MAX038 Maxim, MAX038 Datasheet - Page 11

• Manufacturer Part Number: MAX038
• Description: High-Frequency Waveform Generator
• Manufacturer: Maxim

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 I
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.
The voltage on DADJ controls the waveform duty cycle
(defined as the percentage of time that the output
waveform is positive). Normally, V
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.
FREQUENCY
20k
R
IN
12k
R1
1µF
C1
1nF
C
C3
F
______________________________________________________________________________________
10
1
7
8
5
REF
DADJ
IIN
FADJ
COSC
GND GND
6
IN
Swept Frequency Operation
High-Frequency Waveform Generator
.
2
MAX038
20
9
GND
–5V +5V
V-
17
11
GND GND
V+ A1
DADJ
18
4
DGND
SYNC
OUT
PDO
DV+
PDI
AO
3
19
16
15
14
13
12
= 0V, and the
Duty Cycle
50
R2
N.C.
N.C.
1µF
C2
F
o
=
2 x 2.5V
R
IN
SINE-WAVE
OUTPUT
x C
DADJ can be used to reduce the sine-wave distortion.
The unadjusted duty cycle (V
any deviation from exactly 50% causes even order har-
monics to be generated. By applying a small
adjustable voltage (typically less than ±100mV) to
V
tion can be minimized (see Figure 2).
The voltage on DADJ needed to produce a specific
duty cycle is given by the formula:
or:
where:
Conversely, if V
time are given by:
F
DADJ
V
V
V
dc = duty cycle (in %)
t
t
dc = 50% - (V
t
ON
o
ON
DADJ
DADJ
DADJ
, exact symmetry can be attained and the distor-
= waveform period.
–2.5V
= ON (positive) time
= t
ADJUST R6 FOR MINIMUM SINE-WAVE DISTORTION
o
PRECISION DUTY-CYCLE ADJUSTMENT CIRCUIT
= (50% - dc) x 0.0575
= (0.5 - [t
= DADJ voltage (observe the polarity)
100k
100k
R4
R7
x (0.5 - [V
DADJ
5k
R6
DADJ
ON
is known, the duty cycle and ON
DADJ
x 17.4)
÷ t
100k
100k
R3
R5
o
]) x 5.75
x 0.174])
DADJ
+2.5V
= 0V) is 50% ±2%;
REF
DADJ
MAX038
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