adl5505 Analog Devices, Inc., adl5505 Datasheet - Page 15

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

Multiple RF Inputs
Figure 35 shows a technique for combining multiple RF input
signals to the ADL5505. Some applications can share a single
detector for multiple bands. Three 16.5 Ω resistors in a T network
combine the three 50 Ω terminations (including the ADL5505
with the shunt 75 Ω matching component). The broadband resis-
tive combiner ensures that each port of the T network sees a
50 Ω termination. Because there are only 6 dB of isolation from
one port of the combiner to the other ports, only one band
should be active at a time.
LINEARITY
Because the ADL5505 is a linear responding device, plots of output
voltage vs. input voltage result in a straight line (see Figure 4
and Figure 5) and the dynamic range in decibels (dB) is not
clearly visible. It is more useful to plot the error on a logarith-
mic scale, as shown in Figure 7. The deviation of the plot from
the ideal straight line characteristic is caused by input stage
clipping at the high end and by signal offsets at the low end.
However, offsets at the low end can be either positive or
negative; therefore, the linearity error vs. input level plots (see
Figure 7) can also trend upwards at the low end. Figure 10 to
Figure 12 and Figure 16 to Figure 18 show error distributions
for a large population of devices at specific frequencies over
temperature.
It is also apparent in Figure 7 that the error at the lower portion
of the dynamic range tends to shift up as frequency is increased.
This is due to the calibration points chosen: −14 dBm and +8 dBm
(see the Device Calibration and Error Calculation section).
The input impedance of the ADL5505 varies with frequency,
decreasing the actual voltage across the input stage as the
frequency increases and, thus, reducing the conversion gain.
Similarly, conversion gain is less at frequencies near 450 MHz
because of the small on-chip coupling capacitor. The dynamic
range is near constant over frequency, but with a decrease in
conversion gain as frequency is increased.
DIRECTIONAL
DIRECTIONAL
COUPLER
COUPLER
Figure 35. Combining Multiple RF Input Signals
BAND 1
BAND 2
50Ω
50Ω
16.5Ω
16.5Ω
16.5Ω
75Ω
ADL5505
RFIN
Rev. 0 | Page 15 of 20
Output Swing
At 900 MHz, the VRMS output voltage is nominally 1.86 × the
input rms voltage (a conversion gain of 1.86 V/V rms). The output
voltage swings from near ground to 2.4 V on a 3.0 V supply.
Figure 8 shows the output swings of the ADL5505 to a CW input
for various supply voltages. Only at the lowest supply voltages
(2.5 V and 2.7 V) is there a reduction in the dynamic range as
the input headroom decreases.
Output Offset
The ADL5505 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 (see the Device Calibration and Error
Calculation section for more details). Below an input power of
−16 dBm, the response is no longer linear and begins to lose
accuracy. In addition, depending on the supply voltage, saturation
may limit the detection accuracy above +14 dBm. Choose cali-
bration points in the linear region, avoiding the nonlinear
ranges at the high and low extremes.
Figure 36 shows a distribution of the output response vs. the
input for multiple devices. The ADL5505 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 regres-
sion 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 ADL5505 has a typical output
offset of 10 mV (with a maximum of 100 mV) on VRMS.
0.0001
0.001
0.01
0.1
10
Figure 36. Output vs. Input Level Distribution of 50 Devices;
1
–25
–20
Frequency = 900 MHz; Supply = 3.0 V
–15
–10
INPUT (dBm)
–5
0
5
ADL5505
10
15