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LTC2485

2485fa

Ease of Use

The LTC2485 data output has no latency, filter settling

delay or redundant data associated with the conversion

cycle. There is a one-to-one correspondence between the

conversion and the output data. Therefore, multiplexing

multiple analog voltages is easy.

The LTC2485 performs offset and full-scale calibrations

every conversion cycle. This calibration is transparent to the

user and has no effect on the cyclic operation described

above. The advantage of continuous calibration is extreme

stability of offset and full-scale readings with respect to

time, supply voltage change and temperature drift.

Power-Up Sequence

The LTC2485 automatically enters an internal reset state

when the power supply voltage VCC drops below approxi-

mately 2V. This feature guarantees the integrity of the

conversion result.

When the VCC voltage rises above this critical threshold,

the converter creates an internal power-on-reset (POR)

signal with a duration of approximately 4ms. The POR

signal clears all internal registers. Following the POR

signal, the LTC2485 starts a normal conversion cycle and

follows the succession of states described in Figure 1. The

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 LTC2485 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 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:

SLOPE

SDA

REF

SDA

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 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:

SLOPE

SDA

REF

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

SDA

REF

SDA

()

•

–

•–

273

Figure 11. Internal PTAT Signal vs Temperature

TEMPERATURE (

°C)

–60

500

600

120

2485 F11

400

200

090

–30

300

VCC = 5V

IM = 1

SLOPE = 1.40mV/

°C

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