LTC2420CS8 datasheet(33/36 Pages) LINER

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LTC2420

TYPICAL APPLICATIO S

Using a Low Power Precision Reference

The circuit in Figure 38 shows the connections and by-

passing for an LT1461-2.5 as a 2.5V reference. The

LT1461 is a bandgap reference capable of 3ppm/

°C tem-

perature stability yet consumes only 45

µA of current. The

1k resistor between the reference and the ADC reduces the

transient load changes associated with sampling and

produces optimal results. This reference will not impact

the noise level of the LTC2420 if signals are less than 60%

full scale, and only marginally increases noise approach-

ing full scale. Even lower power references can be used if

only the lower end of the LTC2420 input range is required.

A Differential to Single-Ended Analog Front End

Figure 39 shows the LT1167 as a means of sensing

differential signals. The noise performance of the LT1167

is such that for gains less than 200, the noise floor of the

LTC2420 remains the dominant noise source. At the point

where the noise of the amplifier begins to dominate, the

input referred noise is essentially that of the instrumenta-

tion amplifier. The linearity of the instrumentation ampli-

fier does, however, degrade at higher gains. As a result, if

the full linearity of the LTC2420 is desired, gain in the

instrumentation amplifier should be limited to less than

100, possibly requiring averaging multiple samples to

extend the resolution below the noise floor. The noise level

of the LT1167 at gains greater than 100 is on the order of

50nVRMS, although, 1/f noise and temperature effects may

degrade this below 0.1Hz. The introduction of a filter

between the amplifier and the LTC2420 may improve

noise levels under some circumstances by reducing noise

bandwidth. Note that temperature offset drift effects enve-

lope detection in the input of the LT1167 if exposed to RFI,

thermocouple voltages in connectors, resistors and sol-

dered junctions can all compromise results, appearing as

drift or noise. Turbulent airflow over this circuitry should

be avoided.

LT1461-2.5

GND

2420 F38

µF

16V

TANT

LTC2420

REF

OUT

0.1

µF

CER

–

LT1167

3.5k

×4

µF

Ω

µF

AV =

RECOMMENDED RG: 500Ω, 0.1%

5ppm/

°C

+ 1

49.4k

Ω

OPTIONAL

2428 F39

–5V

LTC2420

Figure 38. Low Power Reference

Figure 39. A Differential to Single-Ended Analog Front End

LTC2420CS8 Datasheet(PDF) 33 Page - Linear Technology

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