THS4511 Datasheet, PDF (22/35 Pages) Texas Instruments – WIDEBAND, LOW NOISE, LOW DISTORTION FULLY DIFFERENTIAL AMPLIFIER

THS4511
SLOS471E – SEPTEMBER 2005 – REVISED NOVEMBER 2009
THS4511 and 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 capacitors 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 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
0.22 mF
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
348 W
0.1 mF
14-bit,
125 MSPS
AIN +
ADS5500
AIN - _CM
Figure 51. THS4511 and ADS5500 Circuit
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THS4511 and 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 and
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 100 MHz (–3 dB).
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 compatible with the
recommended value of the ADS5424 input
common-mode voltage (2.4 V).
From ^VIN
50-W
source
49 .9 W
0.22 mF
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
348 W
0.1 mF
0.1 mF
Figure 52. THS4511 and ADS5424 Circuit
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