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AD9106, no SPI/startup after clock update


I am designing a high speed circuit based on the AD9106 DAC, controlled by a small Arduino board.

Basically I got everything up and running a while back using a LT6904 adjustable clock source. This solution as single-ended and only worked up to ~25MHz, so I upgraded it to a differential clock using an AD8138 amplifier. This released the full potential of the LT-clock, 65MHz.

Only problem was the noise created on the 3.3V supply (and the fact that I would like to increase clock >100MHz).

To improve the circuit further I changed the clock source to a LMK61A2-125 (125MHz, LVDS output, very low jitter).

Unfortunately there was no output on the DAC after the change, nor can I communicate with it over SPI (which also used to work, of course). I have tried writing my old setup to the DAC as well as reading data from the internal registers, but nothing seems to work.

The clock IC is connected as shown in the attached picture, from the AD9106 datasheet, with a 100R resistor across the output (I tried on both sides of the coupling-caps, as shown in the LMK61A2 evaluation board documentation), and i can measure the clock signal on the inputs.

I measure this using two single-ended probes (but the circuits does not work even without the probes), and see ~550mV DC offset on each clock input, with a ~400mVpp clock signal on top of it.

Now, the AD9106 datasheet specifies a DC level of 900mV, but this is internally biased and since the circuit worked before I have to assume this is not the cause(?)

(To put it simply; it will be a major task to change back to the LT6904 circuit - one that I wish to avoid if possible).

Anyway, those are the symptoms, not unlike what a guy in here had experienced (, but changing the clock source and decreasing VCC to 1.8V is not an option for me... But then again; the DAC should be able to run at 3.3V with differential clock, right?

Sorry for the long post, please feel free to ask any questions I have have no been clear enough :-)


David Larsen.

  • Hi -

    Does the  LMK61A2-125 output signal swing meet the specifications for DAC Clock Input in table 5 according to TI's data sheet?


  • Hello Larry,

    thank you so much for getting back to me on this!

    (un)Fortunately I just managed to 'solve' this issue a few hours ago :-)

    I went crazy and replaced the entire clock circuitry with the original version. After a few tests, the DC bias of the AD9106 disappeared completely, i.e clock input had been severely damaged. As this obviously required an IC replacement, I had to bite the bullet and redo/replace the entire circuit on the PCB - much to my dismay!

    But after all my half a day of tedious soldering (had to everything by hand - and the LMK61 is a sensitive little b*tch to solder) everything was replaced and the circuit worked with both the old, single-ended clock and the LMK61 differential clock - Yay...!

    So to sum up, I can confirm that the LMK61 clock source works with the AD9106 DAC, but extreme caution should be exercised when mounting the ICs - especially the LMK61!

    Unfortunately I cannot give a precise fault-indication of this problem, except maybe a slightly low clock bias voltage (~0.5V) which seems to point to a damaged clock input. If no bias at all.. Well, you know... that's bad, obviously...

    Again thank you for your time, and I apologize for not having updated this post, but I pretty much just got this up and running... :-)