Datasheet

2.7pF
0.1 Fm
14-bit,
125 MSPS
A
IN +
A
IN -
CM
ADS5500
4 V
THS 4509
CM
348 W
348 W
100 W
100 W
69.3 W
V
IN
From
50-
source
W
100 W
100 W
-1
V
69.8 W
49.9 W
0.22 Fm
49.9 W
0.22 Fm0.22 Fm
0.1 Fm
V
S+
=3Vto 5V
CM
V
Signal
V
S-
R
F
R
F
R
G
R
G
R
T
R
O
R
O
V
OUT+
V
OUT-
THS 4509
C
R
S
R
T
C
C
R
S
C
65
70
75
80
85
90
10 20 30 40 50 60 70 80 90 100 110
InputFrequency-MHz
SFDR(dBc)
SNR(dBFS)
SFDR(dBc),SNR(dBFS)
THS4509
SLOS454H –JANUARY 2005–REVISED NOVEMBER 2009 www.ti.com
Table 3 is a modification of Table 1 to add the proper of the signal to 115 MHz (–3 dB). For testing, a signal
values with R
PU
assuming a 50-Ω source impedance generator is used for the signal source. The
and setting the input and output common-mode generator is an ac-coupled 50-Ω source. A band-pass
voltage to midsupply. filter is inserted in series with the input to reduce
harmonics and noise from the signal source. Input
Table 3. R
PU
Values for Various Gains termination is accomplished via the 69.8-Ω resistor
and 0.22-μF capacitor to ground in conjunction with
GAIN R
F
R
G
R
IT
R
PU
the input impedance of the amplifier circuit. A 0.22-μF
6 dB 348 Ω 169 Ω 64.9 Ω 200 Ω
capacitor and 49.9-Ω resistor is inserted to ground
10 dB 348 Ω 102 Ω 78.7 Ω 133 Ω
across the 69.8-Ω resistor and 0.22-μF capacitor on
14 dB 348 Ω 61.9 Ω 115 Ω 97.6 Ω
the alternate input to balance the circuit. Gain is a
function of the source impedance, termination, and
20 dB 348 Ω 40.2 Ω 221 Ω 80.6 Ω
348-Ω feedback resistor. Refer to Table 3 for
component values to set proper 50-Ω termination for
There are two drawbacks to this configuration. One is
other common gains. A split power supply of +4 V
that it requires additional current from the power
and –1 V is used to set the input and output
supply. Using the values shown for a gain of 10 dB
common-mode voltages to approximately midsupply
requires 37 mA more current with 5-V supply, and
while setting the input common-mode of the
22 mA more current with 3-V supply.
ADS5500 to the recommended +1.55 V. This
The other drawback is that this configuration also
configuration maintains maximum headroom on the
increases the noise gain of the circuit. In the 10-dB
internal transistors of the THS4509 to insure optimum
gain case, noise gain increases by a factor of 1.5.
performance.
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.
Figure 87. THS4509 and ADS5500 Circuit
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
Figure 88. THS4509 and ADS5500 SFDR and SNR
amplifier circuit provides 10 dB of gain, converts the
Performance versus Frequency
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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