THS4511_14 Datasheet, PDF (21/36 Pages) Texas Instruments – WIDEBAND, LOW-NOISE, LOW-DISTORTION FULLY-DIFFERENTIAL AMPLIFIER

THS4511
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If the signal originates from an ac-coupled 50-Ω
source (see Figure 47), the equivalent dc-source
resistance is an open circuit and R_I = R_G + R_IT.
Table 4 is a modification of Table 1 to add the proper
values with R_PD assuming V_S+ = 3.75 V, an
ac-coupled 50-Ω source impedance, and setting the
output common-mode voltage to midsupply.
Table 4. R_PD Values for Various Gains,
V_S+ = 3.75 V, AC-Coupled Signal Source
GAIN
0 dB
R_F
348 Ω
R_G
340 Ω
R_IT
56.2 Ω
R_PD
390 Ω
6 dB
348 Ω
169 Ω
64.9 Ω
80.6 Ω
C _RS
V Signal
RG
R_IT
R_PD
RF
V_S+ = 3.75 V to 5 V
RG
THS4511
C
RS
R_IT
CM
R_PD
V S-
RO
V OUT-
RO
V OUT+
RF
Figure 47. THS4511 AC-Coupled Single-Source
Supply Range From 3.75 V to 5 V With R_PD Used
To Set V_IC
Video Buffer
Figure 48 shows a possible application of the
THS4508 as a dc-coupled video buffer with a gain of
2. Figure 49 shows a plot of the Y' signal originating
from an HDTV 720p video system. The input signal
includes a 3-level sync (minimum level at –0.3 V) and
the portion of a video signal with maximum amplitude
of 0.7 V. Although the buffer draws its power from a
5-V single-ended power supply, internal level shifters
allow the buffer to support input signals which are as
much as –0.3 V below ground. This allows maximum
design flexibility while maintaining a minimum parts
count. Figure 50 shows the differential output of the
buffer. Note that the dc-coupled amplifier can
introduce a dc offset on a signal applied at its input.
SLOS471E – SEPTEMBER 2005 – REVISED NOVEMBER 2009
Video Source
R_S = 75 W V_IN
175 W
V_Signal
130 W
75 W
175 W
130 W
348 W
V_S+ = 5 V
THS4511
V_S-
V_CM = 2.5 V
R_O
-
R_O ^VOD
+
348 W
Figure 48. Single-Supply Video Buffer, Gain = 2
0.8
0.6
0.4
0.2
0
-0.2
-0.4
0
5
10
15
20
t - Time - ms
Figure 49. Y' Signal with 3-Level Sync and Video
Signal
1.5
1
0.5
0
-0.5
-1
0
5
10
15
20
t - Time - ms
Figure 50. Video Buffer Differential Output Signal
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