18pF

AIN

AIN

VCM

ADS5474

+5V

THS4509

CM

348 W

348 W

100 W

100 W

78.9 W

78.9 W

49.9 W

VIN

From

50

Source

W

49.9 W

49.9 W

49.9 W

0.1

F

m

0.1

F

m

0.22

F

m

0.22

F

m

0.22

F

m

ADS5474

www.ti.com

SLAS525B

– JULY 2007 – REVISED FEBRUARY 2012

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.

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

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.

Copyright

© 2007–2012, Texas Instruments Incorporated

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Product Folder Link(s): ADS5474