Output Buffer

MAX038 output is capable of driving a capacitive load

up to 90pF. The MAX442 amplifier buffers the MAX038

output to a 50

Ω coaxial cable. The MAX442 is set at a

gain of 2V/V, so that the output amplitude remains 1V/V

after the 50

Ω back termination. The EV kit’s OUT pad

provides access to the output of the MAX038 prior to

the MAX442 buffer stage. The MAX442 output connects

to the BNC connector through a 50

Ω resistor to back

terminate a 50

Ω coaxial cable. When a terminated 50Ω

cable is connected, this resistor forms a voltage divider

with the load impedance, which attenuates the signal

by one-half. The MAX442 is operated with a 2V/V

closed-loop gain to provide unity gain at the 50

Ω

cable’s output.

The MAX442 is actually a 2-channel amplifier. A built-

in multiplexer allows either of two input signals to be

selected. TTL-level address pin A0 selects either IN0

or IN1. The MAX038 output is connected to MAX442

input IN0. IN1 is unused and connected to ground; it

may be used by cutting the JU7 trace, thus discon-

necting IN1 from ground. Likewise, the MAX442

address pin A0 can be disconnected from ground by

cutting the JU8 trace. Pull up A0 to +5V to select IN1.

See the MAX442 data sheet for additional operation

details.

Reference Voltage

The MAX038 includes a 2.5V bandgap reference capa-

ble of sourcing 4mA and sinking 100µA. Access to the

reference voltage is provided at the REF pad. The ref-

erence voltage is primarily used to provide stable cur-

rent to IIN and to bias DADJ and FADJ.

Extending the Output

Frequency Range

The components supplied with the EV kit allow an out-

put frequency range of 325kHz to 10MHz. The frequen-

cy range is controlled primarily by the oscillator capaci-

tor (C1) and the input current, which is a function of the

reference voltage and potentiometer R3. The resulting

frequency range can be shifted up or down depending

on the value of C1. Refer to the Output Frequency vs.

Input Current graph which appears in the

Typical

Operating Characteristics of the MAX038 data sheet.

The upper end of the range can be extended by reduc-

ing C1. The lower end of the range can be reduced by

increasing the value of C1. Take care when selecting

alternate capacitors if stable operation over tempera-

ture is desired. Ceramic capacitors with low tempera-

ture coefficients give the best results. Refer to the

Selecting Resistors and Capacitors section of the

MAX038 data sheet for further details.

Sync Output

and Phase-Detector Input

Refer to the

SYNC Output and Phase Detector sections

of the MAX038 data sheet for details of circuit synchro-

nization. Access to the Phase Detector Input (PDI) and

SYNC is provided at pads PDI and SYNC.

High-speed transient currents in DGND and DV+ can

cause a switching spike in the output waveform at the

zero-crossing point. A lowpass output filter, as shown in

Figure 3 of the MAX038 data sheet, may be used to

greatly reduce the spike. Complete LC filter assemblies

(S3LP series) are available from Coilcraft (phone:

708-639-6400). If the SYNC output is not required, dis-

abling the SYNC circuit will eliminate the switching

spike. Cut the trace between the DV+ and +5V pads to

disable the SYNC output.

MAX038 Evaluation Kit

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