REV. 0

ADM1020

–6–

Figure 11 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, provided for tem-

perature monitoring on some microprocessors, but it could

equally well be a discrete transistor. If a discrete transistor is

used, the collector will not be grounded, and should be linked to

the base. To prevent ground noise interfering with the measure-

ment, 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. If the sensor is operating in a noisy environment,

C1 may optionally be added as a noise filter. Its value is typi-

cally 2200 pF but should be no more than 3000 pF. See the

section on Layout Considerations for more information on C1.

To measure

currents of I and N

× I. The resulting waveform is passed

through a 65 kHz low-pass filter to remove noise, hence to a

chopper-stabilized amplifier that performs the functions of am-

plification and rectification of the waveform to produce a dc

voltage proportional to

ADC to give a temperature output in 8-bit twos complement

format. To further reduce the effects of noise, digital filtering is

performed by averaging the results of 16 measurement cycles.

Signal conditioning and measurement of the internal tempera-

ture sensor is performed in a similar manner.

TEMPERATURE DATA FORMAT

One LSB of the ADC corresponds to 1

°C, so the ADC can

theoretically measure from –128

°C to +127°C, although the

practical lowest value is limited to –65

°C due to device maxi-

mum ratings. The temperature data format is shown in Table I.

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.

C1*

D+

D–

REMOTE

SENSING

TRANSISTOR

IN

I

TO ADC

BIAS

DIODE

*CAPACITOR C1 IS OPTIONAL.

IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

C1 = 2.2nF TYPICAL, 3nF MAX.

LOWPASS FILTER

Figure 11. Input Signal Conditioning

Table I. Temperature Data Format

Temperature

Digital Output

–128

°C

1 000 0000

–125

°C

1 000 0011

–100

°C

1 001 1100

–75

°C

1 011 0101

–50

°C

1 100 1110

–25

°C

1 110 0111

–1

°C

1 111 1111

0

°C

0 000 0000

+1

°C

0 000 0001

+10

°C

0 000 1010

+25

°C

0 001 1001

+50

°C

0 011 0010

+75

°C

0 100 1011

+100

°C

0 110 0100

+125

°C

0 111 1101

+127

°C

0 111 1111

ADM1020 REGISTERS

The ADM1020 contains nine registers that are used to store the

results of remote and local temperature measurements, high and

low temperature limits, and to configure and control the device.

A description of these registers follows, and further details are

given in Tables II to IV. It should be noted that the ADM1020’s

registers are dual port, and have different addresses for read and

write operations. Attempting to write to a read address, or to

read from a write address, will produce an invalid result. Regis-

ter addresses above 0F are reserved for future use or used for

factory test purposes and should not be written to.

Address Pointer Register

The Address Pointer Register itself does not have, or require, an

address, as it is the register to which the first data byte of every

write operation is automatically written. This data byte is an

address pointer that sets up one of the other registers for the

second byte of the write operation, or for a subsequent read

operation.