Datasheet LT5538 (Analog Devices) - 9

LT5538
APPLICATIONS INFORMATION OUTPUT INTERFACE
When the part is enabled, the output impedance is about 150Ω. When it is disabled, the output impedance is about The output interface of the LT5538 is shown in Figure 3. 29.5kΩ referenced to ground. 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
EXTERNAL FILTERING AT CAP+, CAP–
output is resistively terminated or open. The full-scaled The CAP+ and CAP– Pins are internally biased at VCC –0.36V 10% to 90% rise and fall times are 100nS and 180nS, via a 200Ω resistor from voltage supply VCC as shown in respectively. The output transient responses at varied Figure 3. These two pins are connected to the differential input power levels are shown in Figure 4. 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 VCC rectifi ed signals (particularly second-order harmonic of + 200 200 the RF signal) from the detector cells are fi ltered and then C6 100μA – 20p the DC output is amplifi ed by the output buffer amplifi er. In C9 OUT some applications, the LT5538 may be used to measure a CAP+ + 150 modulated RF signal with low frequency AM content (lower CAP– – than 20MHz), a large modulation signal may be present + OUTPUT CURRENTS at these two pins due to insuffi cient low-pass fi ltering, FROM RF DETECTORS 200μA – resulting in output voltage fl uctuation at the LT5538’s LT5538 output. Its DC content may also vary depending upon the modulation frequency. To assure stable DC output of the 5538 F03 LT5538, external capacitors C6 and C9 can be connected from CAP+ and CAP– to VCC to fi lter out this low frequency
Figure 3. Simplifi ed Schematic of the Output Interface
AM modulation signal. Assume the modulation frequency of the RF signal is fMOD, the capacitor value in Farads of C6 and C9 can be chosen by the following formula: 3.0 6 AT 880MHZ RF PULSE ON 2.5 2 RF PULSE ENABLE (V) C6 (or C9) ≥ 10/(2π • 200 • fMOD) RF PULSE OFF RF PULSE OFF 2.0 –2 PIN = 0dBm PIN = 10dBm (V) 1.5 PIN = 20dBm –6 OUT Do not connect these two fi ltering capacitors to ground V PIN = 30dBm PIN = 40dBm 1.0 –10 or any other low voltage reference at any time to avoid an PIN = 50dBm abnormal start-up condition. 0.5 –14 0 –18 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 TIME (μs) 5538 F04
Figure 4. Simplifi ed Circuit Schematic of the Output Interface
5538f 9