ADT7490ARQZ-R7 datasheet(20/75 Pages) ONSEMI

20 / 75 page

ADT7490

http://onsemi.com

switched between I and N1 x I, giving

DVBE1, and then

between I and N2 x I, giving

DVBE2. The temperature can

then be calculated using the two

DVBE measurements. This

method can also cancel the effect of any series resistance on

the temperature measurement.

The resulting

DVBE waveforms are passed through a

65 kHz low−pass filter to remove noise and then to a

chopper−stabilized amplifier. This amplifies and rectifies

the waveform to produce a dc voltage proportional to

DVBE.

The ADC digitizes this voltage, and a temperature

measurement is produced. To reduce the effects of noise,

digital filtering is performed by averaging the results of 16

measurement cycles.

The results of remote temperature measurements are

stored in 10−bit, twos complement format, as listed in

Table 11. The extra resolution for the temperature

measurements is held in the Extended Resolution Register 2

(0x77). This gives temperature readings with a resolution of

0.25

°C.

Figure 27. Signal Conditioning for Remote Diode Temperature Sensors

D+

VDD

TO ADC

VOUT+

VOUT–

REMOTE

SENSING

TRANSISTOR

D–

N1 y I

N2 y I

IBIAS

LPF

fC = 65kHz

Series Resistance Cancellation

Parasitic resistance to the ADT7490 D+ and D− inputs

(seen in series with the remote diode) is caused by a variety

of factors, including PCB track resistance and track length.

This series resistance appears as a temperature offset in the

remote sensor’s temperature measurement. This error

typically causes a 0.5

°C offset per ohm of parasitic

resistance in series with the remote diode.

The ADT7490 automatically cancels out the effect of this

series resistance on the temperature reading, giving a more

accurate result without the need for user characterization of

this resistance. The ADT7490 is designed to automatically

cancel, typically up to 1.5 k

W of resistance. By using an

advanced temperature measurement method, this is

transparent to the user. This feature allows resistances to be

added to the sensor path to produce a filter, allowing the part

to be used in noisy environments.

Noise Filtering

For temperature sensors operating in noisy environments,

previous practice was to place a capacitor across the D+ pin

and the D− pin to help combat the effects of noise. However,

large capacitance affect the accuracy of the temperature

measurement, leading to a recommended maximum

capacitor value of 1000 pF. This capacitor reduces the noise,

but does not eliminate it, which makes using the sensor

difficult in a very noisy environment.

The ADT7490 has a major advantage over other devices

for eliminating the effects of noise on the external sensor.

Using the series resistance cancellation feature, a filter can

be constructed between the external temperature sensor and

the part. 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 ADT7490 and the

remote temperature sensor to operate in noisy environments.

Figure 28 shows a low−pass RC filter with the following

values:

R + 100 W,C + 1nF

(eq. 3)

This filtering reduces both common−mode noise and

differential noise.

Figure 28. Filter Between Remote Sensor and ADT7490

D+

1nF

100Ω

REMOTE

TEMPERATURE

SENSOR

D–

100Ω

Factors Affecting Diode Accuracy

Remote Sensing Diode

The ADT7490 is designed to work with either substrate

transistors built into processors or discrete transistors.

Substrate transistors are generally PNP types with the

collector connected to the substrate. Discrete types can be

either PNP or NPN transistors connected as a diode

(base−shorted to the collector). To reduce the error due to

variations in both substrate and discrete transistors, a

number of factors should be taken into consideration:

• The ideality factor, nf, of the transistor is a measure of

the deviation of the thermal diode from ideal behavior.

The ADT7490 is trimmed for an nf value of 1.008. Use

the following equation to calculate the error introduced

at a temperature T (

°C) when using a transistor whose

nf does not equal 1.008. Refer to the data sheet for the

related CPU to obtain the nf values.

DT + (nf * 1.008) 1.008

273.15 K ) T

(eq. 4)

ADT7490ARQZ-R7 Datasheet(PDF) 20 Page - ON Semiconductor

ADT7490ARQZ-R7 Datasheet(HTML) 20 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Datasheet Download

Link URL