AD8314 Datasheet, PDF (10/16 Pages) Analog Devices – 100 MHz-2500 MHz 45 dB RF Detector/Controller

AD8314
1.2
3
V_S = 3V
R_T = 52.3⍀
1.0
2
؎1dB DYNAMIC RANGE
0.8
1
0.6
0
0.4
–1
0.2
–2
؎3dB DYNAMIC RANGE
INTERCEPT
0
–3
–70 –60
–50
–40
–30
–20
–10
0
(–47dBm)
INPUT AMPLITUDE – dBV
(+3dBm)
Figure 29. V_UP and Log Conformance Error vs. Input
Level vs. Input Level at 900 MHz
Transfer Function in Terms of Slope and Intercept
The transfer function of the AD8314 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 AD8314, slope is nominally 21.5 mV/dB. So a 10 dB
change at the input results in a change at the output of approxi-
mately 215 mV. The plot of Log-Conformance (Figure 29) 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 29, for example, the ± 1 dB dynamic range is approxi-
mately 50 dB (from –13 dBV to –63 dBV).
The intercept is the point at which the extrapolated linear
response would intersect the horizontal axis (Figure 29). Using
the slope and intercept, the output voltage can be calculated for
any input level within the specified input range using the equation:
V_UP = V_SLOPE × (P_IN – P_O)
where V_UP is the demodulated and filtered RSSI output, V_SLOPE
is the logarithmic slope, expressed in V/dB, P_IN is the input sig-
nal, expressed in decibels relative to some reference level (either
dBm or dBV in this case) and P_O is the logarithmic intercept,
expressed in decibels relative to the same reference level.
For example, at an input level of –40 dBV (–27 dBm), the
output voltage will be
V_OUT = 0.020 V/dB × (–40 dBV – (–63 dBV )) = 0.46 V
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 pre-
cise, 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), will not follow the rms value
exactly. Since most users specify RF signals in terms of power—
more specifically, in dBm/50 Ω—we use both dBV and dBm 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
will be 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_F (see
Figure 28). The video bandwidth is related to C_F by the equation
Video Bandwidth =
1
2 π × 4.4 kΩ × (10 pF + C _F )
Operating in Controller Mode
Figure 30 shows the basic connections for operation in the con-
troller mode and Figure 31 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 will be a DAC). V_DN will rail
high (2.2 V on a 3.3 V supply, 1.9 V on a 2.7 V supply) when
the applied power is less than the value corresponding to the set-
point voltage. When the input power slightly exceeds this value,
V_DN would, in the absence of the loop via the power amplifier
gain pin, decrease rapidly toward ground. In the closed loop,
however, the reduction in V_DN causes the power amplifier to re-
duce 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 increas-
ing voltage from V_DN will tend to increase gain. The output
swing and current sourcing capability of V_DN are shown in
Figures 19 and 20.
52.3⍀
0.1␮F
INPUT
VSET
1 RFIN
VPOS 8
V_S
2 ENBL
V DN 7
AD8314
3 VSET
V UP 6
4 FLTR
COMM 5
V_S
VDN
C_F
Figure 30. Basic Connections for Operation in Controller
Mode
DIRECTIONAL
COUPLER
52.3⍀
POWER
AMPLIFIER
RF INPUT
C_F
GAIN
CONTROL
VOLTAGE
V UP V DN
FLTR
VSET
AD8314
RFIN
DAC
Figure 31. Typical Controller Mode Application
–10–
REV. 0