AD7818ARM-REEL 查看數據表（PDF） - Analog Devices

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AD7818ARM-REEL

Dual, Low Power CMOS, Analog Front End with DSP Microcomputer

Analog Devices 

Data Sheet

CONTROL BYTE

The AD7817/AD7818 contain two on-chip registers, the address

register and the overtemperature register. These registers can be

accessed by carrying out an 8-bit serial write operation to the devices.

The 8-bit word or control byte written to the AD7817/AD7818 is

transferred to one of the two on-chip registers as follows.

Address Register

If the five MSBs of the control byte are logic zero, the three LSBs

of the control byte are transferred to the address register (see

Figure 6). The address register is a 3-bit-wide register used to

select the analog input channel on which to carry out a conversion.

It is also used to select the temperature sensor, which has the 000

address. Table 6 shows the channel selection. The internal reference

selection connects the input of the ADC to a band gap reference.

When this selection is made and a conversion is initiated, the ADC

output must be approximately midscale. After power-up, the

default channel selection is DB2 = DB1 = DB0 = 0 (temperature

sensor).

Table 6. Channel Selection

DB2 DB1 DB0 Channel Selection

0

0

0

Temperature sensor

0

0

1

Channel 1

0

1

0

Channel 2

0

1

1

Channel 3

1

0

0

Channel 4

1

1

1

Internal reference (1.23 V)

Device

All

All

AD7817

AD7817

AD7817

All

AD7817/AD7818

Overtemperature Register

If any of the five MSBs of the control byte are logic one, the entire

eight bits of the control byte are transferred to the overtemperature

register (see Figure 6). At the end of a temperature conversion,

a digital comparison is carried out between the 8 MSBs of the

temperature conversion result (10 bits) and the contents of the

overtemperature register (8 bits). If the result of the temperature

conversion is greater than the contents of the overtemperature

register (OTR), the overtemperature indicator (OTI) goes logic

low. The resolution of the OTR is 1°C. The lowest temperature

that can be written to the OTR is −95°C and the highest is

+152°C (see Figure 7). However, the usable temperature range of

the temperature sensor is −55°C to +125°C. Figure 7 shows the

OTR and how to set TALARM (the temperature at which the OTI

goes low).

OTR (Dec) = TALARM (°C) + 103°C

For example, to set TALARM to 50°C, OTR = 50 + 103 = 153 Dec

or 10011001 bin. If the result of a temperature conversion exceeds

50°C, OTI goes logic low. The OTI logic output is reset high at the

end of a serial read operation or if a new temperature measurement

is lower than TALARM. The default power on TALARM is 50°C.

DB2 DB1 DB0 ADDRESS REGISTER

IF ANY BIT DB7 TO DB3 ARE LOGIC 0

THEN DB2 TO DB0 ARE WRITTEN TO

THE ADDRESS REGISTER

MSB

LSB

DB7 DB6 DB5 DB4 DB3 DB2 DB1 DB0 CONTROL BYTE

DB7

IF ANY BIT DB7 TO DB3 IS SET TO A

LOGIC 1, THEN THE FULL 8 BITS OF THE

CONTROL WORD ARE WRITTEN TO THE

OVERTEMPERATURE REGISTER

DB6 DB5 DB4 DB3 DB2 DB1

DB0

OVERTEMPERATURE

REGISTER (OTR)

Figure 6. Address and Overtemperature Register Selection

OVERTEMPERATURE REGISTER

MSB

LSB

DB7

DB6

DB5

DB4

DB3

DB2

DB1

DB0

0

0

0

0

1

0

0

0

1

1

1

1

1

1

1

1

OVERTEMPERATURE REGISTER (DEC) = TALARM + 103°C

TALARM RESOLUTION = 18/LSB

Figure 7. The Overtemperature Register (OTR)

MINIMUM TEMPERATURE = –95°C

MAXIMUM TEMPERATURE = +152°C

Rev. D | Page 11 of 20

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