THS4513 Datasheet, PDF (27/34 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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RS
V _Signal
RS
V _S+
R _PU
RG
RF
R_T
V _S+
V _S+
R _PU
R_G
RT
THS 4513
CM
V S-
R_O
V _OUT-
R_O
V _OUT+
R_F
Figure 80. THS4513 DC Coupled Single-Supply
with R_PU Used to Set V_IC
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 mid-supply.
R_S
C
R_G
R_F
V _Signal
R_S
R_T
C
R_G
R_T
V _S+= 3 V to 5 V
THS
CM
V _S-
R_O
V _OUT-
R_O
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
driving 50-Ω test equipment. The circuit as shown
has an overall gain of -6dB 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.
THS4513
SLOS472 – AUGUST 2005
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
THS4513 + ADS5500 Combined Performance
The THS4513 is designed to be a high performance
drive amplifier for high performance data converters
like the ADS5500 14-bit 125-MSPS ADC. Figure 84
shows a circuit combining the two devices. The
THS4513 amplifier circuit provides 10 dB of gain,
converts the single-ended input to differential, and
sets the proper input common-mode voltage to the
ADS5500. The 100-Ω resistors and 2.7-pF capacitor
between the THS4513 outputs and ADS5500 inputs
along with the input capacitance of the ADS5500 limit
the bandwidth of the signal to 115 MHz (–3 dB). For
testing, a signal generator is used for the signal
source. The generator is an AC-coupled 50-Ω source.
A band-pass filter is inserted in series with the input
to reduce harmonics and noise from the signal
source. Input termination is accomplished via the
69.8-Ω resistor and 0.22-µF capacitor to ground in
conjunction with the input impedance of the amplifier
circuit. A 0.22-µF capacitor and 49.9-Ω resistor is
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