OPA690 datasheet(17/40 Pages) TI

17 / 40 page

THS4221, THS4225

THS4222, THS4226

SLOS399G − AUGUST 2002 − REVISED JANUARY 2004

www.ti.com

APPLICATION CIRCUITS

Driving an Analog-to-Digital Converter With the

THS4222

The THS4222 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

THS4222

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 25 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.

THS4222

50 Ω

Source

1.3 kΩ

THS4222

−5 V

649 Ω

4.7 pF

1.3 kΩ

649 Ω

24.9 Ω

4.7 pF

12-Bit, 53 Msps

ADS807

(1:4 Ω)

1:2

5 V

24.9 Ω

237 Ω

Figure 32. 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.

THS4222

1.3 kΩ

ADS807

12-Bit,

53 Msps

+5 V

1.3 kΩ

49.9 Ω

−5 V

50 Ω

Source

RISO 0.1 µF

16.5 Ω

68 pf

0.1 µF

1.82 kΩ

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

differentially. See the THS4500 family of devices for more

information.

Figure 33. Driving an ADC With a

Single-Ended Input

Active Filtering With the THS4222

High-frequency active filtering with the THS4222 is

achievable due to the amplifier’s high slew rate, wide

bandwidth, and voltage feedback architecture. Several

options are available for high-pass, low-pass, bandpass,

and bandstop filters of varying orders. A simple two-pole

low pass filter is presented here as an example, with two

poles at about 25 MHz.

THS4222

49.9 Ω

50 Ω Source

1.3 kΩ

4.7 pF

5 V

−5 V

120 pF

1.3 kΩ

52.3 Ω

Figure 34. A Two-Pole Active Filter With Two

Poles at about 25 MHz

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