I am using the AFE7222 in a SDR design and am having problems with the reference clock input. We chose to use the CDCLVD1213 clock buffer chip to feed the AFE7222 differential LVDS clock input and have found that it does not work reliably. The AFE7222 datasheet does not clearly specify the required clock level but says the following on page 88:
"The clock inputs are versatile. The AFE7225/7222 can be driven by a differential clock, a single-ended
clock or two independent single-ended clocks. Low voltage CMOS for single-ended and LVDS for
differential are supported clock levels."
So we chose the CDCLVD1213 clock buffer as it supplies LVDS outputs and we leave the AFE7222 set for differential clock input. We have found in our testing that the level out of the clock buffer is not quite high enough for the AFE7222 to reliably detect the clock. Sometimes after power up it works and sometimes it does not work. When I probe the clock input pin with the scope I can see when it doesn't work that there is no DC bias on the pins and it does not output anything from the DACs. Sometimes it will detect the clock and start working as I probe the clock pin and I can see the DC bias appear.
I have 0.1uF blocking capacitors between the CDCLVD1213 outputs and the AFE7222 clock inputs. I originally had a 100 ohm termination resistor between the two differential clock input pins, as recommended in the CDCLVD1213 datasheet, but I already removed that to get a higher signal level. It still seems to be not quite enough.
I used the AFE7225EVM to verify this behavior by supplying an external clock input to J18 and testing what clock level is required for the DC bias to appear. It required about 1.8V pp differential on the clock input pins before it detected the clock. This seems to be quite a bit higher that typical LVDS levels.
Can anyone tell me why these two TI parts, which both are supposed to be LVDS, do not work together?
We have already designed our board and built the first prototypes so if we have to change to a different clock buffer it will be a lot of work and a big expense. Any ideas on how to fix this problem?
Russ