AD9283-100 reference voltage

Hi! 

I am using AD9283-100 operating at 50MHz. Ian using internal reference. I am always getting output code as all zeros. When I monitored the Vref at Pin 2 and Pin 3 shorted to ground I am getting 1.82V instead of specified 1.25V. what could be the reason.

  • Hi Satish,

    Internal Reference mode is enabled by strapping VREF OUT (pin 2) to VREF IN (pin 3) as described in the Voltage Reference section on datasheet pg 8, and shown in the eval brd schematic fig 13. The VREF OUT (pin 2) also needs to be bypassed with a 0.1uF capacitor to Ground.

    Please clarify your comment "When I monitored the Vref at Pin 2 and Pin 3 shorted to ground I am getting 1.82V instead of specified 1.25V" VREF pin2, or pin3 should never be shorted to Ground. Please post your AD9283 schematic and/or insure it conforms to the example circuit shown in datasheet fig 13.

    Best Regards,

    TonyM

  • Hi, I have shorted 2 &3 and through 0.1uF connected to ground as shown in evaluation board schematic. Input is single ended AC coupled. When I measured voltage on Pin 6 it reads 1.82V which is same as Vref I monitored. But on pin 7 it shows 0v. Common mode voltage of Vref should be there on both the input pins.

  • Hi Satish,

    Thanks for the additional info regarding your measurements. Please note the Analog Input Common Mode voltage is internally biased at a value of ~0.3 x VD, or 0.9V when operating from a nominal 3V supply. Please refer to datasheet figure 2 and the Analog Input Theory of operation on datasheet page 8. The Analog Input Common mode voltage is not directly influenced/biased from the fixed internal 1.25V VREF OUT voltage. The 1.25V VREF controls the Analog input full-scale span which by default is 1.024Vp-p.

    You can think of the Analog Input Common Mode Voltage (~0.9V with a 3V supply) as the fixed midpoint with the Ain signal swinging +/-0.512V (1.024Vp-p) around this CM midpoint in single-ended (SE) Mode. For SE mode you will typically drive the AIN terminal with your 1.024Vp-p AC coupled signal, while leaving the AIN/ floating, or preferably connected through a 0.1uF cap and 25ohm series resistor to Ground. I'm attaching an Ain Voltage vs output coding table to help illustrate.

    Please verify your supplies and decoupling nodes are properly configured. I will be happy to contact you offline via your EZone registration email if you would prefer to share your AD9283 schematic/layout outside of the public forum. Just let me know...

    Best Regards,

    TonyM

    8b_AD9283_ADC_Output_Coding.xls