ADA2200-EVALZ User Guide
UG-702
Amplitude and Phase Measurements
When the amplitude and phase are unknown, it is necessary to
obtain two orthogonal components of the signal to determine
its amplitude, phase, or both. These two components are in
phase and in quadrature relative to each other; the popular
nomenclature used for these component is I and Q.
A signal with two known rectangular components can be
represented as a phase vector or phasor with an associated
amplitude and phase. This representation is show in Figure 5.
Q
I
A
θ
I
II
IV
III
12359-
005
Figure 5. Signal Represented as Phasor
If the signal amplitude and phase are relatively constant for the
duration of the measurement, it is possible to switch the
to return the I and Q components. On the
this switching is accomplished by toggling the EEPROM_BOOT
switch and pressing the
RESET
button. The dc voltage at the
output represents the I and Q components. Perform the
following calculations to find the amplitude and phase:
2
2
Q
I
A
+
=
θ = sin
−1
(
Q
/
A
)
Or, alternatively:
θ = cos
−1
(
I
/
A
)
The inverse sine or inverse cosine functions involving the I and Q
components linearize the relationship between the phase of the
signal and the measured angle. This calculation also makes it
possible to separate the effects of amplitude and phase variations.
Because the inverse sine and inverse cosine are only defined in
two quadrants, the sign of the I and Q components must be
taken into account to map the result to cover the entire 360°.
It is not recommended to use the inverse tangent function to
extract the phase information, because the function is not defined
at +90° and −90°. This function causes the phase measurement
to become very sensitive to measurement errors and noise.
Rev. 0 | Page 7 of 10
