# URGENT - AD9913 - quartz cristal characteristics

• Hello

AD9913 datasheet does not specify quartz cristal characteristics

What are the absolute maximum for 25MHz cristal characteristics to be used together with 39pF capacitors

- ppm ? 10 - 50 ppm ?

- capacitance ? 4,8, 12, 16, 20 pF ?

- ESR ? 50 / 70 / 100 Ohms ?

STH

typo
[edited by: STH at 12:37 PM (GMT -4) on 10 May 2021]

## Top Replies

Parents
• Hi,

I would recommend using the crystals used in some of the DDS evaluation boards: HC49SD_25MHZ or FOXSDLF/250F-20. You can then use the specifications of these crystals as a guide for the absolute maximum values.

Best regards,

Mark

• Hello

I could find a book that details maths for Pierce oscillator: Les oscillateurs en électronique / Gérard Couturier. ISBN 2-7298-2290-9. Part III provides details about Pierce oscillator equations.

1. Stability Condition

C1 >> Cq and C2 >> Cq. Used capacitors shall be bigger than Quartz

2. Oscillation Condition

C1 = Ce + C1' + Cp with Ce entry capacitance, Cp pcb capacitance and C1' the first capacitance arbitrarily defined

C2 = C2' + Cp

Cq = C1.C2/(C1+C2) or C2 = Cq.C1/(C1-Cq)

with Ce = 3pF, Cp = 1.2pF, C1'=39pF, C2'=39pF.

We get Cq = 43.2 . 40.2 / (43.2 + 40.2) = 20.8pF

Suggested component is 25MHz / 20pF. We do agree

In this case, C2' = 20 . 43.2 / ( 43.2 - 20 ) - 1.2 = 36pF.

I would suggest to add these figures to the datasheet because 20pF Quartz are not always offered by manufacturers. Engineers might choose components with different characteristics because they can not find it.

I could find that 18pF Quartz are much easier to find on the market and thus less expensive.

In this case C2' = 18 . 43.2 / ( 43.2 - 18 ) - 1.2 = 29.6pF.

If someone enventually try this configuration. Thank you to confirm it is working with AD9913.

Thank You

Simon

• Hello

I could find a book that details maths for Pierce oscillator: Les oscillateurs en électronique / Gérard Couturier. ISBN 2-7298-2290-9. Part III provides details about Pierce oscillator equations.

1. Stability Condition

C1 >> Cq and C2 >> Cq. Used capacitors shall be bigger than Quartz

2. Oscillation Condition

C1 = Ce + C1' + Cp with Ce entry capacitance, Cp pcb capacitance and C1' the first capacitance arbitrarily defined

C2 = C2' + Cp

Cq = C1.C2/(C1+C2) or C2 = Cq.C1/(C1-Cq)

with Ce = 3pF, Cp = 1.2pF, C1'=39pF, C2'=39pF.

We get Cq = 43.2 . 40.2 / (43.2 + 40.2) = 20.8pF

Suggested component is 25MHz / 20pF. We do agree

In this case, C2' = 20 . 43.2 / ( 43.2 - 20 ) - 1.2 = 36pF.

I would suggest to add these figures to the datasheet because 20pF Quartz are not always offered by manufacturers. Engineers might choose components with different characteristics because they can not find it.

I could find that 18pF Quartz are much easier to find on the market and thus less expensive.

In this case C2' = 18 . 43.2 / ( 43.2 - 18 ) - 1.2 = 29.6pF.

If someone enventually try this configuration. Thank you to confirm it is working with AD9913.

Thank You

Simon

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