THS4509_14 Datasheet, PDF (28/43 Pages) Texas Instruments – WIDEBAND, LOW-NOISE, LOW-DISTORTION, FULLY-DIFFERENTIAL AMPLIFIER

THS4509
SLOS454H – JANUARY 2005 – REVISED NOVEMBER 2009
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.
Table 3. R_PU Values for Various Gains
GAIN
R_F
R_G
R_IT
R_PU
6 dB
348 Ω
169 Ω
64.9 Ω
200 Ω
10 dB
348 Ω
102 Ω
78.7 Ω
133 Ω
14 dB
348 Ω
61.9 Ω
115 Ω
97.6 Ω
20 dB
348 Ω
40.2 Ω
221 Ω
80.6 Ω
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 10 dB
requires 37 mA more current with 5-V supply, and
22 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.
Figure 86 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
C
RG
RT
C
RG
RT
C
RF
V S+= 3 V to 5 V
THS 4509
CM
V S-
R_F
RO
V _OUT-
RO
V OUT+
Figure 86. THS4509 AC-Coupled Single-Supply
THS4509 and ADS5500 Combined Performance
The THS4509 is designed to be a high-performance
drive amplifier for high-performance data converters
like the ADS5500 14-bit 125-MSPS ADC. Figure 87
shows a circuit combining the two devices, and
Figure 88 shows the combined SNR and SFDR
performance versus frequency with –1-dBFS input
signal level sampling at 125 MSPS. The THS4509
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
THS4509 outputs and ADS5500 inputs along with the
input capacitance of the ADS5500 limit the bandwidth
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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 inserted to ground
across the 69.8-Ω resistor and 0.22-μF capacitor on
the alternate input to balance the circuit. Gain is a
function of the source impedance, termination, and
348-Ω feedback resistor. Refer to Table 3 for
component values to set proper 50-Ω termination for
other common gains. A split power supply of +4 V
and –1 V is used to set the input and output
common-mode voltages to approximately midsupply
while setting the input common-mode of the
ADS5500 to the recommended +1.55 V. This
configuration maintains maximum headroom on the
internal transistors of the THS4509 to insure optimum
performance.
From ^VIN
50-W
source
49.9 W
0.22 mF
100 W
348 W
69.3 W
4V
0.22 mF
100 W
THS 4509
69.8 W
0.22 mF
CM
-1 V
348 W
100 W
100 W^{2.7 pF}
0.1 mF
14 -bit,
125 MSPS
A IN +
ADS5500
A IN - _CM
49.9 W
0.1 mF
Figure 87. THS4509 and ADS5500 Circuit
90
SFDR (dBc)
85
80
75
SNR (dBFS)
70
65
10 20 30 40 50 60 70 80 90 100 110
Input Frequency - MHz
Figure 88. THS4509 and ADS5500 SFDR and SNR
Performance versus Frequency
28
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