Data Sheet

AD8318

USING THE AD8318

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Rev. E | 18 of 24

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 applica-

tion 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.

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, conse-

quently, 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, connect

a ground-referenced capacitor (CFLT) to the CLPF pin, 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).

Figure 41. Lowering the Postdemodulation Bandwidth

CFLT is selected by

CFLT= 1

π×3.13kΩ×VideoBandwidtℎ

−1.5pF (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.

In many log amp applications, it may be necessary to lower the

corner frequency of the postdemodulation filtering to achieve low

output ripple while maintaining a rapid response time to changes

in signal level. For an example of a 4-pole active filter, see the

AD8307 data sheet.

CONTROLLER MODE

The AD8318 provides a controller mode feature at the VOUT pin.

Using VSET for the setpoint voltage, it is possible for the AD8318 to