AD8361
Analysis
Problems caused by impedance mismatch may arise using the
evaluation board to examine the AD8361 performance. One
way to reduce these problems is to put a coaxial 3 dB attenuator
on the RFIN SMA connector. Mismatches at the source, cable,
and cable interconnection, as well as those occurring on the
evaluation board, can cause these problems.
The conversion gain and output reference are derived using the
coefficients of a linear regression performed on data collected
in its central operating range (35 mV rms to 250 mV rms). This
range was chosen to avoid areas of operation where offset
distorts the linear response. Error is stated in two forms error
from linear response to CW waveform and output delta from
2°C performance.
A simple (and common) example of such a problem is triple
travel due to mismatch at both the source and the evaluation
board. Here the signal from the source reaches the evaluation
board and mismatch causes a reflection. When that reflection
reaches the source mismatch, it causes a new reflection, which
travels back to the evaluation board, adding to the original
signal incident at the board. The resultant voltage varies with
both cable length and frequency dependence on the relative
phase of the initial and reflected signals. Placing the 3 dB pad at
the input of the board improves the match at the board and thus
reduces the sensitivity to mismatches at the source. When such
precautions are taken, measurements are less sensitive to cable
length and other fixture issues. In an actual application when
the distance between AD8361 and source is short and well
defined, this 3 dB attenuator is not needed.
The error from linear response to CW waveform is the
difference in output from the ideal output defined by the
conversion gain and output reference. This is a measure of both
the linearity of the device response to both CW and modulated
waveforms. The error in dB uses the conversion gain multiplied
by the input as its reference. Error from linear response to CW
waveform is not a measure of absolute accuracy, since it is
calculated using the gain and output reference of each device.
However, it does show the linearity and effect of modulation on
the device response. Error from 25° C performance uses the
performance of a given device and waveform type as the
reference; it is predominantly a measure of output variation
with temperature.
C4
C2
CHARACTERIZATION SETUPS
Equipment
0.1µF 100pF
AD8361
1
2
3
4
SREF
8
7
6
5
SREF
VRMS
VPOS
VPOS
IREF
RFIN
The primary characterization setup is shown in Figure 72. The
signal source used was a Rohde & Schwarz SMIQ03B, version
3.90HX. The modulated waveforms used for IS95 reverse link,
IS95 nine active channels forward (forward link 18 setting),
and W-CDMA 4-channel and 15-channel were generated using
the default settings coding and filtering. Signal levels were
calibrated into a 50 Ω impedance.
IREF
VRMS
FLTR
C3
RFIN
R1
C1
75Ω
COMM
0.1µF
PWDN
PWDN
Figure 71. Characterization Board
AD8361
CHARACTERIZATION
BOARD
DC OUTPUT
RF SIGNAL
SMIQ038B
RFIN
VRMS
RF SOURCE
3dB
ATTENUATOR
PRUP +V
SREF IREF
S
DC SOURCES
IEEE BUS
DC MATRIX / DC SUPPLIES / DMM
PC CONTROLLER
Figure 72. Characterization Setup
Rev. C | Page 23 of 24
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