Datasheet

Data Sheet AD8432
Rev. C | Page 19 of 32
Table 5. Gain Setting Using a Pin-Strapping Technique and −3 dB Bandwidth for Each Gain Configuration
Differential
Gain (dB)
Single
Gain (dB)
−3 dB
BW (MHz) RG1 (Ω) RG2 (Ω) RG3 (Ω) RG4 (Ω) RG5 (Ω) RG6 (Ω) RG7 (Ω)
12.04 6.02 200 12 12 Connect
GMH to GOH
Connect
GOH to OPH
24 Connect
GML to GOL
Connect
GOL to OPL
18.06 12.04 90 12 12 24 Connect
GOH to OPH
24 24 Connect
GOL to OPL
21.58 15.56 50 12 12 Connect
GMH to GOH
48 24 Connect
GML to GOL
48
24.08 18.06 32 12 12 24 48 24 24 48
The single-ended gain from INH to OPH (see Figure 65) is
defined as
G1
G4G3G2G1
INHOPH
R
RRRR
G
+++
=
−
The single-ended gain from INH to OPL is defined as
G1
G7G6G5
INHOPL
R
RRR
G
++
−=
−
The values of the seven gain resistors were chosen so that both
single-ended gains are equal. For example, to set a gain of
12.04 dB (G = ×4) differentially, the gain from INH to each
output (OPH, OPL) should be 6.02 dB (G = ×2).
INH to OPH: For R
G1
= R
G2
= R
G
, then
2
2
=
×
=
+
=
−
G
G
G1
G2G1
INHOPH
R
R
R
RR
G
INH to OPL: For R
G1
= R
G
and R
G5
= 2 × R
G
, then
2
2
−=
×
−=−=
−
G
G
G1
G5
INHOPL
R
R
R
R
G
ACTIVE INPUT RESISTANCE MATCHING
The AD8432 reduces noise and optimizes signal power transfer
by using active input termination to perform signal source
resistance matching.
The primary purpose of input impedance matching is to optimize
the input signal power transfer. With resistive termination, the
input noise increases due to the thermal noise of the terminating
resistor and the increased contribution of the input voltage
noise generator of the LNA. With active impedance matching,
however, the contributions of both are smaller than they are for
resistive termination by a factor of 1/(1 + ½ LNA) gain. The
noise figure (NF) for the three terminating schemes is shown in
Figure 67.
LNA
R
IN
V
IN
V
OUT
R
S
INH
UNTERMINATED
LNA
R
IN
V
IN
V
OUT
R
S
R
S
INH
RESISTIVE
TERMINATION
LNA
R
IN
V
IN
V
OUT
R
S
R
FB
INH
ACTIVE
IMPEDANCE MATCH
08341-009
Figure 66. Input Resistance Matching
To achieve this active impedance match, connect a feedback
resistor, R
FB
, between the INH and OPL (see Figure 66). R
IN
is
given in Equation 1, where G/2 is the single-ended gain.
2
1
G
R
R
FB
IN
+
=
(1)
In addition, to further reduce the input resistance, there is an
internal resistance of 6.2 kΩ in parallel with the source resistance,
such that
INTERNAL
FB
IN
R
G
R
R
2
1+
=
(2)
Equation 3 should be used to calculate R
FB
accurately for a desired
input resistance and single-ended gain. Refer to Table 6 for
calculated results for R
FB
for several input resistance and gain
combinations.
kΩ2.6,
1
2
1
=
−






+
=⇒
INTERNAL
INTERNAL
IN
IN
FB
R
R
R
G
R
R
(3)