Hig h -Fre q u e n c y Wa ve fo rm Ge n e ra t o r
MAX038
A stable 2.5V reference voltage, REF, allows simple
determination of IIN, FADJ, or DADJ with fixed resistors,
and permits adjustable operation when potentiometers
are connected from each of these inputs to REF. FADJ
and/or DADJ can be grounded, producing the nominal
frequency with a 50% duty cycle.
V
= 0V, the fundamental output frequency (F ) is
FADJ o
given by the formula:
F (MHz) = I (µA) ÷ C (pF)
[1]
[2]
o
F
IN
The period (t ) is:
o
t (µs) = C (pF) ÷ I (µA)
o
F
IN
The outp ut fre q ue nc y is inve rs e ly p rop ortiona l to
where:
capacitor C . C values can be selected to produce
frequencies above 20MHz.
F
F
I
= current injected into IIN (between 2µA and
IN
750µA)
A sine-shaping circuit converts the oscillator triangle
wa ve into a low-d is tortion s ine wa ve with c ons ta nt
amplitude. The triangle, square, and sine waves are
input to a multiplexer. Two address lines, A0 and A1,
control which of the three waveforms is selected. The
C = capacitance connected to COSC and GND
F
(20pF to >100µF).
For example:
0.5MHz = 100µA ÷ 200pF
output amplifier produces a constant 2V
(±1V), regardless of wave shape or frequency.
amplitude
P-P
and
2µs = 200pF ÷ 100µA
The triangle wave is also sent to a comparator that pro-
duces a high-speed square-wave SYNC waveform that
can be used to synchronize other oscillators. The SYNC
circuit has separate power-supply leads and can be
disabled.
Optimum performance is achieved with I between
10µA and 400µA, although linearity is good with I
between 2µA and 750µA. Current levels outside of this
range are not recommended. For fixed-frequency oper-
IN
IN
ation, set I to approximately 100µA and select a suit-
IN
Two other phase-quadrature square waves are gener-
ated in the basic oscillator and sent to one side of an
“exclusive-OR” phase detector. The other side of the
phase-detector input (PDI) can be connected to an
external oscillator. The phase-detector output (PDO) is
a current source that can be connected directly to
FADJ to synchronize the MAX038 with the external
oscillator.
able capacitor value. This current produces the lowest
temperature coefficient, and produces the lowest fre-
quency shift when varying the duty cycle.
The capacitance can range from 20pF to more than
100µF, but stray circuit capacitance must be minimized
by using short traces. Surround the COSC pin and the
trace leading to it with a ground plane to minimize cou-
pling of extraneous signals to this node. Oscillation
a b ove 20MHz is p os s ib le , b ut wa ve form d is tortion
increases under these conditions. The low frequency
limit is set by the leakage of the COSC capacitor and
b y the re q uire d a c c ura c y of the outp ut fre q ue nc y.
Lowest frequency operation with good accuracy is usu-
a lly a c hie ve d with 10µF or g re a te r non-p ola rize d
capacitors.
Wa ve fo rm S e le c t io n
The MAX038 can produce either sine, square, or trian-
gle waveforms. The TTL/CMOS-logic address pins (A0
and A1) set the waveform, as shown below:
A0
X
0
A1
1
WAVEFORM
Sine wave
0
Square wave
Triangle wave
An internal closed-loop amplifier forces IIN to virtual
ground, with an input offset voltage less than ±2mV. IIN
1
0
may be driven with either a current source (I ), or a
IN
voltage (V ) in series with a resistor (R ). (A resistor
X = Don’t care
IN
IN
between REF and IIN provides a convenient method of
generating I : I = V /R .) When using a voltage
in series with a resistor, the formula for the oscillator fre-
quency is:
Waveform switching can be done at any time, without
regard to the phase of the output. Switching occurs
within 0.3µs, but there may be a small transient in the
output waveform that lasts 0.5µs.
IN
IN
REF IN
F (MHz) = V ÷ [R x C (pF)] [3]
IN
IN
F
o
Wa ve fo rm Tim in g
and:
Output Frequency
The outp ut fre q ue nc y is d e te rmine d by the c urre nt
injected into the IIN pin, the COSC capacitance (to
g round ), a nd the volta g e on the FADJ p in. Whe n
t (µs) = C (pF) x R ÷ V
IN
[4]
F
IN
o
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
