LTC2248 datasheet(16/24 Pages) LINER

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LTC2248/LTC2247/LTC2246

224876fa

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 8, the series inductors are impedance matching

elements that maximize the ADC bandwidth.

APPLICATIO S I FOR ATIO

Figure 6. Recommended Front End Circuit for

Input Frequencies Between 70MHz and 170MHz

Ω

0.1

µF

AIN

–

8pF

2.2

µF

VCM

LTC2248/47/46

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

224876 F06

Figure 8. Recommended Front End Circuit for

Input Frequencies Above 300MHz

Ω

0.1

µF

AIN

–

2.2

µF

VCM

LTC2248/47/46

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

224876 F07

Ω

0.1

µF

AIN

–

2.2

µF

VCM

LTC2248/47/46

ANALOG

INPUT

0.1

µF

0.1

µF

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

RESISTORS, CAPACITORS, INDUCTORS

ARE 0402 PACKAGE SIZE

224876 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 < VCM •

1.5V

REFL

2.2

µF

2.2

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

224876 F09

LTC2248/47/46

Ω

DIFF AMP

µF

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

1.1

1.5

Figure 9. Equivalent Reference Circuit

Reference Operation

Figure 9 shows the LTC2248/LTC2247/LTC2246 refer-

ence circuitry consisting of a 1.5V bandgap reference, a

difference amplifier and switching and control circuit. The

internal voltage reference can be configured for two pin

selectable input ranges of 2V (

±1V differential) or 1V

(

±0.5V differential). Tying the SENSE pin to VDD selects

the 2V range; tying the SENSE pin to VCM selects the 1V

range.

The 1.5V bandgap reference serves two functions: its

output provides a DC bias point for setting the common

mode voltage of any external input circuitry; additionally,

the reference is used with a difference amplifier to gener-

ate the differential reference levels needed by the internal

ADC circuitry. An external bypass capacitor is required for

the 1.5V reference output, VCM. This provides a high

frequency low impedance path to ground for internal and

external circuitry.

The difference amplifier generates the high and low refer-

ence for the ADC. High speed switching circuits are

connected to these outputs and they must be externally

bypassed. Each output has two pins. The multiple output

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