LTC2239 datasheet(14/24 Pages) LINER

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LTC2239

2239fa

APPLICATIO S I FOR ATIO

Figure 8, the series inductors are impedance matching

elements that maximize the ADC bandwidth.

Reference Operation

Figure 9 shows the LTC2239 reference circuitry consisting

of a 1.5V bandgap reference, a difference amplifier and

switching and control circuit. The internal voltage refer-

ence can be configured for two pin selectable input ranges

of 2V (

±1Vdifferential)or1V(±0.5Vdifferential).Tyingthe

SENSE pin to VDD selects the 2V range; tying the SENSE

pin to VCM selects the 1V range.

Figure 5. Single-Ended Drive

Ω

0.1

µF

ANALOG

INPUT

VCM

AIN

–

12pF

2239 F05

2.2

µF

Ω

0.1

µF

LTC2239

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

ance seen by the analog inputs should be matched. This

circuit is not recommended if low distortion is required.

The 25

Ωresistorsand12pFcapacitorontheanaloginputs

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 trans-

former gives better high frequency response than a flux

coupled center tapped transformer. The coupling capaci-

tors allow the analog inputs to be DC biased at 1.5V. In

Figure 6. Recommended Front End Circuit for

Input Frequencies Between 70MHz and 170MHz

Ω

0.1

µF

AIN

–

8pF

2.2

µF

VCM

LTC2239

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2239 F06

Ω

0.1

µF

AIN

–

2.2

µF

VCM

LTC2239

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

2239 F07

Ω

0.1

µF

AIN

–

2.2

µF

VCM

LTC2239

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS, INDUCTORS

ARE 0402 PACKAGE SIZE

2239 F08

6.8nH

Figure 7. Recommended Front End Circuit for

Input Frequencies Between 170MHz and 300MHz

VCM

REFH

SENSE

TIE TO VDD FOR 2V RANGE;

TIE TO VCM FOR 1V RANGE;

RANGE = 2 • VSENSE FOR

0.5V < VSENSE < 1V

1.5V

REFL

2.2

µF

2.2

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

2239 F09

LTC2239

Ω

DIFF AMP

µF

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

Figure 9. Equivalent Reference Circuit

Figure 8. Recommended Front End Circuit for

Input Frequencies Above 300MHz

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