ADL5505

Rev. 0 | Page 16 of 20

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.

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COUT CAPACITANCE (nF)

W-CDMA FORWARD LINK (4.6dB CF)

W-CDMA REVERSE LINK (11.7dB CF)

and W-CDMA Forward Link (4.6 dB CF) Waveform

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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.

400mV rms RF INPUT

250mV rms

160mV rms

70mV rms

PULSED RFIN

VRMS

1ms/DIV

Figure 39. Output Response to Various RF Input Pulse Levels; Supply = 3 V;

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).