AD8314 Analog Devices, AD8314 Datasheet - Page 13

• Manufacturer Part Number: AD8314
• Description: 100 MHz to 2.7 GHz, 45 dB RF Detector/Controller
• Manufacturer: Analog Devices

The intercept is the point at which the extrapolated linear
response would intersect the horizontal axis (see Figure 32).
Using the slope and intercept, the output voltage can be
calculated for any input level within the specified input range by
where:
V
V
P
reference level (either dBm or dBV in this case).
P
the same reference level.
For example, at an input level of −40 dBV (−27 dBm), the
output voltage is
dBV VS. dBm
The most widely used convention in RF systems is to specify
power in dBm, that is, decibels above 1 mW in 50 Ω. Specification
of log amp input levels in terms of power is strictly a concession
to popular convention; they do not respond to power (tacitly
power absorbed at the input), but to the input voltage. The use
of dBV, defined as decibels with respect to a 1 V rms sine wave,
is more precise, although this is still not unambiguous because
waveform is also involved in the response of a log amp, which,
for a complex input (such as a CDMA signal), does not follow
the rms value exactly. Since most users specify RF signals in
terms of power (more specifically, in dBm/50 Ω), both dBV and
dBm are used in specifying the performance of the AD8314
showing equivalent dBm levels for the special case of a
50 Ω environment. Values in dBV are converted to
dBm re 50 Ω by adding 13.
FILTER CAPACITOR
The video bandwidth of both V_UP and V_DN is
approximately 3.5 MHz. In CW applications where the input
frequency is much higher than this, no further filtering of the
demodulated signal is required. Where there is a low frequency
modulation of the carrier amplitude, however, the low-pass
corner must be reduced by the addition of an external filter
capacitor, C
to C
IN
O
UP
SLOPE
is the logarithmic intercept, expressed in decibels relative to
is the input signal, expressed in decibels relative to some
is the demodulated and filtered RSSI output.
F
V
V
Video
by
is the logarithmic slope, expressed in V/dB.
UP
OUT
= V
= 0.020 V/dB × [−40 dBV − (−63 dBV)] = 0.46 V
Bandwidth
F
SLOPE
(see Figure 31). The video bandwidth is related
× (P
IN
=
− P
2
π
O
)
×
13
kΩ
×
1
(
3
5 .
pF
+
C
F
)
Rev. B | Page 13 of 20
OPERATING IN CONTROLLER MODE
Figure 33 shows the basic connections for operation in the
controller mode, and Figure 34 shows a block diagram of a
typical controller mode application. The feedback from V_UP
to VSET is broken and the desired setpoint voltage is applied to
VSET from the controlling source (often this is a DAC). V
rails high (2.2 V on a 3.3 V supply, and 1.9 V on a 2.7 V supply)
when the applied power is less than the value corresponding to
the setpoint voltage. When the input power slightly exceeds this
value, V
amplifier gain pin, decrease rapidly toward ground. In the
closed loop, however, the reduction in V
amplifier to reduce its output. This restores a balance between
the actual power level sensed at the input of the AD8314 and
the demanded value determined by the setpoint. This assumes
that the gain control sense of the variable gain element is
positive, that is, an increasing voltage from V_DN tends to
increase gain. The output swing and current sourcing capability
of V_DN are shown in Figure 22 and Figure 25.
DIRECTIONAL
COUPLER
The relationship between the input level and the setpoint
voltage follows from the nominal transfer function of the device
(V
1 V on VSET demands a power level of 0 dBm at RFIN. The
corresponding power level at the output of the power amplifier
is greater than this amount due to the attenuation through the
directional coupler.
UP
INPUT
VSET
vs. input amplitude, see Figure 4). For example, a voltage of
Figure 33. Basic Connections for Operation in Controller Mode
DN
would, in the absence of the loop via the power
Figure 34. Typical Controller Mode Application
52.3Ω
V
S
52.3Ω
1
2
3
4
RFIN
ENBL
VSET
FLTR
AD8314
C
C
F
F
AMPLIFIER
RFIN
FLTR
V_UP
COMM
VPOS
V_DN
V_UP
POWER
AD8314
V_DN
DN
8
7
6
5
causes the power
VSET
0.1µF
GAIN
CONTROL
VOLTAGE
AD8314
RF INPUT
V
V
DAC
S
DN
DN