ad8318acpz-wp Analog Devices, Inc., ad8318acpz-wp Datasheet - Page 14

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

AD8318
Table 5 lists recommended resistors for various frequencies.
These resistors provide the best overall temperature drift based
on measurements of a diverse population of devices.
The relationship between output temperature drift and frequency
is nonlinear and is not easily modeled. Experimentation is
required to choose the correct R
not listed in Table 5.
Table 5. Recommended R
Frequency
900 MHz
1.9 MHz
2.2 GHz
3.6 GHz
5.8 GHz
8 GHz
TEMPERATURE SENSOR
The AD8318 internally generates a voltage that is proportional
to absolute-temperature (V
by a factor of 5, resulting in a 2 mV/°C output at the TEMP pin.
The output voltage at 27°C is typically 600 mV. An emitter
follower drives the TEMP pin, as shown in Figure 30.
The internal pull-down resistance is 5 kΩ. The temperature
sensor has a slope of 2 mV/°C.
The temperature sensor output varies with output current due
to increased die temperature. Output loads less than 1 kΩ draw
enough current from the output stage causing this increase to
occur. An output current of 10 mA results in the voltage on the
temperature sensor to increase by 1.5°C, or ~3 mV.
Best precision from the temperature sensor is obtained when
the supply current to AD8318 remains fairly constant, that is,
no heavy load drive.
INTERNAL
Figure 30. Temp Sensor Interface
TADJ
PTAT
). The V
Resistors
Recommended R
500 Ω
500 Ω
500 Ω
51 Ω
1 kΩ
500 Ω
TADJ
4kΩ
1kΩ
resistor at frequencies
CMIP
VPSI
PTAT
voltage is multiplied
TEMP
TADJ
Rev. B | Page 14 of 24
MEASUREMENT MODE
When the VOUT voltage, or a portion of the VOUT voltage, is
fed back to VSET, the device operates in measurement mode.
As shown in Figure 31, the AD8318 has an offset voltage, a
negative slope, and a VOUT measurement intercept greater
than its input signal range.
The output voltage vs. input signal voltage of the AD8318 is
linear-in-dB over a multidecade range. The equation for this
function is
where:
X is the feedback factor in V
V
V
nominally −25 mV/dB.
V
in-dB transfer function shown in Figure 31.
V
sinusoidal input signal.
The slope of the transfer function can be increased to
accommodate various converter mV per dB (LSB per dB)
requirements. However, increasing the slope can reduce the
dynamic range. This is due to the limitation of the minimum
and maximum output voltages, determined by the chosen
scaling factor X.
The minimum value for V
V
output signal.
INTERCEPT
SLOPE/DEC
INTERCEPT
INTERCEPT
OFFSET
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
V
V
0
–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
OUT
OUT(MIN)
, is equal to 0.5 V and is internally added to the detector
is nominally −500 mV/decade and V
, expressed in dBV, is the x-axis intercept of the linear-
is expressed in V
is 7 dBV (20 dBm, re: 50 Ω or 2.239 V
= X × V
= X × V
Figure 31. Typical Output Voltage vs. Input Signal
= (X × V
SLOPE/DEC
SLOPE/dB
OF SLOPE AND
CALCULATION
OFFSET
INTERCEPT
RANGE OF
× 20 × log
× log
OUT
rms
P
V
ERROR 25°C
)
IN
SET
OUT
.
(dBm)
is X × V
10
= V
25°C
(V
10
IN
OUT
(V
/V
/X.
OFFSET
IN
INTERCEPT
/V
INTERCEPT
. The offset voltage,
0
SLOPE/dB
)
rms
5
) for a
)
10
is
15
INTERCEPT
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
(3)
(4)
(5)