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DAC2904 output issue

pavanbabut
Prodigy 140 points
Other Parts Discussed in Thread: DAC2904, DAC5672, DAC5672A

Hello,

We designed a DAC board using DAC2904 IC and 3.3V power supply for both analog and digital sections. The DAC is responding to digital inputs from 0x0 to 0x3FFF, but the DAC output range on both channels is stuck between 0.28V (digital input is 0x0) and 0.58V (digital input 0x3FFF).

It would be great if someone can help me figure out what the problem is. Please find the attached schematic of analog section for reference.

5736.DAC2904_QuadCh.pdf

The outputs are configured to be single-ended. 

thanks

 

 

 

  • 0
    TI__Genius 17320 points

    Hi,

    From a quick look, I see that you have left the negative current output open (IOUT1-2 and IOUT2-2). This may be the source of your problem. Notice the single-ended configuration in Figure 23 of the datasheet, where the negative output has an equivalent load compared to the positive output. Are you able to try putting an equivalent load on the negative pin?

    Without a load on the negative pin, when the current sources are "steered" toward the negative side the current has no where to go. This will create a high voltage at the source of the PMOS switches, which may force the positive side switch on by creating a positive source-to-gate voltage. This will cause the "negative" current to be steered out of the positive pin. See figure 19 of the datasheet for the equivalent output circuit.

    Regards,
    Matt Guibord 

  • 0 in reply to Matt Guibord
    Prodigy 140 points

    Thanks Matt, I suspect the same after looking at Fig 23 in the datasheet. But thought I would confirm before trying to add that little resistor in there which is not easy now. But will try and see if that solves the problem.

  • 0 in reply to pavanbabut
    Prodigy 140 points

    We would like to replace DAC2904 chips with DAC5672 and on DAC2904 webpage it states that DAC5672  has the SAME FUNCTIONALITY and PINOUT as of DAC2904.

    Can I just replace my DAC2904 chip with DAC5672 and add another trace to connect Pin 48 to DVDD to use Dual bus mode without changing anything else?

    thanks 

  • 0 in reply to pavanbabut
    TI__Genius 17320 points

    Hi,

    I did a quick scan of the datasheets and it appears that this should work fine. In fact, figure 18 of the datasheet indicates that the mode pin has a 100 kOhm internal pullup resistor. So it looks like the part will work even if pin 48 is left floating.

    Also, use the DAC5672A not the DAC5672. The DAC5672 has strict AVDD to DVDD powerup requirements which were removed in the DAC5672A.

    Also, the DAC5672A cannot tolerate 5-V supplies for the analog and digital power supplies.

    Regards,
    Matt Guibord