THS4511 Datasheet, PDF (19/25 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

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THS4511 + ADS5500 Combined Performance
The THS4511 is designed to be a high performance
drive amplifier for high performance data converters
like the ADS5500 14-bit 125-MSPS ADC. Figure 51
shows a circuit combining the two devices. The
THS4511 amplifier circuit provides 0 dB of gain, and
converts the single-ended input signal to a differential
output signal. The default common-mode output of
the THS4511 (2.5 V) is not compatible with the
required common-mode input of the ADS5500 (1.55
V), so dc-blocking capicitors are added (0.22 µF).
Note that a biasing circuit (not shown in Figure 51) is
needed to provide the required common-mode,
dc-input for the ADS5500. The 100-Ω resistors and
2.7-pF capacitor between the THS4511 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 ampli-
fier 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 im-
pedance, 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
340 W
56.2 W
340 W
56.2 W
348 W
5V
0.22 mF 100 W
THS4511
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 51. THS4511 + ADS5500 Circuit
THS4511
SLOS471 – SEPTEMBER 2005
THS4511 + ADS5424 Combined Performance
Figure 52 shows the THS4511 driving the ADS5424
ADC.
As before, the THS4511 amplifier provides 0 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 THS4511 +
ADS5500 circuit.
The 225-Ω resistors and 2.7-pF capacitor between
the THS4511 outputs and ADS5424 inputs (along
with the input capacitance of the ADC) limit the
bandwidth of the signal to about 100MHz (-3dB).
When the THS4511 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 compatable with the
recommended value of the ADS5424 input com-
mon-mode voltage (2.4 V).
From ^VIN
50-W
source
49 .9 W
340 W
56.2 W
340
56.2 W
348 W
5V
225 W
THS4511 _{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 52. THS4511 + ADS5424 Circuit
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