ADT7462ACPZ-R7 datasheet(21/81 Pages) ONSEMI

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ADT7462

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Layout Considerations

Digital boards can be electrically noisy environments.

The ADT7462 measures 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:

• Place the ADT7462 as close as possible to the remote

sensing diode. Provided that the worst noise sources,

such as clock generators, data/address buses, and CRTs,

are avoided, this distance can be 4 inches to 8 inches.

• Route the D+ and D− tracks close together, in parallel,

with grounded guard tracks on each side. To minimize

inductance and reduce noise pickup, a 5 mil track width

and spacing is recommended. If possible, provide a

ground plane under the tracks.

Figure 34. Typical Arrangement of Signal Tracks

GND

D+

D–

GND

5MIL

• Minimize the number of copper/solder joints that can

cause thermo−couple 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

because 1

°C corresponds to about 200 mV, and

thermocouple voltages are about 3 mV/

°C of

temperature difference. Unless there are two

thermocouples with a large temperature differential

between them, thermocouple voltages should be much

less than 200 mV.

• Place a 0.1 mF bypass capacitor close to the VCC pin. In

extremely noisy environments, an input filter capacitor

can be placed across D+ and D− close to the ADT7462.

This capacitance can affect the temperature

measurement, so care must be taken to ensure that any

capacitance seen at D+ and D− is a maximum of

1000 pF. This maximum value includes the filter

capacitance, plus any cable or stray capacitance

between the pins and the sensor diode.

• If the distance to the remote sensor is more than

8 inches, the use of twisted pair cable is recommended.

This works from about 6 feet up to 12 feet.

• For really long distances (up to 100 feet), use shielded

twisted pair, such as Belden No. 8451 microphone

cable. Connect the twisted pair to D+ and D− and the

shield to GND close to the ADT7462. Leave the remote

end of the shield unconnected to avoid ground loops.

• Because the measurement technique uses switched

current sources, excessive cable or filter capacitance

can affect the measurement. When using long cables,

the filter capacitance can be reduced or removed.

Noise Filtering

For temperature sensors operating in noisy environments,

the industry−standard practice is to place a capacitor across

the D+ and D− pins to help combat the effects of noise.

However, large capacitances affect the accuracy of the

temperature measurement, leading to a recommended

maximum capacitor value of 1000 pF. While this capacitor

does reduce noise, it does not eliminate it, making it difficult

to use the sensor in a very noisy environment.

The ADT7462 has a major advantage over other devices

in eliminating the effects of noise on the external sensor. The

series resistance cancellation feature allows a filter to be

constructed between the external temperature sensor and the

device. The effect of any filter resistance seen in series with

the remote sensor is automatically canceled from the

temperature result.

The construction of a filter allows the ADT7462 and the

remote temperature sensor to operate in noisy environments.

Figure 35 shows a low−pass RCR filter, with the following

values:

R = 100

C = 1 nF

This filtering reduces both common−mode noise and

differential noise.

Figure 35. Filter Between Remote Sensor and

ADT7462

100Ω

1nF

REMOTE

SENSOR

D+

D–

Voltage Measurement

The ADT7462 is capable of measuring up to 13 different

voltage inputs at one time. Table 11 is a list of the voltage

measurement inputs and the corresponding input pins. Each

pin can be configured to measure the desired voltage option

using the Pin Configuration 1 (0x10) to Pin Configuration 4

(0x13) registers or the easy configuration options.

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