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2270f

LTC2270

APPLICATIONS INFORMATION

VREF

REFH

SENSE

TIE TO VDD FOR 2.1V RANGE;

TIE TO GND FOR 1.05V RANGE;

SENSE FOR

0.625V < VSENSE < 1.300V

1.25V

REFL

INTERNAL ADC

HIGH REFERENCE

BUFFER

2270 F08a

LTC2270

5Ω

0.84x

DIFF AMP

INTERNAL ADC

LOW REFERENCE

C1: 2.2μF LOW INDUCTANCE

INTERDIGITATED CAPACITOR

TDK CLLE1AX7S0G225M

MURATA LLA219C70G225M

AVX W2L14Z225M

OR EQUIVALENT

1.25V BANDGAP

REFERENCE

0.625V

RANGE

DETECT

AND

CONTROL

2.2μF

1μF

–

Figure 8a. Reference Circuit

SENSE

1.25V

EXTERNAL

REFERENCE

2.2μF

1μF

VREF

2270 F09

LTC2270

Figure 9. Using an External 1.25V Reference

REFH

REFL

2270 F08b

LTC2270

CAPACITORS ARE 0402 PACKAGE SIZE

1μF

2.2μF

1μF

Figure 8b. Alternative REFH/REFL Bypass Circuit

Figure 8c. Recommended Layout for the REFH/REFL

Bypass Circuit in Figure 8a

Alternatively C1 can be replaced by a standard 2.2μF

capacitor between REFH and REFL (see Figure 8b). The

capacitors should be as close to the pins as possible (not

on the back side of the circuit board).

Figure 8c and Figure 8d show the recommended circuit

board layout for the REFH/REFL bypass capacitors. Note

that in Figure 8c, every pin of the interdigitated capacitor

(C1) is connected since the pins are not internally connected

in some vendors’ capacitors. In Figure 8d the REFH and

Figure 8d. Recommended Layout for the REFH/REFL

Bypass Circuit in Figure 8b

Encode Inputs

The signal quality of the encode inputs strongly affects

the A/D noise performance. The encode inputs should

be treated as analog signals – do not route them next to

digital traces on the circuit board. There are two modes

of operation for the encode inputs: the differential encode

mode (Figure 10), and the single-ended encode mode

(Figure 11).

The differential encode mode is recommended for si-

nusoidal, PECL, or LVDS encode inputs (Figure 12 and

Figure 13). The encode inputs are internally biased to 1.2V

through 10kΩ equivalent resistance. The encode inputs

can be taken above VDD (up to 3.6V), and the common

mode range is from 1.1V to 1.6V. In the differential encode

REFL pins are connected by short jumpers in an internal

layer. To minimize the inductance of these jumpers they

can be placed in a small hole in the GND plane on the

second board layer.

LTC2270 F08c

LTC2270 F08d

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