Datasheet

AD7414/AD7415
Rev. F | Page 8 of 20
V
DD
V
OUT
+
V
OUT
–
TO ADC
SENSING
TRANSISTOR
SENSING
TRANSISTOR
II× N
02463-007
Figure 7. Temperature Measurement Technique
Figure 7 shows the method the AD7414/AD7415 use to
measure the ambient device temperature. To measure ΔV
BE
,
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 ΔV
BE
. 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:
4
)(dCodeADC
eTemperaturPositive =
4
512)(
−
=
dCodeADC
eTemperaturNegative
Note that DB9 is removed from the ADC code in the negative
temperature formula.