MAX038 Просмотр технического описания (PDF) - Maxim Integrated
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MAX038 Datasheet PDF : 16 Pages
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High-Frequency Waveform Generator
–5V +5V
PRECISION DUTY-CYCLE ADJUSTMENT CIRCUIT
FREQUENCY
C1
1µF
RIN
20k
R1
12k
1 REF
C3
1nF
20 17 4
V- V+ A1
3
AO
7 DADJ MAX038
10
IIN
8
FADJ
5 COSC
19
OUT
DV+ 16
DGND 15
SYNC 14
13
PDI
12
PDO
GND GND GND GND GND
CF
6 2 9 11 18
C2
1µF
R2
50Ω
N.C.
N.C.
–2.5V R4
100k
SINE-WAVE
OUTPUT
2 x 2.5V
Fo = RIN x CF
R7
100k
R6
5k
R3 +2.5V
100k
REF
MAX038
R5
100k
DADJ
ADJUST R6 FOR MINIMUM SINE-WAVE DISTORTION
Figure 2. Operating Circuit with Sine-Wave Output and 50% Duty Cycle; SYNC and FADJ Disabled
With FADJ disabled, the output frequency can still be
changed by modulating IIN.
Swept Frequency Operation
The output frequency can be swept by applying a vary-
ing signal to IIN or FADJ. IIN has a wider range, slightly
slower response, lower temperature coefficient, and
requires a single polarity current source. FADJ may be
used when the swept range is less than ±70% of the
center frequency, and it is suitable for phase-locked
loops and other low-deviation, high-accuracy closed-
loop controls. It uses a sweeping voltage symmetrical
about ground.
Connecting a resistive network between REF, the volt-
age source, and FADJ or IIN is a convenient means of
offsetting the sweep voltage.
Duty Cycle
The voltage on DADJ controls the waveform duty cycle
(defined as the percentage of time that the output
waveform is positive). Normally, VDADJ = 0V, and the
duty cycle is 50% (Figure 2). Varying this voltage from
+2.3V to -2.3V causes the output duty cycle to vary
from 15% to 85%, about -15% per volt. Voltages
beyond ±2.3V can shift the output frequency and/or
cause instability.
DADJ can be used to reduce the sine-wave distortion.
The unadjusted duty cycle (VDADJ = 0V) is 50% ±2%;
any deviation from exactly 50% causes even order har-
monics to be generated. By applying a small
adjustable voltage (typically less than ±100mV) to
VDADJ, exact symmetry can be attained and the distor-
tion can be minimized (see Figure 2).
The voltage on DADJ needed to produce a specific
duty cycle is given by the formula:
VDADJ = (50% - dc) x 0.0575
[11]
or:
VDADJ = (0.5 - [tON ÷ to]) x 5.75 [12]
where:
VDADJ = DADJ voltage (observe the polarity)
dc = duty cycle (in %)
tON = ON (positive) time
to = waveform period.
Conversely, if VDADJ is known, the duty cycle and ON
time are given by:
dc = 50% - (VDADJ x 17.4)
[13]
tON = to x (0.5 - [VDADJ x 0.174]) [14]
______________________________________________________________________________________ 11
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