ADT7518ARQZ【PDF 17/40 页】IC SENSOR TEMP QD ADC/DAC 16QSOP

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Sheet目录39

13页 14页 15页 16页 [17页]18页 19页 20页 21页

ADT7518

Rev. A | Page 17 of 40

THEORY OF OPERATION

Directly after the power-up calibration routine, the ADT7518

goes into idle mode. In this mode, the device is not performing

any measurements and is fully powered up. All four DAC

outputs are at 0 V.

To begin monitoring, write to the Control Configuration 1

into its power-up default measurement mode, which is round

robin. The device then to take measurements on the VDD chan-

nel, internal temperature sensor channel, external temperature

sensor channel, or AIN1 and AIN2, AIN3, and finally AIN4.

Once it finishes taking measurements on the AIN4 channel, the

device immediately loops back to start taking measurements on

the VDD channel and repeats the same cycle as before. This loop

continues until the monitoring is stopped by resetting Bit C0 of

the Control Configuration 1 register to 0. It is also possible to

continue monitoring as well as switching to single-channel

mode by writing to the Control Configuration 2 register

(Address 19h) and setting Bit C4 = 1. Further explanation of

the single-channel and round robin measurement modes is

given in later sections.

All measurement channels have averaging enabled on them on

power-up. Averaging forces the device to take an average of 16

readings before giving a final measured result. To disable aver-

aging and consequently decrease the conversion time by a factor

of 16, set Bit C5 = 1 in the Control Configuration 2 register.

There are four single-ended analog input channels on the

ADT7518: AIN1 to AIN4. AIN1 and AIN2 are multiplexed with

the external temperature sensor terminals D+ and D. Bits C1

and C2 of the Control Configuration 1 register (Address 18h)

are used to select between AIN1/AIN2 and the external

temperature sensor. The input range on the analog input

channels is dependent on whether the ADC reference used is

the internal V

REF

or V

. To meet linearity specifications, it is

recommended that the maximum V

value is 5 V. Bit C4 of the

Control Configuration 3 register is used to select between the

internal reference or VDD as the analog inputs ADC reference.

Controlling the DAC outputs can be done by writing to the

DACs MSB and LSB registers (Addresses 10h to 17h). The

power-up default setting is to have a low going pulse on the

LDAC

pin (Pin 9) controlling the updating of the DAC outputs

from the DAC registers. Alternatively, one can configure the

updating of the DAC outputs to be controlled by means other

than the LDAC

pin by setting Bit C3 = 1 of the Control Config-

uration 3 register (Address 1Ah). The DAC Configuration

(Address 1Ch) can now be used to control the DAC updating.

These two registers also control the output range of the DACs

and selecting between the internal or external reference. DAC A

and DAC B outputs can be configured to give a voltage output

proportional to the temperature of the internal and external

temperature sensors, respectively.

The dual serial interface defaults to the I

C protocol on power-

up. To select and lock in the SPI protocol, follow the selection

process as described in the Serial Interface Selection section.

The I

C protocol cannot be locked in, while the SPI protocol is

automatically locked in on selection. The interface can be

switched back to be I

C on selection when the device is powered

off and on. When using I

C, the CS pin should be tied to either

or GND.

There are a number of different operating modes on the

ADT7518 devices and all of them can be controlled by the

configuration registers. These features consist of enabling and

disabling interrupts, polarity of the INT/INT

pin, enabling and

disabling the averaging on the measurement channels SMBus

timeout and software reset.

POWER-UP CALIBRATION

It is recommended that no communication to the part be ini-

tiated until approximately 5 ms after V

has settled to within

10% of its final value. It is generally accepted that most systems

take a maximum of 50 ms to power up. Power-up time is

directly related to the amount of decoupling on the voltage

supply line.

During the 5 ms after VDD has settled, the part is performing a

calibration routine. Any communication to the device during

calibration will interrupt this routine, and could cause erro-

neous temperature measurements. If it is not possible to have

at its nominal value by the time 50 ms has elapsed or if

communication to the device has started prior to V

settling, it

is recommended that a measurement be taken on the V

chan-

nel before a temperature measurement is taken. The VDD

measurement is used to calibrate out any temperature measure-

ment error due to different supply voltage values.

CONVERSION SPEED

The internal oscillator circuit used by the ADC has the capa-

bility to output two different clock frequencies. This means that

the ADC is capable of running at two different speeds when

doing a conversion on a measurement channel. Thus, the time

taken to perform a conversion on a channel can be reduced by

setting Bit C0 of the Control Configuration 3 register (Address

1Ah). This increases the ADC clock speed from 1.4 kHz to 22

kHz. At the higher clock speed, the analog filters on the D+ and

D input pins (external temperature sensor) are switched off.

This is why the power-up default setting is to have the ADC

working at the slow speed. The typical times for fast and slow

ADC speeds are given in the specifications.

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