THS4508 Datasheet, PDF (19/29 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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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.
See Table 1 for component values to set proper 50-Ω
termination for other common gains.
VIN
From
50-W
source
49.9 W
100 W
69.8 W
100 W
69.8 W
348 W
5V
0.22 mF 100 W
THS4508
2.7 pF
CM
0.22 mF 100 W
14-bit,
125 MSPS
AIN +
ADS5500
AIN - _CM
0.22 mF
348 W
0.1 mF
Figure 57. THS4508 + ADS5500 Circuit
95
90
85
SFDR
80
75
70
SNR
65
60
10 20 30 40 50 60 70 80 90 100 110
Input Frequency - MHz
Figure 58. THS4508 + ADS5500 SFDR and SNR
Performance versus Frequency
THS4508 + ADS5424 Combined Performance
Figure 59 shows the THS4508 driving the ADS5424
ADC, and Figure 60 shows their combined SNR and
SFDR performance versus frequency with –1 dBFS
input signal level and sampling at 80 MSPS.
As before, the THS4508 amplifier provides 10 dB of
gain, converts the single-ended input to differential,
and sets the proper input common-mode voltage to
the ADS5424. Input termination and circuit testing is
the same as described above for the THS4508 +
ADS5500 circuit.
THS4508
SLAS459A – SEPTEMBER 2005 – REVISED FEBRUARY 2006
The 225-Ω resistors and 2.7-pF capacitor between
the THS4508 outputs and ADS5424 inputs (along
with the input capacitance of the ADC) limit the
bandwidth of the signal to about 100 MHz (-3 dB).
When the THS4508 is operated from a single power
supply with V_S+ = 5 V and V_S- = ground, the 2.5-V
output common-mode voltage is compatible with the
recommended value of the ADS5424 input
common-mode voltage (2.4 V).
From ^VIN
50-W
source
49 .9 W
100 W
69 .8 W
100
69 .8 W
348 W
5V
THS4508
225 W
225 W ^{2.7 pF}
CM
14 -bit,
105 MSPS
A _IN+
ADS5424
AIN– _VBG
49.9 W
0.22 mF
348 W
0.1 mF
0.1 mF
Figure 59. THS4508 + ADS5424 Circuit
95
SFDR
90
85
80
75
SNR
70
65
10
20
30
40
50
60
Input Frequency - MHz
Figure 60. THS4508 + ADS5424 SFDR and SNR
Performance vs Frequency
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