AD8309 Analog Devices, AD8309 Datasheet - Page 13

• Manufacturer Part Number: AD8309
• Description: 5 - 500 Mhz, 100 DB Demodulating Logarithmic Amplifier With Limiter Output
• Manufacturer: Analog Devices

Because of the very high gain bandwidth product of this ampli-
fier considerable care must be exercised in using the limiter
outputs. The minimum necessary bias current and voltage
swings should be used. These outputs are best utilized in a fully-
differential mode. A flux-coupled transformer, a balun, or an
output matching network can be selected to transform these
voltages to a single-sided form. Equal load resistors are recom-
mended, even when only one output pin is used, and these
should always be returned to the same well decoupled node on
the PC board. When the AD8309 is used only to generate an
RSSI output, the limiter should be completely disabled by omit-
ting R
RSSI Output Interface
The outputs from the ten detectors are differential currents,
having an average value that is dependent on the signal input
level, plus a fluctuation at twice the input frequency. The cur-
rents are summed at the internal nodes LGP and LGN shown in
Figure 29. A further current I
the intercept to –108 dBV, by raising the RSSI output voltage
for zero input, and to provide temperature compensation , re-
sulting 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 con-
trolled by a separate feedback stage, having a tightly controlled
transconductance. A small uncertainty in the log slope and
intercept remains (see Specifications); the intercept may be
adjusted (see Applications).
REV. B
DETECTOR
OUTPUTS
LIMITER STAGE
SUMMED
FROM FINAL
I
T
LIM
Figure 29. Simplified RSSI Output Interface
V
and strapping LMHI and LMLO to VPS2.
LOG
TRANSCONDUCTANCE
1.3k
1.3k
DETERMINES SLOPE
Figure 28. Limiter Output Interface
2.6k
VPS2
LGP
LGN
1.3k
250 s
1.3k
1.3k
3.3k
CURRENT
MIRROR
TC
1.3k
is added to LGP, to position
125 A
3.3k
R
LMHI
LIM
ON DEMAND
Q3
Q1
4e
I
LMLO
SOURCE
LMDR
>50mA
3.5pF
FIXED
C1
I
1mA
Q2
4e
SINK
OA
ZERO-TC
400mV
VLOG
20mV/dB
FLTR
VPS2
COMM
COM1
C
F
–13–
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
are:
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 are
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 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 AD8309
The AD8309 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 which 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
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 imped-
ance connection to the common pins, for the decoupling
capacitor(s) used at VPS1 and VPS2, and at the output ground.
It is inadvisable to assume that any ground plane is an equipo-
tential, however, and neither of the signal inputs should be ac-
coupled directly to it, but kept separate, being returned instead
to the “low” associated with the source. This requires isolating
the “low”’ side of an input connector with a small resistance to
the ground plane. Note that COM2 is a special ground pin
serving just the RSSI output.
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 may be increased to improve the isolation and lower
dissipation in the IC. However, since VPS2 supports the RSSI
C
F
12 7 10
.
f
LP
at 150 kHz and 18
–
10
– .
3 5
pF
LP
;
or greater, and at any tempera-
, is nominally 3.5 MHz. This is
f
LP
C
12 7 10
at very high frequencies.
F
.
F
3 5
, between FLTR
.
AD8309
pF
at low frequen-
6
LIM
and
(7)
LP