AD8306 Analog Devices, AD8306 Datasheet - Page 9

• Manufacturer Part Number: AD8306
• Description: 5 - 400 Mhz, 100 DB High Precision Limiting - Logarithmic Amplifier
• Manufacturer: Analog Devices

REV. A
the intercept to –108 dBV, by raising the RSSI output voltage for
zero input, and to provide temperature compensation, resulting
in a stable intercept. For zero signal conditions, all the detector
output currents are equal. For a finite input, of either polarity,
their difference is converted by the output interface to a single-
sided voltage nominally scaled 20 mV/dB (400 mV per decade), at
the output VLOG (Pin 16). This scaling is controlled by a sepa-
rate feedback stage, having a tightly controlled transcon-
ductance. A small uncertainty in the log slope and intercept
remains (see Specifications); the intercept may be adjusted (see
Applications).
The RSSI output bandwidth, f
controlled by the compensation capacitor C1, which may be
increased by adding an external capacitor, C
(Pin 10) and VLOG (Pin 16). An external 33 pF will reduce f
to 350 kHz, while 360 pF will set it to 35 kHz, in each case with
an essentially one-pole response. In general, the relationships
(for f
Using a load resistance of 50
ture, the peak output voltage may be at least 2.4 V when using a
supply of 4.5 V, and at least 2.1 V for a 3 V supply, which is
consistent with the maximum permissible input levels. The incre-
mental output resistance is approximately 0.3
cies, rising to 1
The output is unconditionally stable with load capacitance, but
it should be noted that while the peak sourcing current is
over 100 mA, and able to rapidly charge even large capacitances,
the internally provided sinking current is only 1 mA. Thus, the
fall time from the 2 V level will be as long as 2 s for a 1 nF
load. This may be reduced by adding a grounded load resistance.
USING THE AD8306
The AD8306 exhibits very high gain from 1 MHz to over 1 GHz,
at which frequency the gain of the main path is still over 65 dB.
Consequently, it is susceptible to all signals, within this very
broad frequency range, that find their way to the input termi-
nals. It is important to remember that these are quite indistin-
guishable from the “wanted” signal, and will have the effect of
raising the apparent noise floor (that is, lowering the useful
dynamic range). Therefore, while the signal of interest may be
an IF of, say, 200 MHz, any of the following could easily be
larger than this signal at the lower extremities of its dynamic
LGP
LGN
DETECTOR
OUTPUTS
SUMMED
C
LP
I
T
F
in MHz) are:
Figure 23. Simplified RSSI Output Interface
VLOG
12 7 10
TRANSCONDUCTANCE
1.3k
DETERMINES SLOPE
.
f
LP
at 150 kHz and 18
–
250 s
10
1.3k
– .
3 5
3.3k
pF
CURRENT
MIRROR
LP
;
or greater, and at any tempera-
, is nominally 3.5 MHz. This is
125 A
f
LP
3.3k
ON DEMAND
at very high frequencies.
C
12 7 10
F
I
.
SOURCE
>50mA
3.5pF
FIXED
F
3 5
C1
I
1mA
SINK
, between FLTR
.
pF
at low frequen-
6
VLOG
20mV/dB
FLTR
VPS2
COMM
C
F
(1)
LP
–9–
range: a 60 Hz hum, picked up due to poor grounding tech-
niques; spurious coupling from digital logic on the same PC
board; a strong EMI source; etc.
Very careful shielding is essential to guard against such un-
wanted signals, and also to minimize the likelihood of instability
due to HF feedback from the limiter outputs to the input. With
this in mind, the minimum possible limiter gain should be used.
Where only the logarithmic amplifier (RSSI) function is re-
quired, the limiter should be disabled by omitting R
tying the outputs LMHI and LMLO directly to VPS2. A good
ground plane should be used to provide a low impedance con-
nection to the common pins, for the decoupling capacitor(s)
used at VPS1 and VPS2, and at the output ground. Note that
COM2 is a special ground pin serving just the RSSI output.
The four pins labeled PADL tie down directly to the metallic
lead frame, and are thus connected to the back of the chip. The
process on which the AD8306 is fabricated uses a bonded-wafer
technique to provide a silicon-on-insulator isolation, and there is
no junction or other dc path from the back side to the circuitry
on the surface. These paddle pins must be connected directly to
the ground plane using the shortest possible lead lengths to
minimize inductance.
The voltages at the two supply pins should not be allowed to
differ greatly; up to 500 mV is permissible. It is desirable to
allow VPS1 to be slightly more negative than VPS2. When the
primary supply is greater than 2.7 V, the decoupling resistors R1
and R2 (Figure 24) may be increased to improve the isolation
and lower the dissipation in the IC. However, since VPS2 sup-
ports the RSSI load current, which may be large, the value of
R2 should take this into account.
Basic Connections for Log (RSSI) Output
Figure 24 shows the connections required for most applications.
The AD8306 is enabled by connecting ENBL to VPS1. The
device is put into the sleep mode by grounding this pin. The
inputs are ac-coupled by C1 and C2, which normally should
have the same value (C
with a 52.3
resistance of 1000
50 . Alternatively an input matching network can be used (see
Input Matching section).
The 0.01 F coupling capacitors and the resulting 50
impedance give a high-pass corner frequency of around 600 kHz.
(1/
this corner frequency should be placed as high as possible, to
minimize the coupling of unwanted low frequency signals. In
SIGNAL
INPUTS
Figure 24. Basic Connections for RSSI (Log) Output
0.01 F
0.01 F
RC)), where C = (C1)/2. In high frequency applications,
C2
C1
ENABLE
0.1 F
10
R1
resistor that combines with the AD8306’s input
R
52.3
T
to give a broadband input impedance of
1
2
3
4
5
6
7
8
C
COM2
VPS1
PADL
INHI
INLO
PADL
COM1
ENBL
). The input is, in this case, terminated
AD8306
VLOG
LMLO
LMDR
PADL
PADL
VPS2
FLTR
LMHI
15
13
12
10
16
14
11
9
10
R2
0.1 F
AD8306
LIM
V
S
C
(OPTIONAL
SEE TEXT)
(2.7V TO 6.5V)
F
and
input
RSSI