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ADL5519
Preliminary Technical Data
In this configuration, all four measurements, OUT[A, B, P,
N], are made available simultaneously. A differential
output can be taken from OUTP OUTN, and VLVL can
be used to adjust the common-mode level for an ADC
connection.
Rev. PrB | Page 16 of 27
MEASUREMENT MODE
The ADL5519 requires a single supply of 3.0 V to 5 V. The
supply is connected to the three supply pins, VPSA, VPSB,
and VPSR. Each pin should be decoupled using the two
capacitors with values equal or similar to those shown in
Figure 19. These capacitors must provide a low impedance
over the full frequency range of the input, and they should
be placed as close as possible to the positive supply pins.
Two different capacitors are used in parallel to provide a
broadband ac short to ground.
The device is placed in measurement mode by connecting
OUTA and/or OUTB to VSTA and/or VSTB, respectively. As
seen in Figure 18, the ADL5519 has an offset voltage, a
negative slope, and a V
OUT[A,B]
measurement intercept at the
high end of its input signal range.
Figure 18. Typical Output Voltage vs. Input Signal, Single Channel
The output voltage vs. input signal voltage of the ADL5519
is linear-in-dB over a multidecade range. The equation for
this function is of the form
V
OUT
=
X
×
V
SLOP
E/
DEC
× log
10
(
V
IN
/
V
INTERCEPT
) =
(1)
X
×
V
SLOPE/dB
× 20 × log
10
(
V
IN
/
V
INTERCEPT
)
(2)
where:
X
is the feedback factor in
V
SET
=
V
OUT
/X.
V
SLOPE/DEC
is nominally –440 mV/decade or 22 mV/dB.
V
INTERCEPT
is the x-axis intercept of the linear-in-dB portion
of the
V
OUT
vs.
V
IN
curve (Figure 18).
V
INTERCEPT
is +2 dBV for a sinusoidal input signal.
An offset voltage, V
OFFSET
, of 0.35 V is internally added to
the detector signal, so that the minimum value for V
OUT
is
X ×
V
OFFSET
. So for X = 1, minimum V
OUT
is 0.35 V.
The slope is very stable vs. process and temperature variation.
When base-10 logarithms are used, V
SLOPE/DECADE
represents the
volts/decade. A decade corresponds to 20 dB;
V
SLOPE/DECADE
/20 =
V
SLOPE/dB
represents the slope in volts/dB.
As noted in Equation 1 and Equation 2, the V
OUT
voltage has a
negative
slope. This is also the correct slope polarity to control
the gain of many power amplifiers in a negative feedback
configuration. Because both the slope and intercept vary slightly
with frequency, it is recommended to refer to the Specifications
section for application-specific values for slope and intercept.
Although demodulating log amps respond to input signal voltage,
not input signal power, it is customary to discuss the amplitude
of high frequency signals in terms of power. In this case, the charac-
teristic impedance of the system, Z
0
, must be known to convert
voltages to their corresponding power levels. The following
equations are used to perform this conversion:
P
(dBm) = 10 × log
10
(
V
rms
2
/(
Z
0
× 1 mW))
(3)
P
(dBV) = 20 × log
10
(
V
rms
/1
V
rms
)
(4)
P
(dBm) =
P
(dBV) 10 × log
10
(
Z
0
× 1 mW/1
V
rms
2
)
(5)
For example, P
INTERCEPT
for a sinusoidal input signal expressed in
terms of dBm (decibels referred to 1 mW), in a 50 Ω system is
P
INTERCEPT
(dBm) =
P
INTERCEPT
(dBV) – 10 × log
10
(
Z
0
×
1 mW/1
V
rms
2
) =
(6)
+2 dBV 10 × log
10
(50×10
-3
) = +15 dBm
For a square wave input signal in a 200 Ω system,
P
INTERCEPT
= 1 dBV 10 × log
10
[(200 Ω × 1 mW/1
V
rms
2
)] =
+6 dBm
Further information on the intercept variation dependence upon
waveform can be found in the AD8313 and AD8307 data sheets.
As the input signal to Channel A and Channel B are swept over
their nominal input dynamic range of +10 dBm to 50 dBm,
the output swings from 0.5 V to 1.75 V. The voltages OUTA and
OUTB are also internally applied to a difference amplifier with
a gain of two. So as the dB difference between INA and INB
ranges from approximately 30 dB to +30 dB, the difference
voltage on OUTP and OUTN swings from 0.5 V to 1.75 V.
Input differences larger than ±30 dB can be measured as long as
the absolute input level at INA and INB are within their nominal
ranges of +10 dBm to 50 dBm. However, measurement of large
differences between INA and INB are affected by on-chip signal
leakage. The common-mode level of OUTP and OUTN is set by
the voltage applied to VLVL. These output can be easily biased
up to a common-mode voltage of 2.5 V by connecting VREF to
VLVL. As the gain range is swept, OUTP swings from
approximately 0.5 V to 1.75 V and OUTN swings from 1.75 V
to 0.5 V.