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Original question:

DAC1220: Interfacing DAC1220 with Teensy 3.5 (MCU with 3.3V supply rails)

DAC1220: DAC1220: Consistency problem between boards

Ahmet Inan
Intellectual 450 points
Part Number: DAC1220

Dear TI Technical Support Team,

I design a mini board that only has DAC1220 IC on it. The board is two layer (Bottom is solid GND plane), crystal is 2.5MHz (ECS-25-S-1X ,12pF load cap), , voltage reference is 2.5V (MCP1501). There is not anything else on the board. I have put guard around 10nF and 3.3nF caps and connected it to 2.5V reference. (Attached schematic and board layout picture) I used DAC1220 EVM app note to design it. (Lets call this board BOARD 1)

Previous, I also used DAC1220 IC. The board I used it was also two layer, same clock and etc. But it has many components on it (voltage regulator, ADC, MCU, analog switches etc.) Because the board had may IC on it, I was not able to make routing only on top layer and used bottom layer (GND plane) for some routing, so GND plane had tracks on it and was not solid plane.   (Lets call this board BOARD 2)

I removed all the IC from the BOARD 2, and tested it with my new mcu board (Teensy 3.5) which has 3.3V digital logic levels. I was powering DAC AVDD and DVDD from same power source, which was around 5.04V. (from on board 5V power output of Teensy 3.5, which is actually USB voltage ) The results were somehow satisfying, I was able to adjuct DAC output with around 7mV deviation. (for example, for 1V expected out, DAC was outputing 993 or sth similiar)

I thought the 7mV deviation is due to poor layout, so I designed the Board 1. I also tested it with Teensy 3.5 and also I powered it as I did with Board 2. But this time for Board 1 I am having too much deviation (at least 60mV from expected volt output and always changing). For previous board, the deviation was not that much. When I test it with Board 2, the deviation is 7mV range. 

I am trying to achieve 1mV deviation.  I donot know why this deviation is happening.

Could you please help me to understand the possible causes and solve the problem?

Thank you for the help.

Regards,

Ahmet

  • 0
    Intellectual 450 points

    Hello again,

    I guess I solved problem, it was related to cap on reference voltage output, it was interferening somehow to DAC and the results were inconsistent. Right now, I am able to adjust output within 1mV deviation, which was the thing I wanted. 

    Right now the circuit is working. My last question is whether using same source can have any bad effects? Or is it better to use seperate Avdd and Dvdd for long term stability?

    Thank you for the help

    Regards,

    Ahmet

  • 0 in reply to Ahmet Inan
    TI__Mastermind 19086 points

    Hi,

    Glad that you figured out the issue. I suggest having a proper filtered supply for both AVDD and DVDD to get the best out of these 20bit part.

    USB supply will be noisy, that's the reason your outputs are varying by 1mV or so. I suggest having separate AVDD and DVDD for better performance, if its the same voltage and different source, connect a Schottky diode between these two supplies as recommended in datasheets for proper ramping up.

    Also the capacitors on the Vref pin and C2 should be of high quality ( normal X7R will not give you better performance ).

    Regards,

    AK