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.

The output frequency is inversely proportional to

frequencies above 20MHz.

A sine-shaping circuit converts the oscillator triangle

wave into a low-distortion sine wave with constant

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

(±1V), regardless of wave shape or frequency.

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.

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.

Waveform Selection

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:

X = Don’t care

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.

Waveform Timing

Output Frequency

The output frequency is determined by the current

injected into the IIN pin, the COSC capacitance (to

ground), and the voltage on the FADJ pin. When

given by the formula:

[1]

[2]

where:

750µA)

(20pF to >100µF).

For example:

0.5MHz = 100µA ÷ 200pF

and

2µs = 200pF ÷ 100µA

between 2µA and 750µA. Current levels outside of this

range are not recommended. For fixed-frequency oper-

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

above 20MHz is possible, but waveform distortion

increases under these conditions. The low frequency

limit is set by the leakage of the COSC capacitor and

by the required accuracy of the output frequency.

Lowest frequency operation with good accuracy is usu-

ally achieved with 10µF or greater non-polarized

capacitors.

An internal closed-loop amplifier forces IIN to virtual

ground, with an input offset voltage less than ±2mV. IIN

between REF and IIN provides a convenient method of

in series with a resistor, the formula for the oscillator fre-

quency is:

[3]

and:

[4]

High-Frequency Waveform Generator

_______________________________________________________________________________________

9

A0

A1

WAVEFORM

X

1

Sine wave

0

0

Square wave

1

0

Triangle wave