LTC2221 datasheet(21/32 Pages) LINER

21 / 32 page

LTC2220/LTC2221

22201fa

The AIN+ and AIN– inputs each have two pins to reduce

package inductance. The two AIN+ and the two AIN– pins

should be shorted together.

For input frequencies above 100MHz 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 1.6V. In

Figure 8 the series inductors are impedance matching

elements that maximize the ADC bandwidth.

Figure 7. Recommended Front End Circuit for

Input Frequencies Between 250MHz and 500MHz

Reference Operation

Figure 9 shows the LTC2220/LTC2221 reference circuitry

consisting of a 1.6V 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 differ-

ential). Tying the SENSE pin to VDD selects the 2V range;

typing the SENSE pin to VCM selects the 1V range.

The 1.6V 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 1.6V 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 four pins: two each of REFHA

and REFHB for the high reference and two each of REFLA

and REFLB for the low reference. The multiple output pins

are needed to reduce package inductance. Bypass capaci-

tors must be connected as shown in Figure 9.

Ω

0.1

µF

AIN

–

AIN

–

2pF

2.2

µF

VCM

LTC2220/

LTC2221

ANALOG

INPUT

0.1

µF

0.1

µF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22201 F08

4.7nH

APPLICATIO S I FOR ATIO

Figure 6. Recommended Front End Circuit for

Input Frequencies Between 100MHz and 250MHz

Ω

0.1

µF

AIN

–

AIN

–

8pF

2.2

µF

VCM

LTC2220/

LTC2221

ANALOG

INPUT

0.1

µF

0.1

µF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22201 F06

Figure 8. Recommended Front End Circuit for

Input Frequencies Above 500MHz

Ω

0.1

µF

AIN

–

AIN

–

2.2

µF

VCM

LTC2220/

LTC2221

ANALOG

INPUT

0.1

µF

0.1

µF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22201 F07

VCM

REFHA

REFLB

SENSE

TIE TO VDD FOR 2V RANGE;

TIE TO VCM FOR 1V RANGE;

RANGE = 2 • VSENSE FOR

0.5V < VSENSE < 1V

1.6V

REFLA

REFHB

2.2

µF

2.2

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

22201 F09

LTC2220/LTC2221

Ω

DIFF AMP

µF

0.1

µF

INTERNAL ADC

LOW REFERENCE

1.6V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

Figure 9. Equivalent Reference Circuit

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