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LTC2481

2481f

APPLICATIO S I FOR ATIO

first conversion result following POR is accurate within the

specifications of the device if the power supply voltage is

restored within the operating range (2.7V to 5.5V) before

the end of the POR time interval.

On-Chip Temperature Sensor

The LTC2481 contains an on-chip PTAT (proportional to

absolute temperature) signal that can be used as a tempera-

ture sensor. The internal PTAT has a typical value of 420mV

at 27

°C and is proportional to the absolute temperature

value with a temperature coefficient of 420/(27 + 273) =

1.40mV/

°C (SLOPE), as shown in Figure 11. The internal

PTAT signal is used in a single-ended mode referenced to

device ground internally. The GAIN is automatically set to

one (independent of the values of GS0, GS1, GS2) in order

to preserve the PTAT property at the ADC output code and

avoid an out of range error. The 1x speed mode with auto-

matic offset calibration is automatically selected for the in-

ternal PTAT signal measurement as well.

When using the internal temperature sensor, if the output

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

is calculated using the following formula:

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

SDA

REF

273

•

SLOPE

SDA

REF

SDA

()

•

–

•–

273

Figure 11. Internal PTAT Signal vs Temperature

TEMPERATURE (

°C)

–60

500

600

120

2481 F12

400

200

090

–30

300

VCC = 5V

IM = 1

SLOPE = 1.40mV/

°C

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 absolute

temperature measurements, a one-time calibration is

needed to adjust the SLOPE value. The converter output of

the PTAT signal, R0SDA, is measured at a known tempera-

ture T0 (in

°C) and the SLOPE is calculated as:

This calibrated SLOPE can be used to calculate the

temperature.

Reference Voltage Range

The LTC2481 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. Since the transition noise (600nV) is much less than

the quantization noise (VREF/217), a decrease in the refer-

ence voltage will increase the converter resolution. A

reduced reference voltage will also improve the converter

performance when operated with an external conversion

clock (external FO signal) at substantially higher output

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

≥1.1V to use the internal temperature sensor.

The reference input is differential. The differential refer-

ence input range (VREF = REF+ – REF–) is 100mV to VCC

and the common mode reference input range is 0V to VCC.

SLOPE

SDA

REF

SDA

REF

=°

•

–

in Kelvin

and

in C

273

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