AnsweredAssumed Answered

AD9648 clock output missing

Question asked by dave@skutek.com on Dec 6, 2016
Latest reply on Jan 11, 2017 by DougI

Hello all,

I have two AD9648s that exhibit the same symptoms:

- no DCO output but the clock input is good

- Vref output is ~0.5V instead of 1V

 

It looks like the part is not starting up or something, but the SPI chip identification reads correctly.

 

What is the cause of this and how can it be fixed?   Any suggestions on what to try?

Thanks,
Dave

 

background:

We are using these parts in our product and so far only 2 of ~15 parts have shown this problem.   One of the parts has always had this issue while a second one was working for a few months but now has the same problem.  So, I'm wondering what could have happened to cause the second one to change it's operation.  I initially thought the first one was just a bad part, but now having two with the same symptoms seems interesting.  Especially since the second one didn't have a problem initially.

 

The power and clock input are within the data sheet specifications.  I looked at the power on to the part with an oscilloscope and it shows a nice clean rise time to the final voltage.   The 1.8V regulator does put out ~0.4V for a time and then switches to the full voltage.   But other boards that work do the same, so I don't think that's the problem.   Since the clock is not running when the power is applied, SPI is used to reset the part (see below)

 

The ADC is running at 1.8V for AVdd and 1.8V for DVdd.   The clock is 100MHz single ended with 0.1uF series caps on pins 1 and 2.  

The following sequence is used to initialize the ADC via SPI:

 

Instruction

Word (16b)

Value

(8b)

Description

0x0008

0x03

Digital Reset

0x0008

0x00

Normal Operation

0x0000

0x3C

Soft Reset

0x0014

0xA1

LVDS, interleave,

2's complement

0x0017

0x81

DCO delay = 1.12ns

0x000D

0x00

Output Test Off

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