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Data Sheet

AD8362

Rev. F | Page 21 of 33

An external 100 Ω shunt resistor combines with the internal

100 Ω single-ended input impedance to provide a broadband

50 Ω 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.

PIN (dBm)

–2.0

–55

4.0

2.0

3.5

1.5

3.0

1.0

2.5

0.5

2.0

1.5

–0.5

1.0

–1.0

0.5

–1.5

–50 –45 –40 –35 –30 –25 –20 –15 –10 –5

450MHz

1900MHz

2500MHz

900MHz

2140MHz

Figure 49. Transfer Function at Various Frequencies when the

RF Input is Driven Single-Ended

COMM

CHPF

DECL

INHI

INLO

DECL

PWDN

COMM

ACOM

VREF

VTGT

VPOS

VOUT

VSET

ACOM

CLPF

AD8362

0.01µF

1nF

RF INPUT

2.7nH

4.7nH

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).

3.00

–60 –55

–45

–35

–25

–15

–5

INPUT AMPLITUDE (dBm)

–50

–40

–30

–20

–10

2.75

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

3.0

–3.0

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

2.8GHz

3.45GHz

3.65GHz

Figure 51. Transfer Function at Various Frequencies ≥2.7 GHz when

the RF Input is Driven Single-Ended

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).

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 μF. 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).

More information on the operation of the AD8362 and other

RF power detectors at low frequency is available in AN-691

Application Note, Operation of RF Detector Products at Low

Frequency.

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