EVAL-AD7414SLASH15EBZ datasheet(8/20 Pages) AD

8 / 20 page

AD7414/AD7415

Rev. F | Page 8 of 20

VDD

VOUT+

VOUT–

TO ADC

SENSING

TRANSISTOR

SENSING

TRANSISTOR

× N

Figure 7. Temperature Measurement Technique

Figure 7 shows the method the AD7414/AD7415 use to

measure the ambient 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 format.

TEMPERATURE DATA FORMAT

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

corresponds to 1 LSB of the ADC. The ADC can theoretically

measure a temperature span of 255°C; the lowest practical value

is limited to −40°C due to the device maximum ratings. The

A grade can measure a temperature range of −40°C to +125°C.

(Temperature data format is shown in Table 5.)

Table 5. A Grade Temperature Data Format

Temperature

Digital Output DB9…DB0

−55°C

11 0010 0100

−50°C

11 0011 1000

−25°C

11 1001 1100

−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

+125°C

01 1111 0100

The grade temperature conversion formula follows:

)

Code

ADC

Temperatur

Positive

512

)

(

−

Code

ADC

Temperatur

Negative

Note that DB9 is removed from the ADC code in the negative

temperature formula.

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