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AD9255 evaluation board output jumps (or missing codes?)

I have an AD9255/105 (running with a 100 MHz onboard clock) evaluation board with the HSC-ADC-EVALC board that I am using to capture exponential decays, which have low frequency components. Since the standard 9255 evaluation board is designed for MHz frequencies, I have modified the optional input path (which has a AD5562 amplifier) to take a DC Coupled, single-ended differential input per the circuit in Figure 36 of the AD5562 datasheet (without the AC coupling capacitor). The path between the AD5562 and the AD9255 I modified per Table 10 and figure 69 of the AD9255 datasheet, again without the AC coupling capacitors.I do get a response at lower frequencies, but I am seeing discrete jumps in the output of the AD, as seen on the VisualAnalog software. I have attached a screenshot of the VisualAnalog output screen showing a typical signal. I also see the same effect in sine wave inputs. If I turn on dithering, it masks the effect somewhat, but it is still visible (also attached).

I would appreciate any help.

Thanks,

Greg

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  • Hello,

    Interesting problem.  I am wondering if the problem may be due to incompatible common-mode output voltage of the ADL5562 and the AD9255 when dc coupling.  Looking at the AD9255 EVB schematic, the ADL5562 VCOM pin is left open such that its nominal output common-mode voltage floats to VCC/2 (or 1.65 V with VCC=3.3 V).   The AD9255 operates off of 1.8 V and has a nominal input common-mode that is 1/2 of this supply (or 0.9 V) for best operation.  If the ADL5565 is actually driving the AD9255 with a signal centered at 1.65 V.....................perhaps this may explain the problem you are seeing?

    Note that the ADL5562 datasheet species that the VCOM can be adjusted over a 1.4 to 1.8 V range.  Perhaps you can try setting the VCOM externally to 1.4 (or something less) as well as reduce the AD9255's input span to 1 V  (install R51 on EVB) such that common-mode voltages are more compatible between amplifier output.

    Regards.

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  • Hello,

    Interesting problem.  I am wondering if the problem may be due to incompatible common-mode output voltage of the ADL5562 and the AD9255 when dc coupling.  Looking at the AD9255 EVB schematic, the ADL5562 VCOM pin is left open such that its nominal output common-mode voltage floats to VCC/2 (or 1.65 V with VCC=3.3 V).   The AD9255 operates off of 1.8 V and has a nominal input common-mode that is 1/2 of this supply (or 0.9 V) for best operation.  If the ADL5565 is actually driving the AD9255 with a signal centered at 1.65 V.....................perhaps this may explain the problem you are seeing?

    Note that the ADL5562 datasheet species that the VCOM can be adjusted over a 1.4 to 1.8 V range.  Perhaps you can try setting the VCOM externally to 1.4 (or something less) as well as reduce the AD9255's input span to 1 V  (install R51 on EVB) such that common-mode voltages are more compatible between amplifier output.

    Regards.

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