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DAC8551: datasheet inquiry

Jonathan Navor
Genius 9155 points
Part Number: DAC8551

Hi Experts, 

We noticed a possible mistake in the DAC8551 datasheet.

Figure 46 shows 50kR feedback resistors on the opamp, which would create a gain of 2 at the output.

The other figures on the datasheet (as well as the text) do not mention any gain. No 50kR resistors then?

Best regards,

Jonathan

  • +1
    TI__Mastermind 28488 points

    Hi Jonathan,

    The reference input impedance is 125kΩ:

    Based on this figure it looks like the DAC resistor string uses half of that (62kΩ):

    This would require a gain of 2 for the device output range to be 0 to VREF. 

    Figure 46 is internal to the DAC, I've highlighted the components in the full block diagram.

    Best,

    Katlynne Jones

  • 0 in reply to Katlynne Jones
    TI__Genius 9155 points

    Hi Katlynne,

    Thank you. 

    If I rephrase what I understand from your answer,

    The opamp depicted in Fig 46 is internal to the 16-bit DAC block shown in the functional block diagram (paragraph 7.2).

    In this case, we do not have to worry about the 2x gain, as it belongs to the internals of the chip.

    In this case, however, the datasheet is confusing in the sense that the opamp shown in Fig 46 has a Vout and a VFB output,

    Just like the other opamp (par. 7.2) leading the reader to think that those pins are the output pins of the chip.

    Best regards,

    Jonathan

  • 0 in reply to Jonathan Navor
    TI__Mastermind 28488 points

    Hi Jonathan, 

    The op amp isn't internal to the DAC block, but internal to the DAC8551 device. The DAC register, Register string, and op amp blocks in figure 46 all correspond 1:1 with the DAC register, 16-bit DAC, and op amp blocks that I highlighted in the image in my last reply (also the figure from par 7.2). 

    You are correct though that you do not have to worry about the 2x gain because it is internal to the chip. The resistor sting only uses half of the Vref range (0 to Vref/2), so the op amp applies a 2x gain so the actual device output is using the full output range of 0 to Vref. 

    Best,

    Katlynne Jones