AD9578 Dev board problem

Dear Paul,

The register settings are exactly those loaded by the development software, with the exception that I have enabled the ref out.

I have not yet attempted to change any settings.

I want to modify the XO4,XO3 inputs for an  external differential clock. I have added the balun to the board, however I find the section in the data sheet about the reference/crystal inputs very confusing. Presumably for an external input I should disable the xtal oscillator amplifier. This is the section from the data sheet:

CRYSTAL OSCILLATOR AMPLIFIER ENABLE

The crystal oscillator amplifier is automatically enabled when

either the PLLx or REFOUT bit in Register 2 uses the crystal

oscillator for either Reference 1 or Reference 2. Otherwise, the

crystal oscillator amplifier is disabled if neither the PLLx nor

REFOUT bit selects that input. However, this setting can be

overridden with the enable XTAL1 bit in Register 3 and enable

XTAL2 bit in Register 10. Setting these bits forces the

corresponding crystal oscillator on.

These bits are useful to allow a crystal to power up and stabilize

before it is needed. However, these bits are usually set to 0

under normal operation.

In order to select an external reference on XO4 and XO3 for both PLL, I should set REFSEL1 enable, REFSEL1,REFSEL2 enable, and REFSEL2  to 1. However this appears to leave the xtal oscillator amplifier on, however the XTAL2 bit is set.

What effect does this have on the input characteristics of XO4 and XO3?

Could you please supply an equivalent circuit of the reference inputs.

The development board biases both inputs (XO3 and XO4) at 0.9V, however the specifications for the reference inputs states that these may be AC coupled for differential inputs, with no mention of the need for DC biasing.

best regards

Cosmo Little

Hi cosmolittle 

8 years ago you faced the issue I've now, sometimes the crystal doesn't start at power up, and if I touch it then starts oscillating.
Did you solve it with an external differential clock or did you change the load capacitors or internal settings?
Thanks in advance

Regards

HI,

I do not know anything about this issue and now I am afraid it could be impossible to find out its resolution. Please note the AD9578 is on Last Time Buy status, so you should be looking for an alternative. If you tell me what you want to accomplish (what clock you want to provide and what clocks you need to output), I may offer you an alternative chip. 

Petre

Hi pminc48 ,

I used the AD9578 in an old design and now I'm experiencing that behavior on boards that have been in the warehouse for a few years.
I know that is on the Last Time Buy and I need to find a replacement, but I would like to fix the boards that I've and then proceed with the replacement. An easy replacement is the AD9577 but I would like to have a SPI interface. I need 4 output LVDS to generate video clocks so 74.25MHz, 148.5MHz, 297MHz.

Regards

Hi,

sorry for getting so late back to you.

In my view you should play with the load capacitors on the crystal. This seems the only lever you have. 

Regarding the recommended replacement: The AD9558 is a digital PLL that can create the 4 output LVDS clocks using whatever reference clock you may or may not have. It has SPI and an internal EEPROM.

If you want an analog PLL, the AD9576 may be an alternative. 

Petre

Hi,

sorry for getting so late back to you.

In my view you should play with the load capacitors on the crystal. This seems the only lever you have. 

Regarding the recommended replacement: The AD9558 is a digital PLL that can create the 4 output LVDS clocks using whatever reference clock you may or may not have. It has SPI and an internal EEPROM.

If you want an analog PLL, the AD9576 may be an alternative. 

Petre