AD8362
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
3.0
An external 100 ꢀ shunt resistor combines with the internal
100 ꢀ single-ended input impedance to provide a broadband
50 ꢀ match. The unused input (in this case, INLO) is ac-coupled
to ground. Figure 49 shows the transfer function of the AD8362
at various frequencies when the RF input is driven single-
ended. The results show that transfer function linearity at the
top end of the range is degraded by the single-ended drive.
2.8GHz
2.5
3.45GHz
3.65GHz
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
2.0
450MHz
1900MHz
2500MHz
900MHz
2140MHz
1.5
1.0
0.5
–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
0
5
10 15
INPUT AMPLITUDE (dBm)
0
Figure 51. Transfer Function at Various Frequencies ≥2.7 GHz when
the RF Input is Driven Single-Ended
–0.5
–1.0
–1.5
–2.0
OPERATION AT LOW FREQUENCIES
In conventional rms-to-dc converters based on junction tech-
niques, the effective signal bandwidth is proportional to the
signal amplitude. In contrast, the 3.5 GHz VGA bandwidth in
the AD8362 is independent of its gain. Because this amplifier is
internally dc-coupled, the system is also used as a high accuracy
rms voltmeter at low frequencies, retaining its temperature-
stable, decibel-scaled output (for example, in seismic, audio,
and sonar instrumentation).
–55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
0
5
10
PIN (dBm)
Figure 49. Transfer Function at Various Frequencies when the
RF Input is Driven Single-Ended
AD8362
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
COMM ACOM
0.01µF
1nF
CHPF
DECL
INHI
VREF
VTGT
VPOS
VOUT
VSET
While the AD8362 can be operated at arbitrarily low frequencies,
an ac-coupled input interface must be maintained. In such cases,
the input coupling capacitors should be large enough so that the
lowest frequency components of the signal to be included in the
measurement are minimally attenuated. For example, for a 3 dB
reduction at 1.5 kHz, capacitances of 1 μF are needed because the
input resistance is 100 Ω at each input pin (200 Ω differentially),
and the calculation is 1/(2π × 1.5 kΩ × 100) = 1 ꢁF. In addition, to
lower the high-pass corner frequency of the VGA, a large capaci-
tor must be connected between the CHPF pin and ground (see
the Choosing a Value for CHPF section).
RF INPUT
2.7nH
4.7nH
1nF
1nF
1nF
INLO
DECL
PWDN ACOM
COMM CLPF
Figure 50. Input Matching for Operation at Frequencies ≥2.7 GHz
For operation at frequencies ≥2.7 GHz, some additional
components are required to match the AD8362 input to
50 Ω (see Figure 50). As the operating frequency increases,
there is also corresponding shifting in the operating power
range (see Figure 51).
More information on the operation of the AD8362 and other RF
power detectors at low frequency is available in Application Note
AN-691: Operation of RF Detector Products at Low Frequency.
Rev. D | Page 20 of 32
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