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

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.

20

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