ADT7485A

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10

Figure 15. Signal Conditioning for Remote Diode

Temperature Sensors

C1*

D1+

D1–

I

N1 × IN2 × I

LOW−PASS FILTER

TRANSISTOR

REMOTE

SENSING

*CAPACITOR C1 IS OPTIONAL.

BIAS

DIODE

TO

IN NOISY ENVIRONMENTS.

ADC

IT SHOULD ONLY BE USED

To measure

sensor is switched between three related currents. Figure 15

shows N1 x I and N2 x I as different multiples of the

current I. The currents through the temperature diode are

switched between I and N1 x I, giving

between I and N2 x I, giving

then be calculated using the two

method can also cancel the effect of series resistance on the

temperature measurement. The resulting

are passed through a 65 kHz low−pass filter to remove noise

and then through a chopper−stabilized amplifier to amplify

and rectify the waveform, producing a dc voltage

proportional to

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 is performed in the same manner.

Reading Temperature Measurements

The temperature data returned is two bytes in little endian

format, that is, LSB before MSB. All temperatures can be

read together by using Command Code 0x00 with a read

length of 0x04. The command codes and returned data are

described in Table 10.

Table 10. Temperature Channel Command Codes

Temp

Channel

Command

Code

Returned Data

Internal

0x00

LSB, MSB

External

0x01

LSB, MSB

All Temps

0x00

Internal LSB, Internal MSB;

External LSB, External MSB

SST Temperature Sensor Data Format

The data for temperature is structured to allow values in

the range of

±512°C to be reported. Thus, the temperature

sensor format uses a twos complement, 16−bit binary value

to represent values in this range. This format allows

temperatures to be represented with approximately a

0.016

°C resolution.

Table 11. SST Temperature Data Format

Temperature (5C)

Twos Complement

MSB

LSB

−125

1110 0000

1100 0000

−80

1110 1100

0000 0000

−40

1111 0110

0000 0000

−20

1111 1011

0011 1110

−5

1111 1110

1100 0000

−1

1111 1111

1100 0000

0

0000 0000

0000 0000

+1

0000 0000

0100 0000

+5

0000 0001

0100 0000

+20

0000 0100

1100 0010

+40

0000 1010

0000 0000

+80

0001 0100

0000 0000

+125

0001 1111

0100 0000

Using Discrete Transistors

If a discrete transistor is used, the collector is not grounded

and should be linked to the base. If a PNP transistor is used,

the base is connected to the D− input and the emitter is

connected to the D+ input. If an NPN transistor is used, the

emitter is connected to the D− input and the base is

connected to the D+ input.

Figure 16 shows how to connect the ADT7485A to an NPN

or PNP transistor for temperature measurement. 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

D1− input.

Figure 16. Connections for NPN and PNP Transistors

D1+

D1–

ADT7485A

2N3904

NPN

D1+

D1–

ADT7485A

2N3906

PNP

The ADT7485A shows an external temperature value of

0x8000 if the external diode is an open or short circuit.

Layout Considerations

Digital boards can be electrically noisy environments.

Take the following precautions to protect the analog inputs

from noise, particularly when measuring the very small

voltages from a remote diode sensor:

sensing diode. Provided that the worst noise sources,

such as clock generators, data/address buses, and CRTs,

are avoided, this distance can be four to eight inches.

with grounded guard tracks on each side. Provide a

ground plane under the tracks if possible.