LT5538

9

5538f

APPLICATIONS INFORMATION

OUTPUT INTERFACE

The output interface of the LT5538 is shown in Figure 3.

This output buffer circuit can source 10mA current to the

load and sink 200 μA current from the load. The small-

signal output bandwidth is approximately 4MHz when the

output is resistively terminated or open. The full-scaled

10% to 90% rise and fall times are 100nS and 180nS,

respectively. The output transient responses at varied

input power levels are shown in Figure 4.

When the part is enabled, the output impedance is about

150Ω. When it is disabled, the output impedance is about

29.5kΩ referenced to ground.

Figure 3. These two pins are connected to the differential

outputs of the internal RF detector cells. In combination

with the 20pF in parallel, a low-pass filter is formed with

–3dB corner frequency of 20MHz. The high frequency

rectified signals (particularly second-order harmonic of

the RF signal) from the detector cells are filtered and then

the DC output is amplified by the output buffer amplifier. In

some applications, the LT5538 may be used to measure a

modulated RF signal with low frequency AM content (lower

than 20MHz), a large modulation signal may be present

at these two pins due to insufficient low-pass filtering,

resulting in output voltage fluctuation at the LT5538’s

output. Its DC content may also vary depending upon the

modulation frequency. To assure stable DC output of the

LT5538, external capacitors C6 and C9 can be connected

AM modulation signal. Assume the modulation frequency

C6 and C9 can be chosen by the following formula:

Do not connect these two filtering capacitors to ground

or any other low voltage reference at any time to avoid an

abnormal start-up condition.

Figure 3. Simplified Schematic of the Output Interface

Figure 4. Simplified Circuit Schematic of the Output Interface

200

C9

200

100μA

150

OUT

200μA

OUTPUT CURRENTS

FROM RF DETECTORS

LT5538

5538 F03

20p

+

–

+

–

+

–

C6

5538 F04

TIME (μs)

PIN = 0dBm

PIN = 10dBm

PIN = 20dBm

PIN = 30dBm

PIN = 40dBm

PIN = 50dBm

0

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

0

0.5

1.0

1.5

2.0

3.0

2.5

–18

–14

–10

–6

–2

6

2

AT 880MHZ

RF PULSE OFF

RF PULSE OFF

RF PULSE ON