LTC1151

8

1151fa

S

APPLICATI

I FOR ATIO

LEAD WIRE/SOLDER

COPPER TRACE JUNCTION

NOMINALLY UNNECESSARY

RESISTOR USED TO

THERMALLY BALANCE

OTHER INPUT RESISTOR

OUTPUT

RESISTOR LEAD, SOLDER,

COPPER TRACE JUNCTION

1151 F01

–

+

LTC1151

Figure 1. Extra Resistors Cancel Thermal EMF

Resistors are another source of thermal EMF errors.

Table 1 shows the thermal EMF generated for different

resistors. The temperature gradient across the resistor is

important, not the ambient temperature. There are two

junctions formed at each end of the resistor and if these

junctions are at the same temperature, their thermal EMFs

will cancel each other. The thermal EMF numbers are

approximate and vary with resistor value. High values give

higher thermal EMF.

Table 1. Resistor Thermal EMF

RESISTOR TYPE

THERMAL EMF/

°C GRADIENT

Tin Oxide

>1mV/°C

Carbon Composition

∼450µV/°C

Metal Film

∼20µV/°C

Wire Wound

Evenohm, Manganin

∼2µV/°C

PACKAGE-INDUCED OFFSET VOLTAGE

Package-induced thermal EMF effects are another impor-

tant source of errors. They arise at the junctions formed

when wire or printed circuit traces contact a package lead.

Like all the previously mentioned thermal EMF effects,

they are outside the LTC1151’s offset nulling loop and

cannot be cancelled. The input offset voltage specification

of the LTC1151 is actually set by the package-induced

warm-up drift rather than by the circuit itself. The thermal

time constant ranges from 0.5 to 3 minutes, depending on

package type.

ALIASING

Like all sampled data systems, the LTC1151 exhibits

aliasing behavior at input frequencies near the sampling

frequency. The LTC1151 includes a high frequency cor-

rection loop which minimizes this effect. As a result,

aliasing is not a problem for many applications.

For a complete discussion of the correction circuitry and

aliasing behavior, please refer to the LTC1051/LTC1053

data sheet.

LOW SUPPLY OPERATION

The minimum supply for proper operation of the LTC1151

is typically 4.0V (

±2.0V). In single supply applications,

PSRR is guaranteed down to 4.7V (

±2.35V) to ensure

proper operation at minimum TTL supply voltage of 4.75V.