AD608(RevB) Ver la hoja de datos (PDF) - Analog Devices
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AD608 Datasheet PDF : 12 Pages
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AD608
THEORY OF OPERATION
The AD608 (Figure 20) consists of a mixer followed by a loga-
rithmic IF strip with RSSI and hard limited outputs. Each sec-
tion will be described below.
Mixer
The mixer is a doubly-balanced modified Gilbert cell mixer. Its
maximum input level for linear operation is ± 56.2 mV regard-
less of the impedance across the mixer’s inputs, or –15 dBm for
a 50 Ω input termination. The input impedance of the mixer
can be modeled as a simple parallel RC network; the values ver-
sus frequency are listed in Table I. The bandwidth from the RF
input to the IF output at MXOP pin is –1 dB at 30 MHz and
then falls off rapidly (Figure 4).
Mixer Gain
The mixer’s conversion gain is the product of its transcon-
ductance and the impedance seen at pin MXOP. For a 330 Ω
parallel-terminated filter at 10.7 MHz, the load impedance is
165 Ω, the gain is 24 dB, and the output is 15.85 × 56.2 mV, or
± 891 mV, centered on the midpoint of the supply voltage. For
other load impedances, the expression for the gain in dB is
GdB = 20 log10 (0.0961 RL )
The mixer’s gain can be increased or decreased by changing RL,
the load impedance at pin MXOP. The limitations on the
mixer’s gain are the ± 6 mA maximum output current at MXOP
and the maximum allowable voltage swing at pin MXOP, which
is ± 1.0 V for a 3 V supply or 5 V supply.
24dB MIXER GAIN
RFHI 5
RF INPUT
–95 TO
–15dBm 1.
RFLO 6
±6mA MAX OUTPUT
(±890mV INTO 165Ω)
MIXER
≈
LO
PREAMP
MXO P
7
BPF
DRIVER
VMID
8
LOIP
MID-SUPPL Y
IF BIAS
BIAS
VPS1 COM1
COM2
PRUP
1
2
3
4
16
+2.7V TO 5.5V LO INPUT CMOS LOGIC
–16dBm
INPUT
3dB NOMINAL
INSERTION LOSS
IF INPUT
–75dBm TO
+15dBm 2.
10.7M Hz
BANDPASS
FILTER
IFHI
9
330Ω
100nF
330Ω
10nF
10
100Ω
IFLO
13
18nF
FDBK
110dB LIMITER GAIN
90dB RSSI
7 FULL-WAVE
RECTIFIER CELLS
5-STAGE IF AMPLIFIER
(16dB PER STAGE)
≈
RSSI
RSSI OUTPUT
11 20mV/dB
0.2V TO 1.8V
COM3 12
VPS2
14 +2.7V TO 5.5V
LMOP
15 LIMITER
OUTPUT
FINAL
400mVp-p
LIMITER
AD608
±50µ A
NOTES:
1. –15dBm = ±56mV MAX FOR LINEAR OPERATION
2. 39.76mV RMS TO 396.6mV RMS FOR ±1 dB
RSSI ACCURACY
Figure 20. Functional Block Diagram
Table I. Mixer Input Impedance vs. Frequency
Frequency
(MHz)
Resistance
(Ohms)
Capacitance
(pF)
45
2800
3.1
70
2600
3.1
100
1800
3.1
200
1200
3.1
300
760
3.2
400
520
3.4
500
330
3.6
REV. B
–7–
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