LTC2230UP datasheet(21/32 Pages) LINER

21 / 32 page

LTC2230/LTC2231

22301fb

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.

Reference Operation

Figure 9 shows the LTC2230/LTC2231 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.

APPLICATIO S I FOR ATIO

0.1

μF

AIN

–

AIN

–

8pF

2.2

μF

VCM

LTC2230/

LTC2231

ANALOG

INPUT

0.1

μF

0.1

μF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22301 F06

0.1

μF

AIN

–

AIN

–

2.2

μF

VCM

LTC2230/

LTC2231

ANALOG

INPUT

0.1

μF

0.1

μF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22301 F07

0.1

μF

AIN

–

AIN

–

2pF

2.2

μF

VCM

LTC2230/

LTC2231

ANALOG

INPUT

0.1

μF

0.1

μF

T1 = MA/COM ETC1-1-13

RESISTORS, CAPACITORS

ARE 0402 PACKAGE SIZE

22301 F08

4.7nH

Figure 6. Recommended Front End Circuit for Input

Frequencies Between 100MHz and 250MHz

Figure 7. Recommended Front End Circuit for Input

Frequencies Between 250MHz and 500MHz

Figure 8. Recommended Front End Circuit

for Input Frequencies Above 500MHz

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

22301 F09

LTC2230/LTC2231

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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