LTC2484CDD datasheet(19/40 Pages) LINER

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LTC2484

2484fa

When using the internal temperature sensor, if the output

code is normalized to RSDO = VPTAT/VREF, the temperature

is calculated using the following formula:

SLOPE

SDO

REF

SDO

REF

=°

•

–

in Kelvin

and

in C

273

where SLOPE is nominally 1.4mV/

°C

Since the PTAT signal can have an initial value variation

which results in errors in SLOPE, to achieve better tem-

perature measurements, a one-time calibration is needed

to adjust the SLOPE value. The converter output of the

PTAT signal, R0SDO, is measured at a known temperature

T0 (in

°C) and the SLOPE is calculated as:

SLOPE

SDO

REF

0 273

•

This calibrated SLOPE can be used to calculate the

temperature.

If the same VREF source is used during calibration and

temperature measurement, the actual value of the VREF is

not needed to measure the temperature as shown in the

calculation below:

SLOPE

SDO

REF

SDO

()

•

–

•–

273

Reference Voltage Range

The LTC2484 external reference voltage range is 0.1V to

VCC. The converter output noise is determined by the

thermal noise of the front-end circuits, and as such, its

value in nanovolts is nearly constant with reference volt-

age. A reduced reference voltage will improve the con-

verter performance when operated with an external con-

version clock (external FO signal) at substantially higher

output data rates (see the Output Data Rate section). VREF

must be

≥1.1V to use the internal temperature sensor.

The negative reference input to the converter is internally

tied to GND. GND (Pin 8) should be connected to a ground

plane through as short a trace as possible to minimize

voltage drop. The LTC2484 has an average operational

current of 160

µA and for 0.1Ω parasitic resistance, the

voltage drop of 16

µV causes a gain error of 3.2ppm for

VREF = 5V.

Input Voltage Range

The analog input is truly differential with an absolute/

common mode range for the IN+ and IN– input pins

extending from GND – 0.3V to VCC + 0.3V. Outside

these limits, the ESD protection devices begin to turn on

and the errors due to input leakage current increase

rapidly. Within these limits, the LTC2484 converts the

bipolar differential input signal, VIN = IN+ – IN–, from

– FS to +FS where FS = 0.5 • VREF. Outside this range, the

converter indicates the overrange or the underrange con-

dition using distinct output codes. Since the differential

input current cancellation does not rely on an on-chip

buffer, current cancellation as well as DC performance is

maintained rail-to-rail.

Input signals applied to IN+ and IN– pins may extend by

300mV below ground and above VCC. In order to limit any

fault current, resistors of up to 5k may be added in series

with the IN+ and IN– pins without affecting the perfor-

mance of the devices. The effect of the series resistance on

the converter accuracy can be evaluated from the curves

presented in the Input Current/Reference Current sec-

tions. In addition, series resistors will introduce a tem-

perature dependent offset error due to the input leakage

current. A 1nA input leakage current will develop a 1ppm

offset error on a 5k resistor if VREF = 5V. This error has a

very strong temperature dependency.

APPLICATIO S I FOR ATIO

Figure 4. Internal PTAT Signal vs Temperature

TEMPERATURE (

°C)

–60

500

600

120

2484 F04

400

200

090

–30

300

VCC = 5V

IM = 1

FO = GND

SLOPE = 1.40mV/

°C

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