HA7210_05 Datasheet, PDF (5/15 Pages) Intersil Corporation – 10kHz to 10MHz, Low Power Crystal Oscillator

HA7210
Crystal Selection
For general purpose applications, a Parallel Mode Crystal is
a good choice for use with the HA7210. However for
applications where a precision frequency is required, the
designer needs to consider other factors.
Crystals are available in two types or modes of oscillation,
Series and Parallel. Series Mode crystals are manufactured
to operate at a specified frequency with zero load
capacitance and appear as a near resistive impedance when
oscillating. Parallel Mode crystals are manufactured to
operate with a specific capacitive load in series, causing the
crystal to operate at a more inductive impedance to cancel
the load capacitor. Loading a crystal with a different
capacitance will “pull” the frequency off its value.
The HA7210 has 4 operating frequency ranges. The higher
three ranges do not add any loading capacitance to the
oscillator circuit. The lowest range, 10kHz to 100kHz,
automatically switches in two 15pF capacitors onto OSC IN
and OSC OUT to eliminate potential start-up problems.
These capacitors create an effective crystal loading
capacitor equal to the series combination of these two
capacitors. For the HA7210 in the lowest range, the effective
loading capacitance is 7.5pF. Therefore the choice for a
crystal, in this range, should be a Parallel Mode crystal that
requires a 7.5pF load.
In the higher 3 frequency ranges, the capacitance on OSC
IN and OSC OUT will be determined by package and layout
parasitics, typically 4 to 5pF. Ideally the choice for crystal
should be a Parallel Mode set for 2.5pF load. A crystal
manufactured for a different load will be “pulled” from its
nominal frequency (see Crystal Pullability).
+5V
C₁
C₂
2 ^{XTAL C3}
OSC IN
3
OSC OUT
1
V_DD
+
-
V_REG
HA7210
FIGURE 2.
Frequency Fine Tuning
Two Methods will be discussed for fine adjustment of the
crystal frequency. The first and preferred method (Figure 2),
provides better frequency accuracy and oscillator stability
than method two (Figure 3). Method one also eliminates
start-up problems sometimes encountered with 32kHz
tuning fork crystals.
For best oscillator performance, two conditions must be met:
the capacitive load must be matched to both the inverter and
crystal to provide ideal conditions for oscillation, and the
frequency of the oscillator must be adjustable to the desired
frequency. In Method two these two goals can be at odds
with each other; either the oscillator is trimmed to frequency
by de-tuning the load circuit, or stability is increased at the
expense of absolute frequency accuracy.
Method one allows these two conditions to be met
independently. The two fixed capacitors, C₁ and C₂, provide
the optimum load to the oscillator and crystal. C₃ adjusts the
frequency at which the circuit oscillates without appreciably
changing the load (and thus the stability) of the system.
Once a value for C₃ has been determined for the particular
type of crystal being used, it could be replaced with a fixed
capacitor. For the most precise control over oscillator
frequency, C₃ should remain adjustable.
This three capacitor tuning method will be more accurate
and stable than method two and is recommended for 32kHz
tuning fork crystals; without it they may leap into an overtone
mode when power is initially applied.
Method two has been used for many years and may be
preferred in applications where cost or space is critical. Note
that in both cases the crystal loading capacitors are
connected between the oscillator and V_DD; do not use V_SS
as an AC ground. The Simplified Block Diagram shows that
the oscillating inverter does not directly connect to V_SS but is
referenced to V_DD and V_RN. Therefore V_DD is the best AC
ground available.
+5V
C₁
XTAL ^C2
2
OSC IN
3
OSC OUT
HA7210
1
V_DD
+
-
V_REG
FIGURE 3.
Typical values of the capacitors in Figure 2 are shown
below. Some trial and error may be required before the best
combination is determined. The values listed are total
capacitance including parasitic or other sources. Remember
that in the 10kHz to 100kHz frequency range setting the
HA7210 switches in two internal 15pF capacitors.
CRYSTAL
FREQUENCY
32kHz
1MHz
2MHz
4MHz
LOAD CAPS
C₁, C₂
33pF
33pF
25pF
22pF
TRIMMER CAP
C₃
5pF to 50pF
5pF to 50pF
5pF to 50pF
5pF to 100pF
5