HA7210_05 Datasheet, PDF (6/15 Pages) Intersil Corporation – 10kHz to 10MHz, Low Power Crystal Oscillator

HA7210
Crystal Pullability
Figure 4 shows the basic equivalent circuit for a crystal and
its loading circuit.
C_M
L_M
R_M
C₁
V_DD
C₂
2
C₀
OSC IN
3
OSC OUT
FIGURE 4.
Where:
C_M = Motional Capacitance
L_M = Motional Inductance
R_M = Motional Resistance
C₀ = Shunt Capacitance
C_CL
=
-------------¹-------------


---¹---
C₁
+
-_C--¹--₂- _
=
Equivalent
Crystal Load
If loading capacitance is connected to a Series Mode
Crystal, the new Parallel Mode frequency of resonance may
be calculated with the following equation:
f_P = f_S 1 + -₂---₍---_C----₀--^C---+--^M---_C----_C----_L----₎
Where:
f_P = Parallel Mode Resonant Frequency
f_S = Series Mode Resonant Frequency
In a similar way, the Series Mode resonant frequency may
be calculated from a Parallel Mode crystal and then you may
calculate how much the frequency will “pull” with a new load.
Layout Considerations
Due to the extremely low current (and therefore high
impedance) the circuit board layout of the HA7210 must be
given special attention. Stray capacitance should be
minimized. Keep the oscillator traces on a single layer of the
PCB. Avoid putting a ground plane above or below this layer.
The traces between the crystal, the capacitors, and the OSC
pins should be as short as possible. Completely surround
the oscillator components with a thick trace of V_DD to
minimize coupling with any digital signals. The final
assembly must be free from contaminants such as solder
flux, moisture, or any other potential source of leakage. A
good solder mask will help keep the traces free of moisture
and contamination over time.
Further Reading
Al Little “HA7210 Low Power Oscillator: Micropower Clock
Oscillator and Op Amps Provide System Shutdown for Battery
Circuits”. Intersil Corporation Application Note AN9317.
Robert Rood “Improving Start-Up Time at 32kHz for the
HA7210 Low Power Crystal Oscillator”. Intersil Corporation
Application Note AN9334.
S. S. Eaton “Timekeeping Advances Through COS/MOS
Technology”. Intersil Corporation Application Note
ICAN-6086.
E. A. Vittoz, et. al. “High-Performance Crystal Oscillator
Circuits: Theory and Application”. IEEE Journal of Solid-
State Circuits, Vol. 23, No. 3, June 1988, pp774-783.
M. A. Unkrich, et. al. “Conditions for Start-Up in Crystal
Oscillators”. IEEE Journal of Solid-State Circuits, Vol. 17,
No. 1, Feb. 1982, pp87-90.
Marvin E. Frerking “Crystal Oscillator Design and
Temperature Compensation”. New York: Van Nostrand-
Reinhold, 1978. Pierce Oscillators Discussed pp56-75.
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