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DAC8501 output error and FB line

MINET Julien
Prodigy 100 points
Other Parts Discussed in Thread: DAC8501

Hi,

 I've a board with one DAC8501. There is a basic R/C filter at the output (1ko / 100nF).The FB line of the internal OPAMP of the DAC8501 is connected to the filter output.

 The reference voltage is 4.096V, power suply is 5V.

 Without anything connected at the output, just a high impedance voltmeter (1GO) and 0x4000 for DAC value, the voltmeter indicate :

1.020 Volt => Error :4mV... 

Another problem is the DC current in the filter resistor about 3mV/1k = 3uA....

 FB current input pin ? I can't find any infomation about FB current input in the documentation...

 Any ideas ?

 thanks,

 Julien.

  • 0
    TI__Mastermind 43915 points

    Hi Julien,

    Assuming you have everything connected and set up correctly, a few mV error could certainly come from the stability and filtering of your reference signal. It sounds like you have the RC filter inside of the loop between the internal op amp output and the Rfb resistor. If this is the case, try removing the 1k resistor and replace it with a 0 ohm resistor to see if it helps. Could you attach a drawing or schematic of your setup? I am a little confused on your description of your configuration and a picture will make it a lot clearer.

    Thanks,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 100 points

    Here is the schematic...

  • 0 in reply to MINET Julien
    TI__Mastermind 43915 points

    Julien,

    If you look at the DAC8501 data sheet you will see that the maximum capacitive load what the internal op amp can drive is 470pF typical. In your circuit, you are trying to drive 100nF capacitive load by keeping the 1K resistor inside of the loop. In this case, the internal op amp is more than likely are having stability problems which appear as a 3mV drop when you measure it with a meter. If you move the 1K resistor outside of the feedback loop you will isolate the 100nF capacitor from the op amp output and should not have any problems. Additionally, if you move the resistor outside of the loop, you will not have to worry about a voltage drop across the resistor as you are immediately connected to an external op amp drawing minimal current.

    Regards,

    Tony Calabria

  • 0 in reply to Tony Calabria
    Prodigy 100 points

    Thank you very much. I will try to connect pin 3 and 4 before the resistor and i will tell you the result next week....

    Regards,

    Julien.

  • 0 in reply to Tony Calabria
    Prodigy 100 points

    Here is news about the DAC8501 :

    With the original schematic internal op amp is stable with 1k / 100nF....

    I'm sure that the FB pin is not high impedance input (about 100Ko).

    I moved the 1k resistor outside of the feedback loop by connecting pin 3 and 4, and now the DC ouput error is constant for all DAC values : about 4mV. This is ok for my application.

    Thanks,

    Julien.

  • 0 in reply to MINET Julien
    TI__Mastermind 43915 points

    Julian,

    Thanks for the update. You can also add a 2.2uF - 10uF capacitor to the Vref pin (place close to device in layout) and that will help as well. The 4mV error is probably coming from either reference drooping or reference initial accuracy.

    Regards,

    Tony Calabria