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32kHz crystal (Read 566 times)

analog_wiz

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Re: 32kHz crystal
Reply #15 - Dec 20^th, 2012, 10:06pm

Hi ic_engr, a gain of 7-8 in magnitude should be fine and should be worst case (least no across corners).Thats why i suggest having some way of adding currents (in case your oscillator does not start due to lack of gain).I implemented a 600nA xtal and things are fine.If you are using nmos1v devices then take care of leakages(nmos1v cap gate leakage).Yes the startup times are huge (order of 1-2 sec for 32khz). Just deque the xtal and give a run inorder to see if things are starting up properly across corners.

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ic_engr

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Re: 32kHz crystal
Reply #16 - Feb 8^th, 2013, 7:44am

Hello analog_wiz,

All theoretical calculations for pierce oscillator give minium start-up time using |Rneg| which is computed at gmopt.

t=2*Lm/|Rneg|.

I believe the assumption is at gmopt, the |Rneg| is maximum so we get minimum time.

However, if the actual gm of the circuit is well above gmcritical, but BELOW gmopt, what will be the |Rneg| at the actual gm and how will the max start up time be computed.

Also I am planning to use a Feedback resistor of ~6Meg-7Meg with a bia current of 200nA.

Regard

ic_engr

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ic_engr Senior Member Offline Posts: 135	Re: 32kHz crystal Reply #17 - Mar 12^th, 2013, 12:08pm For transient start-up, I was using a max step of 100nsec AND IC=5nA (in crystal inductor). I see oscillations grown to full amplitude ~400msec. However, if I make ic=0, the oscillations gre to 2mV in 400msec. To predict Start-up time should I use IC=5nA or NOT. Appreciate your feedback Regards ic_engr
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