AD8312_05 AD [Analog Devices], AD8312_05 Datasheet - Page 13

• Manufacturer Part Number: AD8312_05
• Description: 50 MHz to 3.5 GHz, 45 dB RF Detector
• Manufacturer: AD [Analog Devices]

APPLICATIONS
BASIC CONNECTIONS
Figure 22 shows the basic connections for measurement mode.
A supply voltage of 2.7 V to 5.5 V is required. The supply to the
VPOS pin should be decoupled with a low inductance 0.1 µF
surface-mount ceramic capacitor. A series resistor of about 10 Ω
may be added; this resistor slightly reduces the supply voltage to
the AD8312 (maximum current into the VPOS pin is
approximately 5.7 mA). Its use should be avoided in
applications where the power supply voltage is very low (that is,
2.7 V). A series inductor provides similar power supply filtering
with minimal drop in supply voltage.
The AD8312 has an internal input coupling capacitor. This
eliminates the need for external ac coupling. In this example, a
broadband input match is achieved by connecting a
52.3 Ω resistor between RFIN and ground. This resistance
combines with the internal input impedance of approximately
3 kΩ to give an overall broadband input resistance of 50 Ω.
Several other coupling methods are possible; these are
described in the Input Coupling Options section.
The measurement mode is selected by connecting VSET to
VOUT, which establishes a feedback path and sets the
logarithmic slope to its nominal value. The peak voltage range
of the measurement extends from −49 dBm to 0 dBm at
0.9 GHz and is only slightly less at higher frequencies up to
2.5 GHz. At a slope of 21.0 mV/dB, this would amount to an
output span of 1.029 V. Figure 23 shows the transfer function
for VOUT at a supply voltage of 2.7 V and an input frequency of
900 MHz.
The load resistance on VOUT should not be lower than 4 kΩ so
that the full-scale output can be generated with the limited
available current of 1 mA maximum. Figure 23 shows the
logarithmic conformance under the same conditions.
VOUT
Figure 22. Basic Connections for Operation in Measurement Mode
V
S
(SEE TEXT)
OPTIONAL
0.1µF
1
2
3
VPOS
VOUT
VSET
(SEE TEXT)
OPTIONAL
AD8312
C
F
COMM
CFLT
RFIN
6
5
4
52.3Ω
INPUT
Rev. 0| Page 13 of 20
TRANSFER FUNCTION IN TERMS OF SLOPE AND
INTERCEPT
The transfer function of the AD8312 is characterized in terms
of its slope and intercept. The logarithmic slope is defined as the
change in the RSSI output voltage for a 1 dB change at the input.
For the AD8312, the slope is nominally 20 mV/dB. Therefore, a
10 dB change at the input results in a change at the output of
approximately 200 mV. Figure 23 shows the range over which
the device maintains its constant slope. The dynamic range can
be defined as the range over which the error remains within a
certain band, usually ±1 dB or ±3 dB. In Figure 23, for example,
the ±1 dB dynamic range is approximately 51 dB (from
−49 dBm to +2 dBm).
The intercept is the point at which the extrapolated linear
response would intersect the horizontal axis (see Figure 23).
Using the slope and intercept, the output voltage can be
calculated for any input level within the specified input range by
where:
VOUT is the demodulated and filtered RSSI output.
V
P
reference level (dBm in this case).
P
the same reference level.
For example, at an input level of −27 dBm, the output voltage is
IN
O
SLOPE
is the logarithmic intercept, expressed in decibels relative to
is the input signal, expressed in decibels relative to some
1.2
1.0
0.8
0.6
0.4
0.2
VOUT
VOUT
0
–60
is the logarithmic slope, expressed in V/dB.
=
=
Figure 23. VOUT and Log Conformance Error vs.
–50
V
0.020
SLOPE
Input Level vs. Input Level at 900 MHz
V
RT = 52.3Ω
S
= 2.7V
±1dB DYNAMIC RANGE
V/dB
–40
×
(
INTERCEPT
P
IN
×
±3dB DYNAMIC RANGE
−
[
−
–30
P
P
27
O
IN
)
dBm
(dBm)
–20
−
(
−
50
–10
dBm
)
]
=
0
0.46
AD8312
V
10
3
2
1
0
–1
–2
–3