Crystal Startup with ADAU1701
On page 19 of the ADAU1701 specification it describes the hardware components values required for proper crystal operation. The crystal load capacitance is recommended to be 18pF. The calculation for the load capacitance is shown to be the series combination of C1 & C2 plus the stray capacitance, where the stray capacitance is 2pF - 5pF. C1 & C2 normally are of the same value so per my calculations a 33pF would produce a C-load of 16.5pF + C-stray; or about 18.5pF if C-stray is small. If C-stray is 5pF then the values for C1& C2 should be 24pF, which is not a standard value but I would likely choose a 27pF. The application drawing shows a 22pF. Can you explain the reasoning behind this component value selection? When we had C1 & C2 = 22pF the crystal was a little low in frequency so we were able to get the crystal on the correct frequency by using a 33pF.
Another item a little different from the evaluation board schematic is the crystal type we are using is an AT-49 type from Abricom (ABLS-12.288MHZ-B4-T). The evaluation board schematic is using a ABM3B-12.288MHZ-10-1-U-T which is an ABM3B series and has a drive level specification of 10uW TYP and 100uW max. The AT-49 crystal we are using has all the exact specs except for a higher drive level: 100uW TYP and 1mW max. Are there any other precautions we would need to take in order to assure proper crystal starup? Would the series resistor need to be lowered to allow for a higher drive level?
Thanks!
Audio Signal Processors
SigmaDSP Audio Processors
adau1701
The cystal oscillator attempts to drive a constant current into the crystal in order to induce resonance, and different factors on the board can alter the amount of capacitance it sees.
I believe the component selection for our evaluation board was done based on recommendations by the crystal manufacturer. We provided details about the evaluation board (trace length, component placement, board characteristics), as well as the transconductance spec for the ADAU1701 (listed in the datasheet), and they were able to suggest values for the capacitors for us. We then tested the chosen capacitors on an evaluation board with several ADAU1701s and ran various stress tests using freeze spray or a heat gun to check all temperature corners of operation.
In my experience, most ADAU1701 system schematics I have reviewed have used the 22 pF capacitors as shown in the application schematic, and those systems seem to have functioned without issues. However, the particular characteristics of your board and crystal may require changes to the component selection. I would suggest checking with the crystal manufacturer's applications engineers.
The layout of course plays a major role, and minimizing the trace length is important. I would suggest that your layout should keep the crystal and PLL filter components as close as possible to the IC, yielding only to the power supply high-frequency decoupling capacitors in terms of component placement priority.
Brett,
Thanks for your assistance.
Todd
From: BrettG analog@sgaur.hosted.jivesoftware.com
Sent: Wednesday, August 31, 2011 11:17 AM
To: Todd Brandenburg
Subject: New message: "Crystal Startup with ADAU1701"
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Crystal Startup with ADAU1701
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