THS4271D datasheet(21/38 Pages) TI

21 / 38 page

THS4271

THS4275

SLOS397E − JULY 2002 − REVISED JANUARY 2004

www.ti.com

APPLICATION CIRCUITS

Driving an Analog-to-Digital Converter With the

THS4271

The THS4271 can be used to drive high-performance

analog-to-digital converters. Two example circuits are

presented below.

The first circuit uses a wideband transformer to convert a

single-ended input signal into a differential signal. The

differential signal is then amplified and filtered by two

THS4271

amplifiers.

This

circuit

provides

low

intermodulation

distortion,

suppressed

even-order

distortion, 14 dB of voltage gain, a 50-

Ω input impedance,

and a single-pole filter at 100 MHz. For applications

without signal content at dc, this method of driving ADCs

can be very useful. Where dc information content is

required, the THS4500 family of fully differential amplifiers

may be applicable.

THS4271

Ω

Source

249

Ω

−5 V

100

Ω

22 pF

249

Ω

100

Ω

24.9

Ω

22 pF

14-Bit, 62 Msps

ADS5422

(1:4

Ω)

1:2

5 V

24.9

Ω

THS4271

VCM

Figure 78. A Linear, Low Noise, High Gain ADC

Preamplifier

The second circuit depicts single-ended ADC drive. While

not recommended for optimum performance using

converters

with

differential

inputs,

satisfactory

performance

can

sometimes

achieved

with

single-ended input drive. An example circuit is shown here

for reference.

THS4271

249

Ω

ADS807

12-Bit,

53 Msps

+5 V

249

Ω

49.9

Ω

−5 V

Ω

Source

RISO 0.1 µF

16.5

Ω

68 pf

0.1

µF

1.82 k

Ω

NOTE: For best performance, high-speed ADCs should be driven

differentially. See the THS4500 family of devices for more

information.

Figure 79. Driving an ADC With a

Single-Ended Input

Using the THS4271 as a DAC Output Buffer

Two example circuits are presented here showing the

THS4271 buffering the output of a digital-to-analog

converter. The first circuit performs a differential to

single-ended conversion with the THS4271 configured as

a difference amplifier. The difference amplifier can double

as the termination mechanism for the DAC outputs as well.

249

Ω

THS4271

49.9

Ω

100

Ω

249

Ω

100

Ω

+5 V

−5 V

14-Bit,

400 MSps

DAC5675

124

Ω

249

Ω

249

Ω

3.3 V

Figure 80. Differential to Single-Ended

Conversion of a High-Speed DAC Output

For cases where a differential signaling path is desirable,

a pair of THS4271 amplifiers can be used as output

buffers. The circuit depicts differential drive into a mixer’s

IF inputs, coupled with additional signal gain and filtering.

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