Programming DADJ

DADJ is similar to FADJ; it has a 250µA constant cur-

rent sink to V- that must be furnished by the voltage

source. The source is usually an op-amp output, and

the temperature coefficient of the current sink becomes

unimportant. For manual adjustment of the duty cycle, a

250µA current sink’s temperature coefficient becomes

significant. Since external resistors cannot match the

internal temperature-coefficient curve, using external

operation, when the operator can correct for any errors.

voltage source.

(+2.5V) and DADJ provides a convenient means of

manually setting the duty cycle. The resistance value

[15]

observe correct algebraic convention. For example, if

= (4.0V) ÷ 250µA = 16k

Ω

Varying the duty cycle in the range 15% to 85% has

minimal effect on the output frequency—typically less

wideband, and can be modulated at up to 2MHz (see

photos,

Typical Operating Characteristics).

Output

around ground, for all output waveforms. OUT has an

output resistance of under 0.1

Ω, and can drive ±20mA

with up to a 50pF load. Isolate higher output capaci-

tance from OUT with a resistor (typically 50

Ω) or buffer

amplifier.

Reference Voltage

REF is a stable 2.50V bandgap voltage reference capa-

ble of sourcing 4mA or sinking 100µA. It is principally

used to furnish a stable current to IIN or to bias DADJ

and FADJ. It can also be used for other applications

external to the MAX038. Bypass REF with 100nF to min-

imize noise.

Selecting Resistors and Capacitors

The MAX038 produces a stable output frequency over

time and temperature, but the capacitor and resistors

that determine frequency can degrade performance if

they are not carefully chosen. Resistors should be

metal film, 1% or better. Capacitors should be chosen

for low temperature coefficient over the whole tempera-

ture range. NPO ceramics are usually satisfactory.

The voltage on COSC is a triangle wave that varies

between 0V and -1V. Polarized capacitors are generally

not recommended (because of their outrageous tem-

perature dependence and leakage currents), but if they

are used, the negative terminal should be connected to

COSC and the positive terminal to GND. Large-value

capacitors, necessary for very low frequencies, should

be chosen with care, since potentially large leakage

currents and high dielectric absorption can interfere

ble, for a given frequency, use lower IIN currents to

reduce the size of the capacitor.

SYNC Output

SYNC is a TTL/CMOS-compatible output that can be

used to synchronize external circuits. The SYNC output

is a square wave whose rising edge coincides with the

output rising sine or triangle wave as it crosses through

0V. When the square wave is selected, the rising edge

of SYNC occurs in the middle of the positive half of the

output square wave, effectively 90° ahead of the output.

The SYNC duty cycle is fixed at 50% and is indepen-

dent of the DADJ control.

Because SYNC is a very-high-speed TTL output, the

high-speed transient currents in DGND and DV+ can

radiate energy into the output circuit, causing a narrow

spike in the output waveform. (This spike is difficult to

see with oscilloscopes having less than 100MHz band-

width). The inductance and capacitance of IC sockets

tend to amplify this effect, so sockets are not recom-

mended when SYNC is on. SYNC is powered from sep-

arate ground and supply pins (DGND and DV+), and it

can be turned off by making DV+ open circuit. If syn-

chronization of external circuits is not used, turning off

SYNC by DV+ opening eliminates the spike.

Phase Detectors

Internal Phase Detector

The MAX038 contains a TTL/CMOS phase detector that

can be used in a phase-locked loop (PLL) to synchro-

nize its output to an external signal (Figure 3). The

external source is connected to the phase-detector

input (PDI) and the phase-detector output is taken from

PDO. PDO is the output of an exclusive-OR gate, and

produces a rectangular current waveform at the

MAX038 output frequency, even with PDI grounded.

PDO is normally connected to FADJ and a resistor,

of the phase detector, while the capacitor attenuates

high-frequency components and forms a pole in the

phase-locked loop filter.

High-Frequency Waveform Generator

12

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