LTC2188 datasheet(21/36 Pages) LINER

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2188f

LTC2188

applicaTions inForMaTion

VREF

REFH

SENSE

TIE TO VDD FOR 2V RANGE;

TIE TO GND FOR 1V RANGE;

RANGE = 1.6 • VSENSE FOR

0.625V < VSENSE < 1.300V

1.25V

REFL

INTERNAL ADC

HIGH REFERENCE

BUFFER

2188 F08a

LTC2188

0.8x

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

0.1µF

–

Figure 8a. Reference Circuit

SENSE

1.25V

EXTERNAL

REFERENCE

2.2µF

1µF

VREF

2188 F09

LTC2188

Figure 9. Using an External 1.25V Reference

REFH

REFL

2188 F08b

LTC2188

CAPACITORS ARE 0402 PACKAGE SIZE

0.1µF

2.2µF

0.1µF

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)isconnectedsincethepinsarenotinternallyconnected

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.

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