MAX038CSLASHD datasheet(11/16 Pages) MAXIM

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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]

High-Frequency Waveform Generator

______________________________________________________________________________________

MAX038

1µF

GND

COSC

V-

GND GND

N.C.

FADJ

IIN

DADJ

REF

OUT

DV+

DGND

SYNC

PDI

PDO

V+ A1

–5V +5V

1nF

1µF

12k

20k

RIN

FREQUENCY

Ω

N.C.

SINE-WAVE

OUTPUT

2 x 2.5V

RIN x CF

Fo =

MAX038

100k

DADJ

REF

+2.5V

–2.5V

PRECISION DUTY-CYCLE ADJUSTMENT CIRCUIT

ADJUST R6 FOR MINIMUM SINE-WAVE DISTORTION

Figure 2. Operating Circuit with Sine-Wave Output and 50% Duty Cycle; SYNC and FADJ Disabled

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