ADAU1701 crystal startup issue

op-electronics Question

Jun 1, 2019 00:00

I'm experiencing some problems with crystal. This is what's happening, most of the time when i power on the board (my own design) the crystal doesn't startup, if i touch the trace connected to MCLKI pin with multimeter probe the crystal starts and ADAU1701 works fine.

Crystal: ABLS7M-12.288MHZ-B-2-T (shunt capacitance 5p, load capacitance 18p)

caps: 22p

series resistor: 100R

PCB is 4 layer with single ground plane. All traces involved in the oscillator circuit are on TOP layer and kept as short as possible. Caps are gorunded through vias.

I hand soldered the board but i'm sure it's not a soldering issue cause i tested 3 of them and they all share the same problem.

This is my PCB layout:

What do you think about?

Thank you

Omar

EDIT:

I put 4.7pF caps in parallel with 22pF and now the startup seems fine all the time.

Initially i tried a lower value resistor than 100R. I used 60R based on wrong assumption of mine confusing series resistor with internal ESR of crystal, after reading a couple applications notes from Abracon and ADI i understood that was just the internal resistance of the crystal and series resistor is used to set drive level in oscillator circuit. With 60R the startup was "probably" better but still failing from time to time. By the way i replaced 60R with 100R again.

I have a question about load capacitors. I used 22pF as reference value cause i've found it on schematic from evaluation board. By the way the formula on ADAU1701 datasheet for calculating load capacitors CL = ((C1*C2)/(C1+C2))+Cstray would give a range of 32-35pF assuming load capacitance of 18pF and stray capacitance 2-5pF. Should i stick to the formula on ADAU1701 datasheet and forget about the value found on evaluation board schematic?

The crystal i'm using has a shunt capacitance of 5pF while the datasheet suggests 7pF, may the difference contribute to startup fail?

Edit Notes
Updated after further testing

[edited by: op-electronics at 9:35 AM (GMT 0) on 3 Jun 2019]

Audio Signal Processors

crystal

startup

SigmaDSP Audio Processors

adau1701

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DaveThib Reply

Jun 4, 2019 00:00

Hello Omar,

This is a bit of black magic to get crystals to oscillate sometimes. Consulting the crystal datasheet as you did is good and the formula in the datasheet is also good to refer too rather than the evaluation board schematic. Unless you obtained the EXACT brand and model of crystal, the value in the eval board schematic may not work well for your crystal. You indeed may need a different shunt cap. I would keep the 100 ohm series resistor because that has to do with the internal amplifier more than differences in the crystal.

So it seems like you have found a good value for your crystal.

Dave T

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op-electronics Reply

Jun 5, 2019 00:00

Hello Dave,

thank you for your reply. Now i just have to decide if to go with the closest value of 22p+4.7p (27p) or with datasheet formula which gives 33p.

With ADAU1452 board i was using same crystal part number and same load caps (22p) and never had a startup problem.

One funny thing: my device has a mechanical switch, when switch was pushed it would start the crystal. I guess this was happening by vibrations transmitted through the aluminium enclosure.

Regards

Omar

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DaveThib Reply

Jun 6, 2019 00:00

Hello Omar,

I thought you had designed that switch to be the "ON" switch! :)

Dave T

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