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DXP + DAC8871

Armando Govea
Prodigy 170 points
Other Parts Discussed in Thread: DAC8871, DXP, DAC8871EVM, DAC8734EVM, OPA277, OPA211

Hi.

Is there a way to add DAC8871 to DXP tool?. I mean DAC8871 is not in the supported EVM list. I bought the DAC8871 EVM and need a way to test it, it would be great if I can use the DXP.

Regards

AG

  • 0
    TI__Mastermind 46290 points

    Hello Armando,

    DAC8871 is supported by DXP. You can download it using the "Download DAC Devices" utility as described in the DXP User's Guide.

  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Thank you for your answer Kevin.

    I have been searching for MMB0 motherboard part number (Digikey, Mouser, Arrow, etc), but it was not found. How can I get one?.

    Regards.

  • 0 in reply to Armando Govea
    TI__Mastermind 46290 points

    Armando,

    The MMB0 is not sold as an individual orderable. Typically we offer either an "EVM" or an "EVM-PDK" for evaluation of our devices where an EVM is just the daughter-card and the EVM-PDK contains both the daughter-card and MMB0.

    I am not sure why there is not an EVM-PDK offering for the DAC8871. If the e-mail address you registered with the E2E Community is suitable I will send you an email there to get your shipping information and send you an MMB0.

  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Kevin, thank your soon answer .

    DAC8734EVM-PDK does not exist, this DAC is similar to DAC8871, the only difference, it is four channels. The question is:

    As you know the MMB0 motherboard can be used with the EVMs listed in the DXP web page. If I bought the DAC8734EVM, just download the right firmware to MMB0, and the same MMB0 that you will recommend me for DAC8871EVM, will be useful for DAC8734EVM,is this correct?.

  • 0 in reply to Kevin Duke
    Prodigy 170 points

    Hi.

    I am testing DAC8871 EVM with DXP tool. When using the square wave, it looks like the DAC is not meeting the slew rate parameter, mentioned in the data sheet 40V/us.  Maybe I am missing something, could you help?. Attached are the files that describes the measured waveform, DXP configuration and DAC8871 EVM schematic diagram. The signal was measured in J1 connector, pin 2 and pin 6.

    SlewRate_photos.zip
  • 0 in reply to Armando Govea
    TI__Mastermind 46290 points

    Hello Armando,

    There are a few things in play here.

    A critical detail of all of this is that the bill of materials shows U2 as an OPA277 while the schematic seems to indicate an OPA211. Double-checking on the EVM I found that U2 is populated with an OPA277. Thanks for bringing this to our attention, I'll need to go back and fix this. As it concerns your issue at hand, though...

    The datasheet specification shows 40V/us slew rate for the unbuffered voltage output, VOUT, of the DAC8871. The measurement made at J1.2 is the output of the OPA277 installed at U2. The OPA277 is an excellent dc operational amplifier, but it's slew rate is only 0.8V/us, so the signal you see at J1.2 is limited.

    When you probe J1.6 you're looking at the node that is the OPA277 non-inverting terminal and the Vout pin of the DAC8871. The DAC8871 output at the OPA277 non-inverting terminal is going to attempt to slew at 40V/us but the signal at the inverting terminal is controlled by the OPA277 output and its only going to slew at 0.8V/us. A lot of amplifiers feature back to back diodes to protect the input stage from excessive differential voltages, as illustrated below:

    Since the signal at the non-inverting terminal is moving so much faster than the signal at the inverting terminal, the diodes will eventually forward conduct, shorting the OPA277 output to the DAC8871 input and creating the effect that you've observed. 

    To measure the DAC performance alone you need to disconnect the OPA277 non-inverting input from the DAC8871 output. To improve the performance of the whole system you need to replace the OPA277 with a faster amplifier.