AD8361ARMZ datasheet(19/24 Pages) AD

19 / 24 page

AD8361

Rev. C | Page 19 of 24

One of the AD8361s (U2) has a net gain of about 14 dB

preceding it and therefore operates most accurately at low input

signal levels. This is referred to as the weak signal path. U4, on

the other hand, does not have the added gain and provides

accurate response at high levels. The output of U2 is attenuated

by R1 in order to cancel the effect of U2’s preceding gain so that

the slope of the transfer function (as seen at the slider of R1) is

the same as that of U4 by itself.

2.5 GHz. However, it is necessary to calibrate for a given

application to accommodate for the change in conversion gain

at higher frequencies.

Dynamic Range Extension for the AD8361

The accurate measurement range of the AD8361 is limited by

internal dc offsets for small input signals and by square law

conformance errors for large signals. The measurement range

may be extended by using two devices operating at different

signal levels and then choosing only the output of the device

that provides accurate results at the prevailing input level.

The circuit comprising U3, U5, and U6 is a crossfader, in which

the relative gains of the two inputs are determined by the output

currents of a fuzzy comparator made from Q1 and Q2.

Assuming that the slider of R2 is at 2.5 V dc, the fuzzy

comparator commands full weighting of the weak signal path

when the output of U2 is below about 2.0 V dc, and full

weighting of the strong signal path when the output of U3

exceeds about 3.0 V dc. U3 and U5 are OTAs (operational

transconductance amplifiers).

Figure 60 depicts an implementation of this idea. In this circuit,

the selection of the output is made gradually over an input level

range of about 3 dB in order to minimize the impact of

imperfect matching of the transfer functions of the two

AD8361s. Such a mismatch typically arises because of the

variation of the gain of the RF preamplifier U1 and both the

gain and slope variations of the AD8361s with temperature.

AD8361

0.1

µF

100pF

0.01

µF

Ω

ERA-3

20dB

RFC

270

Ω

12V

6dB

PAD

6dB

SPLITTER

INPUT

12V

20k

Ω

10k

Ω

2N3906

16k

Ω

CA3080

+12V

–5V

20k

Ω

CA3080

+12V

–5V

20k

Ω

10k

Ω

–5V

+5V

12k

Ω

AD8361

0.1

µF

100pF

0.01

µF

Ω

AD820

8.2nF

VOUT

100

Ω

Figure 60. Range Extender Application

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