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ADS9224REVM-PDK: EVM schematic - FD amplifier

user6145812
Prodigy 10 points
Part Number: ADS9224REVM-PDK
Other Parts Discussed in Thread: ADS9224R

Greetings,

I have a question regarding the connection of NC pins on the THS4551IRUNR FD amplifier in the schematic of the evaluation board for the ADS9224R ADC. NC pin no. 2 is connected to the VA rail via 10k resistor, and NC pin no. 8 is connected to the REFP/2 (which is used for setting output common mode voltage) via RC filter.

Is there a reason for this since the datasheet for this amplifier doesn't mention where NC pins should be connected (it only notes that there is no internal connection)?

Can I leave NC pins floating, or connect them to other potential, for example GND?

Best regards,

Domagoj

  • 0
    TI__Guru 57296 points

    Hello Domagoj,

    Welcome to our forum and thank you for your post.

    Pins 2 and 8 on the THS4551IRUN are supposed to be left floating as there is no internal connection. I believe the EVM has provisional connections intended for alternative devices with a different pin out. It will not affect the performance of the FDA to leave the connections as-is, but you can leave them floating in your circuit.

    The output common-mode voltage is actually connected to pin 7. For the ADS9224R, the expected input common-mode is mid-reference (REFP/2). For signals near ADC full-scale, I recommend using the internal feature to increase the REFP/2 common-mode by ~100 mV to improve the linear output swing of the FDA circuit. This feature is enabled by setting bit 0 in register 07h. For more information about the benefits of this feature, please check out the following TI TechNote:

    https://www.ti.com/lit/an/sbaa406/sbaa406.pdf

    Best regards,