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ADL5511ACPZ Datasheet(PDF) 18 Page - Analog Devices |
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ADL5511ACPZ Datasheet(HTML) 18 Page - Analog Devices |
18 / 28 page ADL5511 Data Sheet Rev. A | Page 18 of 28 OUTPUT DRIVE CAPABILITY AND BUFFERING The envelope output of the ADL5511 is presented on the VENV pin as a single-ended buffered output with low output imped- ance. To achieve high envelope bandwidth, this output is not ground referenced, unlike the VRMS output, which is ground referenced. The VENV output has a no signal dc value of about 1.1 V. This dc reference is temperature dependent and is presented as a standalone reference voltage on the EREF pin and as a buffered output. The true envelope at any instant of time is simply (VENV − VEREF), but these two pins do not constitute a differential output. EREF is a fixed dc voltage and VENV carries all the envelope information. The VENV output is capable of supporting a parallel load of 500 Ω and 10 pF at full-scale envelope output and maximum bandwidth. Lighter loads (higher R and lower C) are always recommended whenever possible to minimize power consumption and achieve maximum possible bandwidth. The maximum source/sink current capacity of the VNEV output is 15 mA peak and load conditions should be such that this is not exceeded. The maximum output voltage at this pin is approximately (VPOS − 1.5) V. For the case of ac coupling only, the VENV output can drive a 50 Ω load, as long as the maximum signal swing does not exceed an amplitude of approximately 1.5 V p-p. This corre- sponds to the peak signal current of 15 mA into the 50 Ω load. If a 50 Ω drive capability is desired, the maximum input signal to ADL5511 should be adjusted, such that this output swing condition is not exceeded. A 50 Ω load should never be dc coupled to the VENV output, as it presents a current draw of >20 mA even for no-signal condition corresponding to 1.1 V nominal dc voltage at the VENV pin. The VRMS buffered output can source a maximum current of 3 mA, but is not designed to sink any appreciable amount of current. If current sink capability is desired at this pin, a shunt resistance to ground can be connected. The VRMS output has an on-chip series resistance of 100 Ω, to allow a low-pass filtering of the residual ripple using a single shunt capacitor at this pin. Large shunt capacitors at this pin may also require a shunt resistor to be placed to allow fast discharging of the capacitor. The internal shunt resistance on the VRMS pin is 10 kΩ. Note that any shunt resistance placed on this pin creates a resistive divider with the on-chip 100 Ω series resistance. The EREF output buffer also has 3 mA current sourcing capability. The internal shunt resistance on this pin through which any current must be sunk, is 12 kΩ. A capacitor to ground can be placed on this pin to eliminate any RF or envelope ripple at this pin to ensure that voltage at this pin acts as a clean reference for the VENV output for all possible carrier and envelope frequencies. Viewing the Envelope on an Oscilloscope When viewing the VENV output on an oscilloscope, use a low capacitive FET probe. This reduces the capacitance presented to the VENV output and avoids the corresponding effects of larger capacitive loads. |
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