AD8318
Table 5. Recommended T
ADJ
Resistors
Rev. 0 | Page 14 of 24
Frequency
Recommended T
ADJ
900 MHz
500
1.9 MHz
500
2.2 GHz
500
3.6 GHz
51
5.8 GHz
1 k
8 GHz
500
TEMPERATURE SENSOR
The AD8318 internally generates a voltage that is proportional-
to-absolute-temperature (V
PTAT
). The V
PTAT
voltage is multiplied
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 28.
0
1k
4k
CMIP
INTERNAL
VPSI
TEMP
Figure 28. Temp Sensor Interface
The internal pull-down resistance is 5 k. The temperature
sensor has a slope of +2 mV/
°
C.
The temp sensor output will vary with output current due to
increased die temperature. Output loads less than 1 k
will draw
enough current from the output stage causing this increase to
occur. An output current of 10 mA will result in the voltage on
the temp sensor to increase by 1.5
°
C, or ~3 mV.
To get the best precision from the temperature sensor, ensure
that supply current to AD8318 remains fairly constant (i.e., no
heavy load drive).
MEASUREMENT MODE
When the V
OUT
voltage or a portion of the V
OUT
voltage is fed
back to VSET, the device operates in measurement mode. As
seen in Figure 29, the AD8318 has an offset voltage, a negative
slope, and a V
OUT
measurement intercept greater than its input
signal range.
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
0
–0.5
–1.0
–1.5
–2.0
–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
P
IN
(dBm)
0
5
INTERCEPT
0
E
V
O
ERROR 25
°
C
V
OUT
25
°
C
OF SLOPE AND INTERCEPT
Figure 29. Typical Output Voltage vs. Input Signal
The output voltage versus input signal voltage of the
AD8318 is linear-in-dB over a multidecade range. The
equation for this function is of the form
V
OUT
=
X
×
V
SLOPE/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
INTERCEPT
is expressed in V
rms
.
V
SLOPE/DEC
is nominally –500 mV/decade or 25 mV/dB.
V
INTERCEPT
expressed in dBV is the x-axis intercept of the
linear-in-dB transfer function shown in Figure 29.
V
INTERCEPT
is +7 dBV (+20 dBm, re: 50
or 2.239 V
rms
)
for a
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 may 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
OUT
is X × V
OFFSET
. An offset
voltage, V
OFFSET
, of 0.5 V is internally added to the detector
signal.
V
OUT(MIN)
= (
X
×
V
OFFSET
)
The maximum output voltage is 2.1 V × X, and cannot
exceed 400 mV below the positive supply.