ad8432acpz Analog Devices, Inc., ad8432acpz Datasheet - Page 9

• Manufacturer Part Number: ad8432acpz
• Description: Dual Lna With Selectable Gain And Input Impedance
• Manufacturer: Analog Devices, Inc.

Preliminary Technical Data
The single-ended gain from INHx to OPLx is defined as
Again, different gain configurations can be realized by
externally shorting the GMLx, GOLx, and OPLx pins.
The values of the seven gain resistors are chosen so that both
single-ended gains are equal. For example, to set a gain G = 4
(12.04 dB) differentially, the gain from INHx to each output
(OPHx, OPLx) should be ×2 (6.02 dB).
INHx to OPHx: if R
resistor value, then
INHx to OPLx: if R
By shorting the different external pins, four gain (4, 8, 12, and
16) configurations can be realized. The gains are set by first
selecting a value for R
other resistors in terms of R
indicated pins.
ACTIVE INPUT RESISTANCE MATCHING
The AD8432 reduces noise by using active input termination to
perform signal source resistance matching. To achieve this,
connect a feedback resistor R
(see Figure 10). R
single-ended gain.
G
G
G
R
OPL
OPH
OPL
IN
=
−
−
−
INH
INH
V
V
V
INH
1
IN
IN
IN
R
+
FB
=
=
=
G
2
−
−
Figure 10. Input Resistance Matching
R
IN
R
R
R
R
R
R
G
S
S
S
G1
is given in Equation 1, where G/2 is the
1
G
G1
G
G
R
5
1
+
5
G
= R
G1
R
R
R
= R
+
1
R
=
IN
IN
IN
and then determining the values of the
R
G
R
−
G
2
G
G
G
* 2
and R
6
1
and R
=
G1
R
+
FB
R
, and accordingly shorting the
* 2
R
G
R
S
INH
INH
INH
R
G
between the INHx and OPLx
G
R
G
G5
7
R
G2
=
G
FB
= 2 × R
IMPEDANCE MATCH
−
= R
LNA
LNA
LNA
=
UNTERMINATED
2
TERMINATION
2
RESISTIVE
G
ACTIVE
, where R
G
, then
V
V
V
OUT
OUT
OUT
G
= any
Rev. PrA | Page 9 of 12
(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
Equation 3 should be used to calculate accurately R
desired input resistance and single-ended gain. Refer to Table 5
for calculated results for R
gain combinations.
There is a feedback capacitor (C
the dc levels of the positive output and the positive input are
different. At higher frequencies, the value of the feedback
capacitor needs to be considered. The user must determine the
level of matching accuracy and adjust R
enhances the stability at higher frequencies and prevents high
peaking especially for the lowest gain, 12.04 dB. (See the
Applications Information section for the selection of C
The unterminated bandwidth (R
has a low input referred voltage noise of 0.8 nV/√Hz at the
lowest gain, 12.04 dB (unterminated configuration). To achieve
such low noise, the amplifier consumes 21.4 mA, resulting in a
power consumption of 107 mW.
The primary purpose of the input impedance matching is to
improve the system transient response. With resistive termination,
the input noise increases due to the thermal noise of the matching
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 would
be for resistive termination by a factor of 1/(1 + LNA Gain).
⇒
R
IN
R
=
FB
1 +
R
=
FB
1
G
2
R
−
IN
R
R
⎛ +
⎜
⎝
INTERNAL
INTERNAL
1
R
IN
G
2
⎞
⎟
⎠
FB
,
for several input resistance and
R
INTERNAL
FB
FB
) in series with R
= ∞) is 200 MHz. The LNA
FB
=
6
accordingly. C
2 .
k
Ω
FB
FB
AD8432
because
for a
SH
SH
).
also
(2)
(3)