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PMOD AD2 Reference Voltage problem



I am currently working with the FPGA Board ZYBO Z7020. Part of my current task is: Apply a voltage to the AD2, the FPGA contains some logic and has to output the same Voltage with a DA2. I configured a VHDL I2C Master for communication with the AD2. I also attached an image with how I added the pullup Resistor in the schematic. I am using the internal voltage reference, which equals VCC = 3,3V. I am also watching the SDA Line with an oscilloscope to have a look at the data the ADC sends to me. I can apply an input voltage (DC) to the AD2 with a Waveform Generator. The AD2 works in High Speed mode (1MHz Clock). The configuration I use is: "00010011". The big problem with the SDA Data from the AD2 is: It reaches its maximum at 1,65V Input Voltage, so at 1,65V input voltage, all bits of the 12 bit data are '1'. I just can't figure out, why that's the case. The jumper on the AD2 board is set to "REF", the configuration tells the AD to use this reference and not an external one and the VCC at the PMOD surely delivers 3,3V.

Any idea about what I am doing wrong?

EDIT: I am only using Channel 0 for conversion.


Edited by Alex77

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Hi @Alex77,

Welcome to the Digilent Forums!

We would suggest potentially using 10K OHM's for the Pull up's. Also we would suggest putting a pull up on both SDA and SCL.

It looks like you are using the VCC instead of the VREF although they are the same value based on your configuration byte since you set REF_SEL (D3) to 0, which selects the supply voltage to be used rather than an external reference voltage.

Please attach your output so we can better understand the issue. Is data output below 1.65 V accurate?

When you say 1 MHz high speed are you referring to 1.7 MHz or are you referring to fast mode plus?

best regards,


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