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AFE7225: Auxillary DAC output is not proper as per the datasheet.

Pradeep Prabhu G
Intellectual 560 points
Part Number: AFE7225

Hi Team,

We have configured AFE AUX DAC output as per below register settings.

Rint =200ohm

Rext = 240ohm

E register configuration :

Register 0x20A --> 0x2E ---> To enable SPI SDOUT function

Register 0x242 --> 0x03 ---> To enable AUX DAC1 and DAC 2

Register 0x248/0x249    ---> To set DAC output current and voltage

Auxiliary DAC output voltage:

SL NO

Register value
0x248/0x249

DAC output current

DAC output voltage
(Channel 2)

1

0xF

2.5mA

0.218V

2

0xC

3mA

0.259V

3

0xD

3.5mA

0.300V

4

0x2

4mA

0.342V

5

0x3

4.5mA

0.383V

6

0x0

5mA

0.423V

7

0x1

5.5mA

0.465V

8

0x6

6mA

0.503V

9

0x7

6.5mA

0.544V

10

0x4

7mA

0.585V

11

0x5

7.5mA

0.627

But in datasheet mentioned the maximum AUX DAC swing is 0 to 1.5V. But we are not getting this value output in any of the above configuration. Please let us know is there any other setting to get maximum output swing.

regards,

pradeep

  • 0
    Intellectual 560 points

    Hi Team,

    Also please let us know is the any settings to get linear output in auxiliary DAC.

  • 0 in reply to Pradeep Prabhu G
    TI__Mastermind 18095 points

    Are you using the TI EVM or is it your own PCB/circuit design.  If the latter, please provide the schematic.  My initial guess is that the output load is not set-up properly.

    --RJH

  • 0 in reply to RJ Hopper
    Intellectual 560 points

    Hi RJH,

    We are testing in our custom board not TI EVK.

    Please find custom board schematic connection attached here. In attached schematic i have changed R599 and R593 resistor value to 200ohm and tested. and output of the AUX DAC is connected to OPAMP for amplification.

    Please let me know the required output connection and AFE configuration to get full scale output and also let me know possibility of get leaner output in AUX DAC.

    3301.schematic.pdf

  • 0 in reply to Pradeep Prabhu G
    TI__Mastermind 18095 points

    As you have mentioned, registers 0x248/0x249 set the Aux DAC full scale current.  The maximum current is b0101 corresponding to 7.5 mA.  Register 0x242 sets the internal differential load resistor that should be equal to (or close to) the external resistor load.  In your schematics provided the load is 50 ohms which is too low for the desired maximum swing. You mentioned above that the load resistor was changed to 200 ohms which corresponds to the default internal load setting (b0).  With that load, you should be able to achieve 7.5 mA * 200 ohms = 1.5V max.

    -RJH 

  • 0 in reply to RJ Hopper
    Intellectual 560 points

    Hi RJH,

    Below are the settings i have done in hardware and software:

    Hardware wise,  external load resistor changed to 200ohm

     

    software wise, 

    Register 0x20A --> 0x2E ---> To enable SPI SDOUT function

    Register 0x242 --> 0x03 ---> To enable AUX DAC1 and DAC 2 and set internal load resistor as 200 (default)

    Register 0x248 --> 0x05 ---> To set DAC output current 7.5mA

    Register 0x249 --> 0x05 ---> To set DAC output current 7.5mA

    After above configuration, Still i am getting only 0.627V output at AUX DAC pin.

    Please let me know something i missed to get 1.5V swing output??

  • 0 in reply to Pradeep Prabhu G
    TI__Mastermind 18095 points

    Pradeep:

    I think the missing step is actually loading the desired DAC level into the register.  This is done in registers 0x243 and 0x244 (for Aux DACA). The default will be 0.  0xFFF corresponds to a maximum value. Then you have to enable the loaded register data to the Aux DAC.  The procedure is outlined in section 10.10 of the datasheet.  Essentially the address is b0101 followed by b1100 corresponding to a 12-bit address of x5C0.  You can reference the AFE7225 GUI available on ti.com to use as a reference on how the Aux DAC is programmed.

    --RJH