ADM1021A

–5–

REV. D

4

0

2

FREQUENCY – Hz

10mV p-p

100k

1M

10M

100M

1G

1

3

TPC 7. Temperature Error vs. Differential-Mode Noise

Frequency

CONVERSION RATE – Hz

250

0.125

0.25

0.5

8

300

350

400

550

4

0.0625

450

500

200

150

100

50

5V

2

1

3.3V

TPC 8. Operating Supply Current vs. Conversion Rate

0

20

SUPPLY VOLTAGE – V

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

40

60

80

100

–20

TPC 9. Standby Supply Current vs. Supply Voltage

TIME – Seconds

0

25

50

75

100

125

REMOTE

TEMPERATURE

INT

TEMPERATURE

02

3

4

567

89

10

1

TPC 10. Response to Thermal Shock

FUNCTIONAL DESCRIPTION

The ADM1021A contains a two-channel A-to-D converter with

special input-signal conditioning to enable operation with remote and

on-chip diode temperature sensors. When the ADM1021A is operat-

ing normally, the A-to-D converter 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 stored in the Local and Remote Temperature Value

Registers as 8-bit, twos complement words.

The measurement results are compared with local and remote,

high and low temperature limits, stored in four on-chip registers.

Out-of-limit comparisons generate flags that are stored in the

status register, and one or more out-of-limit results will cause

the

ALERT output to pull low.

The limit registers can be programmed, and the device con-

trolled and configured, via the serial System Management Bus.

The contents of any register can also be read back via the SMBus.

Control and configuration functions consist of:

mode.

On initial power-up, the remote and local temperature values

default to –128

°C. Since the device normally powers up converting,

a measurement of local and remote temperature is made and these

values are then stored before a comparison with the stored limits

is made. 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 out-

put. This may be cleared in one of two ways:

1. Change both the local and remote lower limits to –128

°C

and read the status register (which in turn clears the

ALERT

output).

2. Take the part out of standby and read the status register

(which in turn clears the

ALERT output). This will work only

if the measured values are within the limit values.

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, operated at constant current. Unfortu-

nately, this technique requires calibration to null out the effect