AD7416ARMZ datasheet(12/24 Pages) AD

12 / 24 page

AD7416/AD7417/AD7418

Rev. I | Page 12 of 24

An external 2.5 V reference can be connected to the REFIN pin.

This has the effect of shutting down the on-chip reference

circuitry.

REFIN

SW1

26kΩ

2.5V

24kΩ

1.2V

EXTERNAL

REFERENCE

DETECT

BUFFER

Figure 11. On-Chip Reference

TEMPERATURE MEASUREMENT

A common method of measuring temperature is to exploit the

negative temperature coefficient of a diode, or the base-emitter

voltage of a transistor, operated at a constant current. Unfortu-

nately, this technique requires calibration to null out the effect

of the absolute value of VBE, which varies from device to device.

The technique used in the AD7416/AD7417/AD7418 is to

measure the current change in VBE when the device is operated

at two different currents.

This is given by

()

/ ×

where:

K is Boltzmann’s constant.

q is the charge on the electron (1.6 × 10−19 Coulombs).

T is the absolute temperature in Kelvins.

N is the ratio of the two currents.

SENSING

TRANSISTOR

TO ADC

SENSING

TRANSISTOR

VOUT+

VDD

VOUT–

IN × I

Figure 12. Temperature Measurement Technique

Figure 12 shows the method the AD7416/AD7417/AD7418 use

to measure the device temperature. To measure ΔVBE, the

sensor (substrate transistor) is switched between operating

currents of I and N × I. The resulting waveform is passed through

a chopper-stabilized amplifier that performs the functions of

amplification and rectification of the waveform to produce a dc

voltage proportional to ΔVBE. This voltage is measured by the ADC

to give a temperature output in 10-bit twos complement form.

The temperature resolution of the ADC is 0.25°C, which corres-

ponds to 1 LSB of the ADC. The ADC can theoretically measure a

temperature span of 255°C; the guaranteed temperature range is

−40°C to +125°C. The result of the conversion is stored in the

temperature value register (0x00) as a 16-bit word. The 10 MSBs

of this word store the temperature measurement (see Table 9

and Table 10).

The temperature conversion formulas using the 10 MSBs of the

temperature value register are

Positive Temperature = ADC Code/4

(1)

Negative Temperature = (ADC Code − 512)/4

(2)

The MSB is removed from ADC Code in Equation 2.

INTERNAL REGISTER STRUCTURE

The AD7417/AD7418 have seven internal registers, as shown in

Figure 13. Six of these are data registers and one is an address

pointer register. The AD7416 has five internal registers (the

ADC and Config2 registers are not applicable to the AD7416).

TEMPERATURE

VALUE

(READ-ONLY

ADDRESS 0x00)

CONFIGURATION

(READ/WRITE

ADDRESS 0x01)

THYST SETPOINT

(READ/WRITE

ADDRESS 0x02)

TOTI SETPOINT

(READ/WRITE

ADDRESS 0x03)

ADC VALUE

(READ-ONLY

ADDRESS 0x04)

CONFIG2

(READ/WRITE

ADDRESS 0x05)

SDA

DATA

SCL

ADDRESS POINTER

(SELCTS DATA REGISTER

FOR READ/WRITE)

ADDRESS

SERIAL BUS INTERFACE

Figure 13. AD7417/AD7418 Register Structure

Address Pointer Register

The address pointer register is an 8-bit register that stores an

address that points to one of the six data registers. The first data

byte of every serial write operation to the AD7416/AD7417/

AD7418 is the address of one of the data registers, which is

stored in the address pointer register, and selects the data

three LSBs of the address pointer register are used to select a

data register.

Table 7. Address Pointer Register

P71

P61

P51

P41

P31

1 P3 to P7 must be set to 0.

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