High-Frequency Waveform Generator

16

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Layout Considerations

Realizing the full performance of the MAX038 requires

careful attention to power-supply bypassing and board

layout. Use a low-impedance ground plane, and con-

nect all five GND pins directly to it. Bypass V+ and V-

directly to the ground plane with 1µF ceramic capaci-

tors or 1µF tantalum capacitors in parallel with 1nF

ceramics. Keep capacitor leads short (especially with

the 1nF ceramics) to minimize series inductance.

If SYNC is used, DV+ must be connected to V+, DGND

must be connected to the ground plane, and a second

1nF ceramic should be connected as close as possible

between DV+ and DGND (pins 16 and 15). It is not

necessary to use a separate supply or run separate

traces to DV+. If SYNC is disabled, leave DV+ open.

Do not open DGND.

Minimize the trace area around COSC (and the ground

plane area under COSC) to reduce parasitic capaci-

tance, and surround this trace with ground to prevent

coupling with other signals. Take similar precautions

to the ground plane is close to pin 6 (GND).

__________Applications Information

Frequency Synthesizer

Figure 6 shows a frequency synthesizer that produces

accurate and stable sine, square, or triangle waves with

a frequency range of 8kHz to 16.383MHz in 1kHz incre-

ments. A Motorola MC145151 provides the crystal-con-

trolled oscillator, the ÷N circuit, and a high-speed phase

detector. The manual switches set the output frequency;

opening any switch increases the output frequency.

Each switch controls both the ÷N output and an

MX7541 12-bit DAC, whose output is converted to a cur-

rent by using both halves of the MAX412 op amp. This

current goes to the MAX038 IIN pin, setting its coarse

frequency over a very wide range.

Fine frequency control (and phase lock) is achieved

from the MC145151 phase detector through the differ-

ential amplifier and lowpass filter, U5. The phase detec-

tor compares the ÷N output with the MAX038 SYNC

output and sends differential phase information to U5.

U5’s single-ended output is summed with an offset into

the FADJ input. (Using the DAC and the IIN pin for

coarse frequency control allows the FADJ pin to have

very fine control with reasonably fast response to switch

changes.)

A 50MHz, 50

Ω lowpass filter in the output allows pas-

sage of 16MHz square waves and triangle waves with

reasonable fidelity, while stopping high-frequency noise

generated by the ÷N circuit.

V+

PDI

SYNC

AO

DADJ

PDO

FADJ

0.118"

(2.997mm)

0.106"

(2.692mm)

A1

COSC

GND

IIN

GND

GND

DGND

DV+

GND

GND

REF

V-

OUT

TRANSISTOR COUNT: 855

SUBSTRATE CONNECTED TO GND

___________________Chip Topography