ADM1023

http://onsemi.com

7

Theory of Operation

Functional Description

The ADM1023 contains a two−channel analog−to−digital

converter (ADC) with special input−signal conditioning to

enable

operation

with

remote

and

on−chip

diode

temperature sensors. When the ADM1023 is operating

normally, the ADC operates in a free−running mode. The

analog input multiplexer alternately selects either the

on−chip temperature sensor to measure its local temperature

or the remote temperature sensor. These signals are digitized

by the ADC, and the results are stored in the local and remote

temperature value registers. Only the eight most significant

bits (MSBs) of the local temperature value are stored as an

8−bit binary word. The remote temperature value is stored

as an 11−bit binary word in two registers. The eight MSBs

are stored in the remote temperature value high byte register

at Address 0x01. The three least significant bits (LSBs) are

stored, left justified, in the remote temperature value low

byte register at Address 0x10.

Error sources such as PCB track resistance and clock noise

can introduce offset errors into measurements on the remote

channel. To achieve the specified accuracy on this channel,

these offsets must be removed, and two offset registers are

provided for this purpose at Address 0x11 and Address

0x12.

An offset value may automatically be added to or

subtracted from the measurement by writing an 11−bit, twos

complement value to Register 0x11 (high byte) and Register

0x12 (low byte, left−justified).

The offset registers default to 0 at powerup and have no

effect if nothing is written to them.

The measurement results are compared with local and

remote, high and low temperature limits, stored in six

on−chip limit registers. As with the measured value, the

local temperature limits are stored as 8−bit values and the

remote temperature limits as 11−bit values. Out−of−limit

comparisons generate flags that are stored in the status

register, and one or more out−of−limit results cause the

ALERT output to pull low.

Registers can be programmed, and the device controlled

and configured, via the serial system management bus

(SMBus). The contents of any register can also be read back

via the SMBus.

Control and configuration functions consist of:

standby mode.

On initial powerup, the remote and local temperature

values default to −128

°C. The device normally powers up

converting, making a measure of local and remote

temperature. These values are then stored before making a

comparison with the stored limits. However, if the part is

powered up in standby mode (STBY pin pulled low), no new

values are written to the register before a comparison is

made. As a result, both RLOW and LLOW are tripped in the

status register, thus generating an ALERT output. This may

be cleared in one of two ways:

–128

°C and read the status register (which in turn

clears the ALERT output).

(which in turn clears the ALERT output). This works

only when the measured values are within the limit

values.

Figure 13. Input Signal Conditioning

D+

D–

REMOTE

SENSING

TRANSISTOR

I

N y I

TO ADC

BIAS

DIODE

LOW−PASS FILTER

1

C1 = 1000pF MAX.

CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.

Measurement Method

A simple method of measuring temperature is to exploit

the negative temperature coefficient of a diode, or the base

emitter voltage of a transistor, operating at constant current.

Thus, the temperature may be obtained from a direct

(eq. 1)

V

nKT

q

1n

I

C

I

S

This technique, however, requires calibration to nullify

device to device.

The technique used in the ADM1023 is to measure the

collector currents.