.  

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Guru 21791 points

    Hi Alec,

    Thank you for your reply.

    (1) I'll ask my customers to refer to the data sheet.

    (2) I understand. I'll share with the customer. 

    Alec Saebeler said:
    This statement is referring to the DC voltage of the input signal being within +/- 0.2V of the midsupply reference.  This is only a limitation on the DC signal reference, not the AC components of the signal.  When using 5V single supply operation, the midsupply reference would be 2.5V.  A valid input signal would be a 2Vpk-pk sine wave with a DC offset (or reference at 2.5V; the AC sine signal swings between 1.5V and 3.5V, but the DC component of the signal is within +/-0.2V from the midsupply (2.5V).

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 21791 points

    Hi Alec,

    The customer has an additional question.

    Could you please advise?

    ---

    Alec Saebeler said:
    2) This statement is referring to the DC voltage of the input signal being within +/- 0.2V of the midsupply reference.  This is only a limitation on the DC signal reference, not the AC components of the signal.  When using 5V single supply operation, the midsupply reference would be 2.5V.  A valid input signal would be a 2Vpk-pk sine wave with a DC offset (or reference at 2.5V; the AC sine signal swings between 1.5V and 3.5V, but the DC component of the signal is within +/-0.2V from the midsupply (2.5V).

    If the midpoint voltage is kept at 2.5V,  it will function normally even if a 2Vpk-pk DC signal is input, for example, 3.5V to the + terminal and 1.5V to the - terminal, is it correct?

    ---

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • +1 in reply to Shinichi Inoue
    TI__Mastermind 23669 points

    Hello Inoue-san,

    Your understanding is correct, the common mode voltage needs to be at or near mid-supply, but the AC or DC signal voltage levels can be as you described above.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Guru 21791 points

    Hi Alec,

    Thank you for your reply.

    I understand.

    I'll share you answer with the customer.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi