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PGA for 24bit ADC

Hello,

I already asked this quiestion in the Precision ADC group, but they gave me the hint to try it here in parallel.
(https://ez.analog.com/data_converters/precision_adcs/f/q-a/121547/pga-for-ad7768-1/367471#367471)

I have to measure +/-10V single ended signals up to 200kHz with the AD7768-1 (24bit @ 512ksps, 16bit @ 1Msps).

Now, my customer asks for a PGA with gain = 1, 2 and 4.

Which signal conditoning solution do you suggest that maintains the noise and distortion levels of the AD7768?

I am looking forward to your answers.

Regards, Niels



typo correction
[edited by: Niels at 7:45 AM (GMT 0) on 18 Mar 2020]
  • Hi Niels,

    ADA4945-1, or other devices shown in AD7768-1 datasheet Table 25, would seem most capable of achieving the distortion / SNR specs to not impact the AD7768-1 performance:

    ADA4945-1 Specs:

    Low harmonic distortion
    −133 dBc HD2 and −140 dBc HD3 at 1 kHz
    −133 dBc HD2 and −116 dBc HD3 at 100 kHz

    To add Programmable Gain Amplifier (PGA) capability on the analog input, one crude possibility would be to use analog switches / relays to switch the resistive attenuation or the external resistor ratios of the ADC driver stage. Even then, the added parasitics of the relay / traces might affect your performance?

    I'm thinking with the performance level that you need from your driver, you may not have many choice of devices that can do the gain select values you need as well. At least I could not locate any such ADC driver.

    Regards,

    Hooman

  • Hi Niels,

    I agree with Hooman that ADA4945-1 is your best bet for driving AD7768-1 while preserving THD and SNR. I will be reaching out to you via email shortly with more details.

    Best regards,

    Catherine

  • Hi Catherine and Niels,

    FYI>>

    ADA4896-2 datasheet has a PGA that alleviates some of the concerns with gain error associated with analog switches, as shown below:

    This circuit achieves gain of +4 and +2 depending on the switch position.

    Regards,

    Hooman

  • In addition, if the signal chain before the driver is fully differential then an very high common mode rejection differential amplifier is an LT5401 with the ADA4945-1.

  • Thank you for your valuable input.
    Unfortunately, my signal comes from a source with unknown output impedance. Thus, I am in need for an input stage with high input impedance above the mega-ohm range.

    Do you have any suggestions?

    Regards, Niels

  • Hello Niels.

    For an ultra-high input impedance use a FET input op amp like an ADA4625-1.

    For a single-ended signal use the ADA4625-1 as non-inverting gain stage or for

    a differential signal as an instrumentation amplifier with two FET op amps (ADA4625-2).

  • Hello Philip,

    thanks for this suggestion. The ADA4625 looks great.
    Do you have more information about THD in the range between 20kHz and 200kHz?
    I wonder why the diagram from the data sheet stops at 20kHz:

    Regards, Niels