XMEGA32A4 / DAC's external reference source at PB0

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Hi, I am having a problem that voltage, applied to PB0 (reference input) is not stable for the integrated D/A converter. 1K ohm VR, which is biased by 3.3V creates the voltage and if I disconnect PB0 from VR, VR's voltage, of course becomes stable. I set PB0 pin as input just in case, but the same result. It sometimes becomes stable. There are not any components between VR and PB0 and both are very close. Does anyone know what is going on? I originally thought the internal circuit of PB0 was broken, so I changed the Xmega, but got the same problem. I also changed VR, but same problem I got ...

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How is it not stable? Oscillations? Dips? Spikes?

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I use the same microcontroller in the same configuration. PB0 is connected to Vref 1,25V. DAC is very stable. Did you place capacitor between PB0 and ground?

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Hi, everybody
Thank you for your responses. Yes, I had the caps at both PB0 and AVCC pins. However, I encountered another problem that PDI programing occasionally stops working. So I looked at PDI pin's pull up resistor of 10K and found that it was disconnected. After fixed it, the voltage fluctuation was stopped. Anyway, I appreciate your supports on this.

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masaavr wrote:
So I looked at PDI pin's pull up resistor of 10K and found that it was disconnected.
I am confused as usual. I've never seen anything about resistors on either PDI pin. Have I been looking in all the wrong places?