AD8318

Rev. B | Page 18 of 24

PULSED RF

INPUT

1nF

+5V

40Ω

100Ω

100Ω

50Ω

50Ω

1nF

52.3Ω

INHI

INLO

VOUT

VPOS

GND

AD8318

VSET

ADCMP563

+5V

–5.2V

–5.2V

50Ω

50Ω

COMPARATOR

OUTPUT

AD8318

OUTPUT

Figure 39. AD8318 Operating with the High Speed ADCMP563 Comparator

RESPONSE TIME CAPABILITY

The AD8318 has a 10 ns rise/fall time capability (10% to 90%)

for input power switching between the noise floor and 0 dBm.

This capability enables RF burst measurements at repetition

rates beyond 45 MHz. In most measurement applications, the

AD8318 has an external capacitor connected to CLPF to

provide additional filtering for VOUT. However, using the

CLPF capacitor slows the response time as does stray capacitance

on VOUT. For an application requiring maximum RF burst

detection capability, the CLPF pin is left unconnected. In this

case, the integration function is provided by the 1.5 pF on-chip

capacitor.

There is a 10 Ω internal resistor in series with the output driver.

Because of this resistor, it is necessary to add an external 40 Ω

back-terminating resistor in series with the output when driving

a 50 Ω load. Place the back-terminating resistor close to the

VOUT pin. The AD8318 has the drive capability to drive a 50 Ω

load at the end of a coaxial cable or transmission line when back

terminated (see Figure 39).

The circuit diagram in Figure 39 shows the AD8318 used with a

high speed comparator circuit. The 40 Ω series resistor at the

output of the AD8318 combines with an internal 10 Ω to

properly match to the 50 Ω input of the comparator.

0

100

200

300

400

500

600

700

800

TIME (ns)

AD8318

OUTPUT

COMPARATOR

OUTPUT

PULSED RF

INPUT

–50dB

–30dB

–20dB

–10dB

Figure 40. Pulse Response of AD8318 and Comparator for RF Pulses

of Varying Amplitudes

Figure 40 shows the response of the AD8318 and the comparator

for a 500 MHz pulsed sine wave of varying amplitudes. The

output level of the AD8318 is the signal strength of the input

signal. For applications where these RF bursts are very small,

the output level does not change by a large amount. Using a

comparator is beneficial in this case because it turns the output

of the log amp into a limiter-like signal. While this configuration

does result in the loss of received signal power level, it does

allow for presence-only detection of low power RF bursts.

OUTPUT FILTERING

For applications in which maximum video bandwidth and,

consequently, fast rise time are desired, it is essential that the

CLPF pin be left unconnected and free of any stray capacitance.

To reduce the nominal output video bandwidth of 45 MHz,

as shown in Figure 41. Generally, this is done to reduce output

ripple (at twice the input frequency for a symmetric input

waveform, such as sinusoidal signals).

+4

3.13kΩ

VOUT

CLPF

AD8318

1.5pF

Figure 41. Lowering the Postdemodulation Bandwidth

()

pF

5

.

1

kΩ

3.13

π

1

−

×

×

=

idth

VideoBandw

C

FLT

(19)

Set the video bandwidth to a frequency equal to about one-

tenth the minimum input frequency. This ensures that the

output ripple of the demodulated log output, which is at twice

the input frequency, is well filtered.