adl5505 Analog Devices, Inc., adl5505 Datasheet - Page 16

• Manufacturer Part Number: adl5505
• Description: 450 Mhz To 6000 Mhz Trupwr Detector Adl5505
• Manufacturer: Analog Devices, Inc.

ADL5505
OUTPUT DRIVE CAPABILITY AND BUFFERING
The ADL5505 is capable of sourcing a VRMS output current of
approximately 3 mA. The output current is sourced through the
on-chip, 100 Ω series resistor; therefore, any load resistor forms
a voltage divider with this on-chip resistance. It is recommended
that the ADL5505 VRMS output drive high resistance loads to
preserve output swing. If an application requires driving a low
resistance load (as well as in cases where increasing the nominal
conversion gain is desired), a buffering circuit is necessary.
SELECTING THE OUTPUT LOW-PASS FILTER
The internal filter capacitor of the ADL5505 provides averaging
in the square domain but leaves some residual ac on the output.
Signals with high peak-to-average ratios, such as W-CDMA or
CDMA2000, can produce ac residual levels on the ADL5505
VRMS dc output. To reduce the effects of these low frequency
components in the waveforms, some additional filtering is
required.
The output of the ADL5505 can be filtered directly by placing a
capacitor between VRMS (Pin 4) and ground. The combination
of the on-chip, 100 Ω output series resistance and the external
shunt capacitor forms a low-pass filter to reduce the residual ac.
Figure 37 show the effects on the residual ripple vs. the output
filter capacitor value at two communication standards with high
peak-to-average ratios. Note that there is a trade-off between ac
residual and response time. Large output filter capacitances
increase the turn-on and pulse response times, as shown in
Figure 38.
Figure 37. AC Residual vs. C
400
350
300
250
200
150
100
50
0
1
and W-CDMA Forward Link (4.6 dB CF) Waveform
OUT
COUT CAPACITANCE (nF)
10
, W-CDMA Reverse Link (11.7 dB CF) Waveform
W-CDMA FORWARD LINK (4.6dB CF)
W-CDMA REVERSE LINK (11.7dB CF)
100
1000
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The turn-on time and pulse response are strongly influenced by
the sizes of the output shunt capacitor. Figure 39 shows a plot of
the output response to an RF pulse on the RFIN pin, with a 0.1 μF
output filter capacitor. The falling edge is particularly dependent on
the output shunt capacitance, as shown in Figure 39.
To improve the falling edge of the enable and pulse responses,
a resistor can be placed in parallel with the output shunt capacitor.
The added resistance helps to discharge the output filter capacitor.
Although this method reduces the power-off time, the added
load resistor also attenuates the output (see the Output Drive
Capability and Buffering section).
Figure 39. Output Response to Various RF Input Pulse Levels; Supply = 3 V;
Frequency = 900 MHz; Square-Domain Filter Open; C
250
200
150
100
50
0
1
Figure 38. Effect of C
C
10
OUT
CAPACITANCE (nF)
1ms/DIV
400mV rms RF INPUT
250mV rms
160mV rms
70mV rms
OUT
on Response Time
PULSED RFIN
100
VRMS
OUT
= 0.1 μF
1000