ADM1021AARQ View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADM1021AARQ

Low-Cost Microprocessor System Temperature Monitor

Analog Devices 

ADM1021A

VDD

I

N؋I

IBIAS

REMOTE

SENSING

TRANSISTOR

D+

C1*

D–

BIAS

DIODE

LOWPASS FILTER

fC = 65kHz

VOUT+

TO ADC

VOUT–

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

C1 = 2.2nF TYPICAL, 3nF MAX.

Figure 2. Input Signal Conditioning

The technique used in the ADM1021A is to measure the change

in VBE when the device is operated at two different currents.

This is given by:

where:

∆VBE = KT/q × ln(N)

K is Boltzmann’s constant,

q is charge on the electron (1.6 × 10–19 coulombs),

T is absolute temperature in kelvins,

N is ratio of the two currents.

Figure 2 shows the input signal conditioning used to measure the

output of an external temperature sensor. This figure shows the

external sensor as a substrate transistor, provided for tempera-

ture monitoring on some microprocessors, but it could equally

well be a discrete transistor. If a discrete transistor is used, the

collector will not be grounded and should be linked to the base.

To prevent ground noise interfering with the measurement, the

more negative terminal of the sensor is not referenced to ground,

but is biased above ground by an internal diode at the D– input.

If the sensor is operating in a noisy environment, C1 may optionally

be added as a noise filter. Its value is typically 2200 pF, but should

be no more than 3000 pF. See the section on layout considerations

for more information on C1.

To measure ∆VBE, the sensor is switched between operating currents

of I and N × I. The resulting waveform is passed through a 65 kHz

low-pass filter to remove noise, then to a chopper-stabilized ampli-

fier that performs the functions of amplification and rectification of

the waveform to produce a dc voltage proportional to ∆VBE. This

voltage is measured by the ADC to give a temperature output in

8-bit twos complement format. To reduce the effects of noise

further, digital filtering is performed by averaging the results of

16 measurement cycles.

Signal conditioning and measurement of the internal tempera-

ture sensor is performed in a similar manner.

DIFFERENCES BETWEEN THE ADM1021 AND THE

ADM1021A

Although the ADM1021A is pin-for-pin compatible with the

ADM1021, there are some differences between the two devices.

Below is a summary of these differences and reasons for the changes.

1. The ADM1021A forces a larger current through the remote

temperature sensing diode, typically 205 µA versus 90 µA

for the ADM1021. The main reason for this is to improve

the noise immunity of the part.

2. As a result of the greater Remote Sensor Source Current the

operating current of the ADM1021A is higher than that of

the ADM1021, typically 205 mA versus 160 mA.

3. The temperature measurement range of the ADM1021A is

0°C to 127°C, compared with –128°C to +127°C for the

ADM1021. As a result, the ADM1021 should be used if

negative temperature measurement is required.

4. The power-on reset values of the remote and local tempera-

ture values are –128°C in the ADM1021A as compared with

0°C in the ADM1021. As the part is powered up converting

(except when the part is in standby mode, i.e., Pin 15 is

pulled low) the part will measure the actual values of remote

and local temperature and write these to the registers.

5. The four MSBs of the Revision Register may be used to

identify the part. The ADM1021 Revision Register reads

0xh and the ADM1021A reads 3xh.

6. The power-on default value of the Address Pointer Register

is undefined in the ADM1021A and is equal to 00h in the

ADM1021. As a result, a value must be written to the Address

Pointer Register before a read is done in the ADM1021A.

The ADM1021 is capable of reading back local temperature

without writing to the Address Pointer Register as it defaulted

to the local temperature measurement register at power-up.

7. Setting the mask bit (Bit 7 Config Reg) on the ADM1021A

will mask current and future ALERTs. On the ADM1021

the mask bit will only mask future ALERTs. Any current

ALERT will have to be cleared using an ARA.

TEMPERATURE DATA FORMAT

One LSB of the ADC corresponds to 1°C, so the ADC can theo-

retically measure from –128°C to +127°C, although the device

does not measure temperatures below 0°C so the actual range is

0°C to 127°C. The temperature data format is shown in Table I.

The results of the local and remote temperature measurements

are stored in the local and remote temperature value registers,

and are compared with limits programmed into the local and

remote high and low limit registers.

–6–

REV. D

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