ADT7462ACPZ-R7 datasheet(19/81 Pages) ONSEMI

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ADT7462

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constant current. Unfortunately, this technique requires

calibration to cancel the effect of the absolute value of VBE,

which varies from device to device.

The technique used in the ADT7462 is to measure the

change in VBE when the device is operated at three different

currents. Previous devices have used only two operating

currents; use of a third current allows automatic cancellation

of any resistances in series with the external temperature

sensor.

Figure 33 shows the input signal conditioning used to

measure the output of an external temperature sensor. This

figure shows the external sensor as a substrate transistor, but

it could equally be a discrete transistor. If a discrete

transistor is used, the collector is not grounded and should

be linked to the base. To prevent ground noise from

interfering with the measurement, the more negative

terminal of the sensor is not referenced to ground but is

biased above ground by an internal diode at the D− input. C1

can optionally be added as a noise filter (recommended

maximum value 1000 pF). However, a better option in noisy

environments is to add a filter, as described in the Noise

Filtering section.

To measure

DVBE, the operating current through the

sensor is switched among three related currents. As shown

in Figure 33, N1

× I and N2 × I are different multiples of the

Current I. The currents through the temperature diode are

switched between I and N1

× I, giving DVBE1, and then

between I and N2

× I, giving DVBE2. The temperature can

then be calculated using the two

DVBE measurements. This

method can also be shown to cancel the effect of any series

resistance on the temperature measurement.

The resulting

DVBE waveforms are passed through a

65 kHz low−pass filter to remove noise and then to a

chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to

DVBE.

The ADC digitizes this voltage, and a temperature

measurement is produced. To reduce the effects of noise,

digital filtering is performed by averaging the results of 16

measurement cycles for low conversion rates.

Signal conditioning and measurement of the internal

temperature sensor are performed in the same manner (see

Figure 33).

Temperature Measurement Results

The results of the local and remote temperature

measurements are stored in the local and remote temperature

value registers and are compared with limits programmed

into the local and remote high and low limit registers.

Table 8. Temperature Measurement Registers

Temperature Value

Local Temperature, LSB

Local Temperature, MSB

Remote 1 Temperature, LSB

Remote 1 Temperature, MSB

Remote 2 Temperature, LSB

Remote 2 Temperature, MSB

Remote 3 Temperature, LSB

Remote 3 Temperature, MSB

Figure 33. Input Signal Conditioning

C1*

D+

BIAS

DIODE

*CAPACITOR C1 IS OPTIONAL. IT SHOULD ONLY BE USED IN NOISY ENVIRONMENTS.

VCC

TO ADC

VOUT+

VOUT–

REMOTE

SENSING

TRANSISTOR

D–

N1 y I

N2 y I

IBIAS

LOW−PASS FILTER

fC = 65kHz

The temperature value is stored in two registers. The MSB

has a resolution of 1

°C. Only two bits in the temperature

LSB register are used, Bit 7 and Bit 6, giving a temperature

measurement resolution of 0.25

°C. The temperature

measurement range for both local and remote measurements

is from −64

°C to +191°C. However, the ADT7462 itself

should never be operated outside its operating temperature

range, which is from −40

°C to +125°C. For the remote

diode, the user should refer to the data sheet of the diode.

Table 9. Temperature Data Format

Temperature Value

MSB

LSB

−64°C

0000 0000

−50.25°C

0000 1110

0100 0000

−25°C

0010 0111

0000 0000

0°C

0100 0000

0000 0000

+25°C

0101 1001

0000 0000

+50.25°C

0111 0010

0100 0000

+100°C

1010 0100

0000 0000

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