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Topic: Crystal behaving badly (Read 898 times) previous topic - next topic

npulver

I've pieced together a 24-bit analog to digital conversion circuit and I'm having some trouble making it work.  I've got a crystal oscillator (P/N: ECS-49-18-5PXEN-TR) connected to the MCLKOUT and MCLKIN pins of the AD7730R Analog to Digital Converter.  The problem I'm having is that I'm getting 4.9152 MHz on the MCLKOUT pin, but nothing on the MCLKIN pin.  The crystal is not polarized and I assume that I should be getting excitation on one side of the crystal and a steady MHz output on the other.  I've attached a snapshot of the circuit with the section in question highlighted.  Can anyone see any reason why the crystal is not working as expected?

KeithRB

How are you measuring it? The MCLK in might be high impedance and easily disturbed. If you are getting 5 MHZ at the MClock out you are golden.

npulver

I'm just sticking an O-scope probe on the pin.  4.9152MHz on MCLKOUT, 0 Hz on MCLKIN.

pwillard

Do you have 12pF caps at each of the XTAL pins to GND?  Datasheet - Page 32 says they may be optional...  but in reality, it depends on your specific crystal.  If I read it right, based my interpretation of the datasheet, you will ONLY see the clock frequency on MCLKOUT pin... even with a crystal.

Personally, I would use TCXO and feed ttl level clock to pin MCLKIN


mauried

The oscillator is a parallel mode type, and the impedance on the input side is very high.
Its likley that the CRO probe is causing the oscillator to stop when you probe the input side.
You should also have the parallel caps installed .

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