ADT7485A

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9

Table 6. Maximum Reported Input Voltages

Voltage Channel

Full−Scale Voltage

12 V

16 V

5.0 V

8.0 V

4.0 V

2.5 V

4.0 V

4.0 V

Input Circuitry

The internal structure for the analog inputs is shown in

Figure 14. The input circuit consists of an input protection

diode and an attenuator, plus a capacitor that forms a

first−order, low−pass filter to provide input immunity to

high frequency noise.

Figure 14. Internal Structure of Analog Inputs

17.5k

52.5k

35pF

45k

94k

30pF

68k

71k

30pF

93k

47k

30pF

120k

20k

30pF

MUX

W

W

W

W

W

W

W

W

W

W

Voltage Measurement Command Codes

The voltage measurement command codes are detailed in

Table 7. Each voltage measurement has a read length of two

bytes in little endian format (LSB followed by MSB). All

voltages can be read together by addressing Command Code

0x10 with a read length of 0x10. The data is retrieved in the

order listed in Table 7.

Table 7. Voltage Measurement Command Code

Voltage Channel

Command Code

Returned Data

12 V

0x10

LSB, MSB

5.0 V

0x11

LSB, MSB

0x12

LSB, MSB

2.5 V

0x13

LSB, MSB

0x14

LSB, MSB

Voltage Data Format

The returned voltage value is in twos complement, 16−bit,

binary format. The format is structured so that voltages in

the range of

±32 V can be reported. In this way, the reported

value represents the number of 1/1024 V in the actual

reading, allowing a resolution of approximately 1 mV.

Voltage

Twos Complement

MSB

LSB

12

0011 0000

0000 0000

5.0

0001 0100

0000 0000

3.3

0000 1101

0011 0011

3.0

0000 1100

0000 0000

2.5

0000 1010

0000 0000

1.0

0000 0100

0000 0000

0

0000 0000

0000 0000

Temperature Measurement

The ADT7485A has two dedicated temperature

measurement channels: one for measuring the temperature

of an on−chip band gap temperature sensor, and one for

measuring the temperature of a remote diode, usually

located in the CPU or GPU.

The ADT7485A monitors one local and one remote

temperature channel. Monitoring of each of the channels is

done in a round−robin sequence. The monitoring sequence

is in the order shown in Table 9.

Table 9. Temperature Monitoring Sequence

Channel

Number

Measurement

Conversion

Time (ms)

0

Local temperature

12

1

Remote 1 temperature

38

Temperature Measurement Method

A simple method for measuring temperature is to exploit

the negative temperature coefficient of a diode by measuring

constant current. Unfortunately, this technique requires

which varies from device to device.

The technique used in the ADT7485A measures the

currents.

Figure 15 shows the input signal conditioning used to

measure the output of a remote temperature sensor. This

figure shows the remote sensor as a substrate transistor,

which is provided for temperature monitoring on some

microprocessors, but it could also 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 D1− input.

If the sensor is operating in an extremely noisy environment,

C1 can be added as a noise filter. Its value should not exceed

1000 pF.