THS4513_13 Datasheet, PDF (27/41 Pages) Texas Instruments – WIDEBAND, LOW-NOISE, LOW-DISTORTION,FULLY-DIFFERENTIAL AMPLIFIER

THS4513
www.ti.com
V_IC is the desired input common-mode voltage, V_CM =
CM, and R_IN = R_G+ R_S || R_T. To set to midsupply,
make the value of R_PU = R_G+ R_S || R_T.
Table 3 is a modification of Table 1 to add the proper
values with R_PU assuming a 50-Ω source impedance
and setting the input and output common-mode
voltage to midsupply.
There are two drawbacks to this configuration. One is
that it requires additional current from the power
supply. Using the values shown for a gain of 0 dB
requires 14 mA more current with 5-V supply, and 8.2
mA more current with 3-V supply.
The other drawback is that this configuration also
increases the noise gain of the circuit. In the 10-dB
gain case, noise gain increases by a factor of 1.5.
Gain
0 dB
6 dB
Table 3. R_PU Values for Various Gains
R_F
348 Ω
R_G
340 Ω
R_IT
56.2 Ω
R_PU
365 Ω
348 Ω
168 Ω
64.9 Ω
200 Ω
Figure 81 shows ac-coupling to the source. Using
capacitors in series with the termination resistors
allows the amplifier to self-bias both input and output
to midsupply.
RS
C
V _SIGNAL
RS
RG
RT
C
RG
RT
RF
V S+= 3 V to 5 V
THS4513
CM
V S-
RO
V _OUT-
RO
V OUT+
C
C
R_F
Figure 81. THS4513 AC-Coupled Single-Supply
Low-Pass Filter
One application for the THS4513 is as a unity-gain
buffer with low-pass filtering. Figure 82 shows a
circuit that is driven by an ac-coupled 50-Ω source. A
1:1 transformer converts the differential output of the
THS4513 into a single-ended output capable of
SLOS472E – AUGUST 2005 – REVISED JANUARY 2013
driving 50-Ω test equipment. The circuit as shown
has an overall gain of –6 dB due to the voltage
divider on the device output, and has a roll-off
frequency of approximately 60 MHz. The measured
gain versus frequency response of the overall circuit
is shown in Figure 83. The low-frequency roll-off is
due to losses in the output transformer at those
frequencies.
6.8 pF
50-W
V_IN
Single-
Ended
AC-Coupled
Source
50 W
0.22 mF
340 W
56.2 W
0.22 mF
340 W
56.2 W
0.22 mF
348 W
2.5 V
25 W
THS4513
CM
-2.5 V
348 W
25 W
0.1 mF
To 50-W Test
Equipment
1:1
V_OUT
0.1 mF
6.8 pF
Figure 82. 60-MHz Low-Pass Filter
0
-2
-4
-6
-8
-10
-12
-14
-16
-18
-20
-22
-24
-26
0.1
1
10
100
f - Frequency (MHz)
1000
Figure 83. Low-Pass Filter Measured
Frequency Response
Copyright © 2005–2013, Texas Instruments Incorporated
Product Folder Links: THS4513
Submit Documentation Feedback
27