lt5538 Linear Technology Corporation, lt5538 Datasheet - Page 9

• Manufacturer Part Number: lt5538
• Description: 40mhz To 3.8ghz Rf Power Detector With 75db Dynamic Range
• Manufacturer: Linear Technology Corporation

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.
Figure 4. Simplifi ed Circuit Schematic of the Output Interface
C9
Figure 3. Simplifi ed Schematic of the Output Interface
C6
CAP
CAP
V
CC
+
–
3.0
2.5
0.5
2.0
1.5
1.0
0
0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
RF PULSE OFF
OUTPUT CURRENTS
FROM RF DETECTORS
200
RF PULSE ON
LT5538
20p
TIME (μs)
200
+
–
RF PULSE OFF
PIN = 0dBm
PIN = 10dBm
PIN = 20dBm
PIN = 30dBm
PIN = 40dBm
PIN = 50dBm
100μA
AT 880MHZ
5538 F04
+
–
200μA
6
2
–14
–18
–2
–6
–10
+
–
150
5538 F03
OUT
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.
EXTERNAL FILTERING AT CAP
The CAP
via a 200Ω resistor from voltage supply V
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 fi lter is formed with
–3dB corner frequency of 20MHz. The high frequency
rectifi ed signals (particularly second-order harmonic of
the RF signal) from the detector cells are fi ltered and then
the DC output is amplifi ed by the output buffer amplifi er. 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 insuffi cient low-pass fi ltering,
resulting in output voltage fl uctuation 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
from CAP
AM modulation signal. Assume the modulation frequency
of the RF signal is f
C6 and C9 can be chosen by the following formula:
Do not connect these two fi ltering capacitors to ground
or any other low voltage reference at any time to avoid an
abnormal start-up condition.
C6 (or C9) ≥ 10/(2π • 200 • f
+
and CAP
+
and CAP
–
Pins are internally biased at V
–
MOD
to V
, the capacitor value in Farads of
CC
to fi lter out this low frequency
+
MOD
, CAP
)
–
CC
LT5538
as shown in
CC
–0.36V
9
5538f