Datasheet

AD8432 Data Sheet
Rev. C | Page 20 of 32
08341-267
0
1
2
3
4
5
6
7
8
10050 1k
NOISE FIGURE (dB)
R
S
(Ω)
(SIMULATED RESULTS)
ACTIVE IMPEDANCE
MATCH
UNTERMINATED
RESISTIVE TERMINATION
(R
S
= R
IN
)
Figure 67. Noise Figure vs. R
S
for Resistive, Active Match, and
Unterminated Inputs
08341-268
0
2
6
4
8
10
12
14
16
18
10050 1k
NOISE FIGURE (dB)
R
S
(Ω)
(SIMULATED RESULTS)
R
IN
= UNTERMINATED
R
IN
= 50Ω
R
IN
= 75Ω
R
IN
= 100Ω
R
IN
= 200Ω
R
IN
= 1kΩ
Figure 68. Noise Figure vs. R
S
for Various Values of R
IN
, Actively Matched
The user must determine the level of matching accuracy desired
and adjust R
FB
accordingly. The R
FB
and C
FB
network presents a
load to OPL that OPH does not see. The user can add an identical
load on OPH to improve slightly the distortion caused by this
imbalance.
There is a feedback capacitor (C
FB
) in series with R
FB
(see
Figure 65) because the dc levels of the positive output and the
positive input are different. At higher frequencies, the value
of the feedback capacitor must be considered.
The unterminated bandwidth (R
FB
= ∞) is 200 MHz. The AD8432
has a low input-referred voltage noise of 0.85 nV/√Hz at the
lowest gain, 12.04 dB (unterminated configuration). To achieve
such low noise, the dual amplifier consumes 24 mA, resulting in
a power consumption of 120 mW.
Table 6. Feedback Resistance for Several R
IN
and Gain Combinations
Desired R
IN
(Ω) Differential Gain (V/V)
Single-Ended Gain,
G/2 (V/V)
Exact R
FB
(Ω),
Equation 2
R
FB
(Ω), 1% Standard Value
Actual R
IN
(Ω),
Equation 2
50 4 2 151.2 150 49.6
75 4 2 227.8 226 74.4
100 4 2 304.9 301 98.7
200 4 2 620 619 199.7
1 k 4 2 3.58 k 3.57 k 998.4
50 8 4 252 250 49.6
100 8 4 508.2 511 100.5
50 12 6 352.9 357 50.6
100 12 6 711.5 715 100.5
50 16 8 453.7 453 49.9
100 16 8 914.8 909 99.4