ADL5501AKSZ-R21 datasheet(21/28 Pages) AD

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ADL5501

Rev. B | Page 21 of 2

The square-domain filter at FLTR can be reduced to improve

response time, and the remaining ac residual can be decreased

by using the output filter, which has a smaller time constant.

OUTPUT DRIVE CAPABILITY AND BUFFERING

The ADL5501 is capable of sourcing an output current of approx-

imately 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 ADL5501 drive high resistive loads

to preserve output swing. If an application requires driving a low

resistance load, a simple buffering circuit can be used, as shown

in Figure 49. Similar circuits can be used to increase or decrease

the nominal conversion gain (see Figure 47 and Figure 48). In

Figure 48, the AD8031 buffers a resistive divider to give half of

the slope. In Figure 47, the op amp gain of two doubles the slope.

Using other resistor values, the slope can be changed to an arbitrary

value. The AD8031 rail-to-rail op amp, used in these examples,

can swing from 50 mV to 4.95 V on a single 5 V supply and

operates at supply voltages down to 2.7 V. If high output current

is required (>10 mA), the AD8051, which also has rail-to-rail

capability, can be used down to a supply voltage of 3 V. It can

deliver up to 45 mA of output current.

100pF

0.1µF

0.01µF

ADL5501

VRMS

VPOS

COMM

5kΩ

12.6V/V rms

AD8031

Figure 47. Output Buffering Options, Slope of 12.6 V/V rms at 900 MHz

100pF

0.1µF

0.01µF

ADL5501

VRMS

VPOS

COMM

3.2V/V rms

AD8031

4kΩ

5kΩ

Figure 48. Output Buffering Options, Slope of 3.2 V/V rms at 900 MHz

100pF

0.1µF

0.01µF

ADL5501

VRMS

VPOS

COMM

6.3V/V rms

AD8031

Figure 49. Output Buffering Options, Slope of 6.3 V/V rms at 900 MHz

VRMS OUTPUT OFFSET

The ADL5501 has a ±1 dB error detection range of about 30 dB,

as shown in Figure 10 to Figure 12 and Figure 16 to Figure 18.

The error is referred to the best-fit line defined in the linear region

of the output response. Below an input power of −20 dBm, the

response is no longer linear and begins to lose accuracy. In addi-

tion, depending on the supply voltage, saturation of the output

limits the detection accuracy above 10 dBm. Calibration points

should be chosen in the linear region, avoiding the nonlinear

ranges at the high and low extremes.

Figure 50 shows the distribution of the output response vs. the

input power for multiple devices. The ADL5501 loses accuracy at

low input powers as the output response begins to fan out. As the

input power is reduced, the spread of the output response increases

along with the error. Although some devices follow the ideal linear

response at very low input powers, not all devices continue the

ideal linear regression to a near 0 V y-intercept. Some devices

exhibit output responses that rapidly decrease, and some flatten

out. With no RF signal applied, the ADL5501 has a typical output

offset of 50 mV (with a maximum of 150 mV).

0.01

0.1

–40 –35 –30 –25 –20 –15 –10

–5

INPUT (dBm)

Figure 50. Output vs. Input Level Distribution of 50 Devices,

Frequency = 900 MHz, Supply = 5.0 V

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