AD8432
Data Sheet
Rev. C | Page 20 of 32
08341-
267
0
1
2
3
4
5
6
7
8
100
50
1k
N
OIS
E
FIGU
R
E
(
dB
)
RS ()
(SIMULATED RESULTS)
ACTIVE IMPEDANCE
MATCH
UNTERMINATED
RESISTIVE TERMINATION
(RS = RIN)
Figure 67. Noise Figure vs. RS for Resistive, Active Match, and
Unterminated Inputs
08341-
268
0
2
6
4
8
10
12
14
16
18
100
50
1k
N
OIS
E
FIGU
R
E
(
dB
)
RS ()
(SIMULATED RESULTS)
RIN = UNTERMINATED
RIN = 50
RIN = 75
RIN = 100
RIN = 200
RIN = 1k
Figure 68. Noise Figure vs. RS for Various Values of RIN, Actively Matched
The user must determine the level of matching accuracy desired
and adjust RFB accordingly. The RFB and CFB 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 (CFB) in series with RFB (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 (RFB = ∞) 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 RIN and Gain Combinations
Desired RIN (Ω)
Differential Gain (V/V)
Single-Ended Gain,
G/2 (V/V)
Exact R
FB (Ω),
Equation 2
RFB (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