ADS5474 Datasheet, PDF (20/38 Pages) Texas Instruments – 14-Bit, 400-MSPS Analog-to-Digital Converter

ADS5474
SLAS525 – JULY 2007
www.ti.com
Applications Information (continued)
For applications requiring dc-coupling with the signal source, a differential input/differential output amplifier such
as the THS4509 (shown in Figure 34) provides good harmonic performance and low noise over a wide range of
frequencies.
VIN
From
50 W
Source
100 W
78.9 W
348 W
+5V
49.9 W
0.22 mF
100 W
78.9 W
THS4509
CM
49.9 W
49.9 W ^{18 pF}
AIN
ADS5474
AIN VCM
49.9 W
0.22 mF
0.22 mF
348 W
0.1 mF
0.1 mF
Figure 34. Using the THS4509 or THS4520 With the ADS5474
In this configuration, 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 ADS5474 by utilizing the VCM output pin of
the ADC. The 50-Ω resistors and 18-pF capacitor between the THS4509 outputs and ADS5474 inputs (along
with the input capacitance of the ADC) limit the bandwidth of the signal to about 70 MHz (–3 dB). Input
termination is accomplished via the 78.9-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 are inserted to ground across
the 78.9-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 the THS4509 data sheet for further
component values to set proper 50-Ω termination for other common gains. Because the ADS5474
recommended input common-mode voltage is 3.1 V, the THS4509 operates from a single power-supply input
with V_S+ = 5 V and V_S– = 0 V (ground). This configuration has the potential to slightly exceed the recommended
output voltage from the THS4509 of 3.6V due to the ADC input common-mode of 3.1V and the +0.55V full-scale
signal. This will not harm the THS4509 but may result in a degradation in the harmonic performance of the
THS4509. An amplifier with a wider recommended output voltage range is the THS4520, which is optimized for
low noise and low distortion in the range of frequencies up to ~20MHz. Applications that are not sensitive to
harmonic distortion could consider either device at higher frequencies.
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