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LT5538 Datasheet(PDF) 9 Page - Linear Technology |
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LT5538 Datasheet(HTML) 9 Page - Linear Technology |
9 / 12 page 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. EXTERNAL FILTERING AT CAP+, CAP– The CAP+ and CAP– Pins are internally biased at VCC –0.36V via a 200Ω resistor from voltage supply VCC as shown in 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 from CAP+ and CAP– to VCC to filter out this low frequency 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: C6 (or C9) ≥ 10/(2π • 200 • fMOD) 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 CAP+ C9 CAP– 200 100μA 150 OUT 200μA OUTPUT CURRENTS FROM RF DETECTORS LT5538 VCC 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 |
Similar Part No. - LT5538_15 |
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Similar Description - LT5538_15 |
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