ad8318acpz-wp Analog Devices, Inc., ad8318acpz-wp Datasheet - Page 15

• Manufacturer Part Number: ad8318acpz-wp
• Description: 1 Mhz To 8 Ghz, 70 Db Logarithmic Detector/controller
• Manufacturer: Analog Devices, Inc.

The maximum output voltage is 2.1 V × X, and cannot exceed
400 mV below the positive supply.
V
V
When X = 1, the typical output voltage swing is 0.5 V to 2.1 V.
The output voltage swing is modeled using Equation 5 to
Equation 7 and restricted by Equation 8:
When X = 4 and V
For X = 4, slope = −100 mV/dB; V
usable dynamic range is reduced to 26 dB from 0 dBm to –26 dBm.
The slope is very stable vs. process and temperature variation.
When base-10 logarithms are used, V
output voltage per decade of input power. One decade is equal
to 20 dB; V
slope in V/dB.
As noted in Equation 3, the V
This is the correct slope polarity to control the gain of many
power amplifiers and other VGAs in a negative feedback
configuration. Because both the slope and intercept vary
slightly with frequency, refer to Table 1 for application-specific
values for the 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
known to convert voltages to corresponding power levels.
Beginning with the definitions of dBm and dBV,
When Equation 9 is expanded
and given Equation 10, Equation 11 can be rewritten as
For example, P
in terms of dBm (decibels referred to 1 mW), in a 50 Ω system is
OUT(MAX)
OUT(MAX)
V
(X × V
(4 × 0.5 V) < V
2 V < V
P (dBm) = 10 × log
V (dBV) = 20 × log
P (dBm) = 20 × log
P (dBm) = V (dBV) − 10 × log
P
7 dBV − 10 × log
− 10 × log
INTERCEPT
OUT(MIN)
= (2.1 V × X) when X < (V
= (V
SLOPE/DEC
OFFSET
OUT
< V
POS
(dBm) = V
INTERCEPT
10
< 4.6 V
) < V
(Z
− 400 mV) when X ≥ (V
OUT
/20 = V
POS
0
OUT
× 1 mW) =
< V
OUT
10
= 5 V,
for a sinusoidal input signal, expressed
(50 × 10
< (2.1 V × 4)
10
10
10
OUT(MAX)
< (V
INTERCEPT
(V
(V
(V
SLOPE/dB
rms
rms
rms
POS
OUT
2
) − 10 × log
/1 V
/(Z
−3
− 400 mV)
) = 20 dBm
represents the output voltage
voltage has a negative slope.
(dBV)
0
OUT
rms
× 1 mW))
10
POS
(Z
)
SLOPE/DECADE
can swing 2.6 V, and the
− 400 mV)/(2.1 V)
0
× 1 mW)
POS
10
(Z
− 400 mV)/ (2.1 V)
0
× 1 mW)
represents the
0
, must be
(10)
(11)
(12)
(13)
Rev. B | Page 15 of 24
(6)
(7)
(8)
(9)
For further information on the intercept variation dependence
upon waveform, refer to the AD8313 and
DEVICE CALIBRATION AND ERROR CALCULATION
The measured transfer function of the AD8318 at 2.2 GHz is
shown in Figure 32. The figure shows plots of both output
voltage vs. input power and calculated log conformance error
vs. input power.
As the input power varies from −65 dBm to 0 dBm, the output
voltage varies from 2 V to about 0.5 V.
Because the slope and intercept vary from device to device,
board-level calibration is performed to achieve high accuracy.
The equation can be rewritten for output voltage, from the
Measurement Mode section, using an intercept expressed
in dBm.
In general, the calibration is performed by applying two known
signal levels to the AD8318 input and measuring the corre-
sponding output voltages. The calibration points are generally
chosen to be within the linear-in-dB operating range of the
device (see Figure 32). Calculation of the slope and intercept is
done by:
Once the slope and intercept are calculated, an equation can be
written to allow calculation of an (unknown) input power based
on the output voltage of the detector.
V
V
OUT2
OUT1
V
Slope = (V
Intercept = P
P
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
IN
OUT
–65 –60 –55
(unknown) = V
VOUT
SLOPE = (V
INTERCEPT = P
ERROR (dB) = (V
= Slope × (P
IDEAL
OUT1
Figure 32. Transfer Function at 2.2 GHz
P
OUT1
= SLOPE × (P
IN2
IN1
− V
IN1
− V
– V
OUT
–45 –40 –35 –30 –25 –20 –15
IN
– (V
OUT2
OUT2
OUT
× VOUT
– Intercept)
OUT1
OUT1
)/(P
(measured)/Slope + Intercept
IN
)/(P
/Slope
– INTERCEPT)
/SLOPE)
P
IN1
IDEAL
IN
IN1
(dBm)
– P
)/SLOPE
− P
IN2
)
IN2
)
AD8307
P
IN1
V
V
V
ERROR +25°C
ERROR –40°C
ERROR +85°C
OUT
OUT
OUT
–5
+25°C
–40°C
+85°C
data sheets.
0
AD8318
5
INTERCEPT
2.5
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
(14)
(15)
(16)
(17)