LTC2285 datasheet(14/24 Pages) LINER

14 / 24 page

LTC2285

2285fb

APPLICATIONS INFORMATION

Figure 4 demonstrates the use of a differential ampliﬁer to

convert a single ended input signal into a differential input

signal. The advantage of this method is that it provides

low frequency input response; however, the limited gain

bandwidth of most op amps will limit the SFDR at high

input frequencies.

Figure 5 shows a single-ended input circuit. The impedance

seen by the analog inputs should be matched. This circuit

is not recommended if low distortion is required.

The 25Ω resistors and 12pF capacitor on the analog

inputs serve two purposes: isolating the drive circuitry

from the sample-and-hold charging glitches and limiting

the wideband noise at the converter input.

For input frequencies above 70MHz, the input circuits of

Figure 6, 7 and 8 are recommended. The balun transformer

gives better high frequency response than a ﬂux coupled

center tapped transformer. The coupling capacitors allow

the analog inputs to be DC biased at 1.5V. In Figure 8, the

series inductors are impedance matching elements that

maximize the ADC bandwidth.

Figure 4. Differential Drive with an Ampliﬁer

Figure 5. Single-Ended Drive

Figure 6. Recommended Front End Circuit for

Input Frequencies Between 70MHz and 170MHz

Figure 7. Recommended Front End Circuit for

Input Frequencies Between 170MHz and 300MHz

Figure 8. Recommended Front End Circuit for

Input Frequencies Above 300MHz

25Ω

12pF

2.2μF

VCM

2285 F04

–

ANALOG

INPUT

HIGH SPEED

DIFFERENTIAL

AMPLIFIER

AIN

–

LTC2285

25Ω

0.1μF

ANALOG

INPUT

VCM

AIN

–

12pF

2285 F05

2.2μF

25Ω

0.1μF

LTC2285

25Ω

12Ω

0.1μF

AIN

–

8pF

2.2μF

VCM

ANALOG

INPUT

0.1μF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2285 F06

LTC2285

25Ω

0.1μF

AIN

–

2.2μF

VCM

ANALOG

INPUT

0.1μF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2285 F07

LTC2285

25Ω

0.1μF

AIN

–

2.2μF

VCM

ANALOG

INPUT

0.1μF

T1 = MA/COM, ETC 1-1-13

RESISTORS, CAPACITORS, INDUCTORS

ARE 0402 PACKAGE SIZE

2285 F08

8.2nH

LTC2285

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