THS4211 Datasheet, PDF (30/50 Pages) Texas Instruments – LOW-DISTORTION HIGH-SPEED VOLTAGE FEEDBACK AMPLIFIER

THS4211
THS4215
SLOS400E – SEPTEMBER 2002 – REVISED SEPTEMBER 2009 ................................................................................................................................... www.ti.com
Evaluation Fixtures, SPICE Models, and
Applications Support
Texas Instruments is committed to providing its
customers with the highest quality of applications
support. To support this goal, evaluation boards have
been developed for the THS4211 operational
amplifier. Three evaluation boards are available: one
THS4211 and one THS4215, both configurable for
different gains, and a third for untiy gain (THS4211
only). These boards are easy to use, allowing for
straightforward evaluation of the device. These
evaluation boards can be ordered through the Texas
Instruments web site at www.ti.com, or through your
local Texas Instruments sales representative.
Schematics for the evaluation boards are shown
below.
The THS4211/THS4215 EVM board shown in
Figure 95 through Figure 99 accommodates different
gain configurations. Its default component values are
set to give a gain of 2. The EVM can be configured
for unity-gain; however, it is strongly not
recommended. Evaluating the THS4211/THS4215 in
unity-gain using this EVM may cause the device to
become unstable. The stability of the device can be
controlled by adding a large resistor in the feedback
path, but performance is sacrificed. Figure 93 shows
the small-signal frequency response of the THS4211
with different feedback resistors in the feedback path.
Figure 94 is the small frequency response of the
THS4211 using the unity-gain EVM.
17
15
_ ^Rf
13
+
499 Ω
11
49.9 Ω
9
7
R_f = 100 Ω
5
3 ^{Rf = 200 Ω}
1
R_f = 0 Ω
R_f = 50 Ω
-1
-3 ^PIN ^{= -7 dBm}
-5 ^{VS = ±5 V}
10 M
100 M
1G
f - Frequency - Hz
10 G
Figure 93. Frequency Response vs Feedback
Resistor Using the EDGE #6439527 EVM
5
4
_
3
+
499 Ω
2
49.9 Ω
1
0
-1
-2
-3 ^{PIN = -7 dBm}
V_S = ±5 V
-4
100 k 1 M 10 M 100 M 1 G
f - Frequency - Hz
10 G
Figure 94. Frequency Response Using the
EDGE #6443547 G = +1 EVM
The frequency-response peaking is due to the lead
inductance in the feedback path. Each pad and trace
on a PCB has an inductance associated with it, which
in conjunction with the inductance associated with the
package may cause frequency-response peaking,
causing the device to become unstable.
In order to achieve the maximum performance of the
device, PCB layout is very critical. Texas Instruments
has developed an EVM for the evaluation of the
THS4211 configured for a gain of 1. The EVM is
shown in Figure 100 through Figure 104. This EVM is
designed to minimize peaking in the unity-gain
configuration.
Minimizing the inductance in the feedback path is
critical for reducing the peaking of the frequency
response in unity-gain. The recommended maximum
inductance allowed in the feedback path is 4 nH. This
inductance can be calculated using Equation 8:
ƪ ƫ L(nH)
+ Kȏ
ln
2ȏ
W)
T
)
0.223
W
)
ȏ
T
)
0.5
where
W = Width of trace in inches.
ȏ = Length of the trace in inches.
T = Thickness of the trace in inches.
K = 5.08 for dimensions in inches, and K = 2 for dimensions
in cm.
(8)
space
30
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