Datasheet

AD8317
Rev. B | Page 12 of 20
The slope is given by
−I
D
× 2x × 1.5 kΩ = −22 mV/dB × x
For example, if a resistor divider to ground is used to generate a
V
SET
voltage of V
OUT
/2, x = 2. The slope is set to −880 V/decade
or −44 mV/dB.
TEMPERATURE COMPENSATION OF OUTPUT
VOLTAGE
The primary component of the variation in V
OUT
vs. temperature,
as the input signal amplitude is held constant, is the drift of the
intercept. This drift is also a weak function of the input signal
frequency; therefore, provision is made for the optimization of
internal temperature compensation at a given frequency by
providing Pin TADJ.
COMM COMM
I
COMP
V
INTERNAL
TADJ
R
TADJ
05541-026
1.5k Ω
AD8317
Figure 26. TADJ Interface
R
TADJ
is connected between TADJ and ground. The value of
this resistor partially determines the magnitude of an analog
correction coefficient, which is used to reduce intercept drift.
The relationship between output temperature drift and
frequency is not linear and cannot be easily modeled. As a
result, experimentation is required to choose the correct
TADJ resistor. Table 4 shows the recommended values for
some commonly used frequencies.
Table 4. Recommended R
TADJ
Values
Frequency Recommended R
TA DJ
50 MHz 18 kΩ
100 MHz 18 kΩ
900 MHz 18 kΩ
1.8 GHz 8 kΩ
1.9 GHz 8 kΩ
2.2 GHz 8 kΩ
3.6 GHz 8 kΩ
5.3 GHZ 500 Ω
5.8 GHz 500 Ω
8 GHz Open
MEASUREMENT MODE
When the V
OUT
voltage or a portion of the V
OUT
voltage is fed
back to the VSET pin, the device operates in measurement
mode. As seen in Figure 27, the AD8317 has an offset voltage,
a negative slope, and a V
OUT
measurement intercept at the high
end of its input signal range.
0
0.25
0.50
0.75
1.00
1.25
1.50
2.00
V
OUT
(V)
–1.5
–1.0
–0.5
0
0.5
1.0
1.5
2.0
–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0 5 10 15
P
IN
(dBm)
05541-027
RANGE FOR
CALCULATION OF
SLOPE AND INTERCEPT
V
OUT
IDEAL
V
OUT
25°C
ERROR 25°C
1.75
INTERCEPT
Figure 27. Typical Output Voltage vs. Input Signal
The output voltage vs. input signal voltage of the AD8317 is
linear-in-dB over a multidecade range. The equation for this
function is
V
OUT
= X × V
SLOPE/DEC
× log
10
(V
IN
/V
INTERCEPT
) (3)
= X × V
SLOPE/dB
× 20 × log
10
(V
IN
/V
INTERCEPT
) (4)
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. P
IN
curve (see Figure 27).
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
; therefore, for X = 1, the 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 3 and Equation 4, 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 con-
figuration. 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 characteristic 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)) (5)
P [dBV] = 20 × log
10
(V
RMS
/1 V
RMS
) (6)
P [dBm] = P [dBV] − 10 × log
10
(Z
0
× 1 mW/1 V
RMS
2
) (7)