ADT7518 datasheet(25/40 Pages) AD

Part #	ADT7518

Description	SPI/I2C Compatible, Temperature Sensor, 4-Channel ADC and Quad Voltage Output DAC

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ADT7518

Rev. A | Page 25 of 40

5.

Place 0.1 µF bypass and 2,200 pF input filter capacitors

close to the ADT7518.

6.

If the distance to the remote sensor is more than 8 inches,

the use of twisted-pair cable is recommended. This will

work up to about 6 feet to 12 feet.

7.

For long distances (up to 100 feet), use shielded twisted-

pair cable, such as Belden #8451 microphone cable.

Connect the twisted pair to D+ and D– and the shield to

GND close to the ADT7518. Leave the remote end of the

shield unconnected to avoid ground loops.

Because the measurement technique uses switched current

sources, excessive cable and/or filter capacitance can affect the

measurement. When using long cables, the filter capacitor may

be reduced or removed.

Cable resistance can also introduce errors. Series resistance of

1 Ω introduces about 0.5°C error.

Temperature Value Format

One LSB of the ADC corresponds to 0.25°C. The ADC can

theoretically measure a temperature span of 255°C. The internal

temperature sensor is guaranteed to a low value limit of –40°C.

It is possible to measure the full temperature span using the

external temperature sensor. The temperature data format is

shown in Table 9.

The result of the internal or external temperature measure-

ments is stored in the temperature value registers, and is com-

pared with limits programmed into the internal or external high

and low registers.

Table 9. Temperature Data Format (Internal and External

Temperature)

Temperature

Digital Output

–40°C

11 0110 0000

–25°C

11 1001 1100

–10°C

11 1101 1000

–0.25°C

11 1111 1111

0°C

00 0000 0000

+0.25°C

00 0000 0001

+10°C

00 0010 1000

+25°C

00 0110 0100

+50°C

00 1100 1000

+75°C

01 0010 1100

+100°C

01 1001 0000

+105°C

01 1010 0100

+125°C

01 1111 0100

Temperature Conversion Formula:

Positive Temperature = ADC Code/4

Negative Temperature = (ADC Code* – 512)/4

*

where DB9 is removed from the ADC code.

Interrupts

The measured results from the internal temperature sensor,

external temperature sensor, VDD pin, and AIN inputs are

compared with the THIGH/VHIGH (greater than comparison) and

TLOW/VLOW (less than or equal to comparison) limits. An inter-

rupt occurs if the measurement exceeds or equals the limit

registers. These limits are stored in on-chip registers. Note that

the limit registers are 8 bits long while the conversion results are

10 bits long. If the limits are not masked, any out-of-limit com-

parisons generate flags that are stored in the Interrupt Status 1

register (Address 00h) and Interrupt Status 2 register

(Address 01h). One or more out-of-limit results will cause the

INT/INT output to pull either high or low depending on the

output polarity setting. It is good design practice to mask out

interrupts for channels that are of no concern to the application.

Figure 49 shows the interrupt structure for the ADT7518. It

gives a block diagram representation of how the various

measurement channels affect the INT/INT pin.

ADT7518 REGISTERS

The ADT7518 contains registers that are used to store the

results of external and internal temperature measurements, VDD

value measurements, analog input measurements, high and low

temperature limits, supply voltage and analog input limits, to set

output DAC voltage levels, to configure multipurpose pins, and

generally to control the device. A description of these registers

follows.

The register map is divided into registers of 8 bits. Each register

has its own individual address, but some consist of data that is

linked to other registers. These registers hold the 10-bit conver-

sion results of measurements taken on the temperature, VDD,

and AIN channels. For example, the eight MSBs of the VDD

measurement are stored in Register Address 06h, while the two

LSBs are stored in Register Address 03h. These types of registers

are linked such that when the LSB register is read first, the MSB

registers associated with that LSB register are locked to prevent

any updates. To unlock these MSB registers, the user has only to

read any one of them, which will have the effect of unlocking all

previously locked MSB registers. So, for the preceding example,

if Register 03h was read first, MSB Registers 06h and 07h would

be locked to prevent any updates to them. If Register 06h were

read, this register and Register 07h would be subsequently

unlocked.

LOCK ASSOCIATED

MSB REGISTERS

FIRST READ

COMMAND

LSB

REGISTER

OUTPUT

DATA

Figure 51. Phase 1 of 10-Bit Read

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