ADT7488A

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9

voltage and therefore has adequate headroom to cope with

overvoltage. The full−scale voltage that can be recorded for

each channel is shown in Table 6.

Table 6. Maximum Reported Input Voltages

Voltage Channel

Full−Scale Voltage

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

35pF

30pF

30pF

MUX

68kΩ

71kΩ

45kΩ

94kΩ

17.5kΩ

52.5kΩ

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

0x12 with a read length of 0x06. The data is retrieved in the

order listed in Table 7.

Table 7. Voltage Measurement Command Code

Voltage Channel

Command Code

Returned Data

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

LSB

MSB

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 ADT7488A has three dedicated temperature

measurement channels: one for measuring the temperature

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

measuring the temperature of a remote diode, usually

located in the CPU or GPU.

The ADT7488A monitors one local and two remote

temperature channels. 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

2

Remote 2 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 ADT7488A 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.

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