ADM1021

–11–

REV. 0

APPLICATIONS INFORMATION

FACTORS AFFECTING ACCURACY

Remote Sensing Diode

The ADM1021 is designed to work with substrate transistors

built into processors, or with discrete transistors. Substrate

transistors will generally be PNP types with the collector con-

nected to the substrate. Discrete types can be either PNP or

NPN, connected as a diode (base shorted to collector). If an

NPN transistor is used then the collector and base are con-

nected to D+ and the emitter to D–. If a PNP transistor is used

then the collector and base are connected to D– and the emitter

to D+.

The user has no choice in the case of substrate transistors, but if

a discrete transistor is used the best accuracy will be obtained by

choosing devices according to the following criteria:

1. Base-emitter voltage greater than 0.25 V at 6

µA, at the high-

est operating temperature.

2. Base-emitter voltage less than 0.95 V at 100

µA, at the lowest

operating temperature.

3. Base resistance less than 100

Ω.

Transistors such as 2N3904, 2N3906 or equivalents in SOT-23

package are suitable devices to use.

Thermal Inertia and Self-Heating

Accuracy depends on the temperature of the remote-sensing

diode and/or the internal temperature sensor being at the same

temperature as that being measured, and a number of factors

can affect this. Ideally, the sensor should be in good thermal

contact with the part of the system being measured, for example

the processor. If it is not, the thermal inertia caused by the mass

of the sensor will cause a lag in the response of the sensor to a

temperature change. In the case of the remote sensor this should

not be a problem, as it will be either a substrate transistor in the

processor or a small package device such as SOT-23 placed in

close proximity to it.

The on-chip sensor, however, will often be remote from the

processor and will only be monitoring the general ambient tem-

perature around the package. The thermal time constant of the

QSOP-16 package is about 10 seconds.

In practice, the package will have electrical, and hence thermal,

connection to the printed circuit board, so the temperature rise

due to self-heating will be negligible.

LAYOUT CONSIDERATIONS

Digital boards can be electrically noisy environments, and the

ADM1021 is measuring very small voltages from the remote

sensor, so care must be taken to minimize noise induced at the

sensor inputs. The following precautions should be taken:

1. Place the ADM1021 as close as possible to the remote sens-

ing diode. Provided that the worst noise sources such as

clock generators, data/address buses and CRTs are avoided,

this distance can be four to eight inches.

2. Route the D+ and D– tracks close together, in parallel, with

grounded guard tracks on each side. Provide a ground plane

under the tracks if possible.

3. Use wide tracks to minimize inductance and reduce noise

pickup. 10 mil track minimum width and spacing is recom-

mended.

GND

D+

D-

GND

10 mil.

10 mil.

10 mil.

10 mil.

10 mil.

10 mil.

10 mil.

Figure 17. Arrangement of Signal Tracks

4. Try to minimize the number of copper/solder joints, which

can cause thermocouple effects. Where copper/solder joints

are used, make sure that they are in both the D+ and D–

path and at the same temperature.

Thermocouple effects should not be a major problem as 1

°C

corresponds to about 240

µV, and thermocouple voltages are

about 3

µV/°C of temperature difference. Unless there are

two thermocouples with a big temperature differential be-

tween them, thermocouple voltages should be much less

than 240

µV.

5. Place a 0.1

µF bypass capacitor close to the V

2200 pF input filter capacitors across D+, D– close to the

ADM1021.

6. If the distance to the remote sensor is more than eight inches,

the use of twisted pair cable is recommended. This will work

up to about 6 to 12 feet.

7. For really long distances (up to 100 feet), use shielded twisted

pair such as Belden #8451 microphone cable. Connect the

twisted pair to D+ and D– and the shield to GND close to

the ADM1021. Leave the remote end of the shield uncon-

nected to avoid ground loops.

Because the measurement technique uses switched current

sources, excessive cable and/or filter capacitance can affect the

measurement. When using long cables, the filter capacitor may

be reduced or removed.

Cable resistance can also introduce errors. 1

Ω series resistance

introduces about 0.5

°C error.