ADT7516 datasheet(19/44 Pages) AD

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ADT7516/ADT7517/ADT7519

Rev. B | Page 19 of 44

THEORY OF OPERATION

Directly after the power-up calibration routine, the ADT7516/

ADT7517/ADT7519 go into idle mode. In this mode, the

devices are not performing any measurements and are fully

powered up. All four DAC outputs are at 0 V.

To begin monitoring, write to the Control Configuration 1

ADT7517/ADT7519 go into the power-up default measurement

mode (round robin). The devices proceed to take measurements

on the VDD channel, internal temperature sensor channel,

external temperature sensor channel (AIN1 and AIN2), AIN3,

and finally AIN4. After they finish taking measurements on the

AIN4 channel, the devices immediately loop back to start

taking measurements on the VDD channel and repeat the same

cycle as before. This loop continues until the monitoring is

stopped by resetting Bit C0 of the Control Configuration 1

It is also possible to continue monitoring as well as switching to

single-channel mode by writing to the Control Configuration 2

tion of the single-channel and round robin measurement modes

is given in later sections. All measurement channels have

averaging enabled on them at power-up. Averaging forces the

devices to take an average of 16 readings before giving a final

measured result. To disable averaging 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

ADT7516/ADT7517/ADT7519, AIN1 to AIN4. AIN1 and

AIN2 are multiplexed with the external temperature sensor

terminals (D+ and D−). Bit C1 and Bit C2 of the Control

Configuration 1 register (Address 0x18) 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 VREF or VDD. To

meet linearity specifications, it is recommended that the maximum

VDD value is 5 V. Bit C4 of the Control Configuration 3 register

be used to select between the internal reference and VDD as the

ADC reference of the analog inputs.

Controlling the DAC outputs can be done by writing to the MSB

and LSB registers of the DAC (Address 0x10 to Address 0x17).

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

Configuration 3 register (Address 0x1A). The DAC configura-

tion register (Address 0x1B) and the LDAC configuration

updating. These two registers also control the output range of

the DACs and select 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 I2C 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 I2C protocol cannot be locked in, though the SPI protocol

is automatically locked in on selection. The interface can be

switched back to be I2C on selection when the device is powered

off and on. When using I2C, the CS pin should be tied to either

VDD or GND.

There are a number of different operating modes on the

ADT7516/ADT7517/ADT7519 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

initiated until approximately 5 ms after VDD 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 interrupts this routine, and can cause erroneous

temperature measurements. If it is not possible to have VDD at its

nominal value by the time 50 ms has elapsed or if communication

to the device has started prior to VDD settling, it is recommended

that a measurement be taken on the VDD channel before a

temperature measurement is taken. The VDD measurement is

used to calibrate out any temperature measurement error due to

different supply voltage values.

CONVERSION SPEED

The internal oscillator circuit used by the ADC has the capability

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 0x1A).

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 sensors) 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 section.

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