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AFE7225: what is the pullup and pulldown value resistor is required for DAC output pins to achieve full scale swing and 20mA current sink if we connect 5V pullup

Pradeep Prabhu G
Intellectual 560 points
Part Number: AFE7225
Other Parts Discussed in Thread: TRF370333, THS3217

Hi team,

The DAC part of the AFE7225 is a current sink architecture. So a external pull up resistor is required for each DAC output pin.

The voltage divider resistors are to be selected to present the proper impedance that maintains proper current sink current for max voltage swing for DAC output. 

We are setting 20mA DAC current sink by connecting recommended BIAS resistors.

The query is ,What is the pull-up and pull-down resistor is required to achieve maximum output swing in each DAC output channel if we connect 5V pullup to DAC output pins. 

Currently we have used 115R pullup and 634R pulldown as per AFE7225 reference schematic. Is below connection and resistor values are fine to achieve maximum output swing and sink current??

  • 0
    TI__Mastermind 18095 points

    Pradeep:

    Be careful. The reference schematic is proper for interfacing to the TRF370333 modulator.  If that is the intent, then the circuit is good.  There are a couple of additional resistors that impact the AC load so that compliance voltage is kept within range.  Check out this app note that discusses the interface with RF modulators: https://www.ti.com/lit/pdf/slaa399. 

    If the load is high impedance, then you are concerned about presenting the proper load resistance that translates to the proper Vcm and swing.  The load resistance should be 50 ohms and the Vcm set to 3V.  With a 5V supply, you can use (ideally) 71.5 ohm pullup to 5V and 167 ohm pull-down to ground.

    --RJH

  • 0 in reply to RJ Hopper
    Intellectual 560 points

    Hi,

    We are connecting the AFE7225 DAC output directly to THS3217 D2S opamp and given +5V pull-up to DAC output pins. Then in our case what is the value pull-up and pulldown resistors we need to use?

  • 0 in reply to Pradeep Prabhu G
    TI__Mastermind 18095 points

    Treat THS3217 as a high impedance op-amp.  The aforementioned values are good: 71.5 ohm pullup to 5V and 167 ohm pull-down to ground.

    --RJH