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ADM1022ARQ Datasheet(PDF) 10 Page - Analog Devices |
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ADM1022ARQ Datasheet(HTML) 10 Page - Analog Devices |
10 / 19 page REV. 0 ADM1022 –10– LOW-PASS FILTER fC = 65kHz BIAS DIODE REMOTE SENSING TRANSISTOR I N IIBIAS D+ D– VOUT+ VOUT– TO ADC VDD Figure 12. Signal Conditioning If a discrete transistor is used, the collector will not be grounded, and should be linked to the base. If a PNP transistor is used the base is connected to the D– input and the emitter to the D+ input. If an NPN transistor is used, the emitter is connected to the D– input and the base to the D+ input. Table II. Temperature Data Format Temperature Digital Output –128 °C 1000 0000 –125 °C 1000 0011 –100 °C 1001 1100 –75 °C 1011 0101 –50 °C 1100 1110 –25 °C 1110 0111 –1 °C 1111 1111 0 °C 0000 0000 +1 °C 0000 0001 +10 °C 0000 1010 +25 °C 0001 1001 +50 °C 0011 0010 +75 °C 0100 1011 +100 °C 0110 0100 +125 °C 0111 1101 +127 °C 0111 1111 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 used in a very noisy environment, a capacitor of value up to 1000 pF may be placed between the D+ and D– inputs to filter the noise. 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, thence to a chopper- stabilized amplifier that performs the functions of amplification and rectification of the waveform to produce a dc voltage pro- portional to ∆V BE. This voltage is measured by the ADC to give a temperature output in 8-bit twos complement format. To fur- ther reduce the effects of noise, digital filtering is performed by averaging the results of 16 measurement cycles. An external temperature measurement takes nominally 9.6 ms. LAYOUT CONSIDERATIONS Digital boards can be electrically noisy environments, and care must be taken to protect the analog inputs from noise, particu- larly when measuring the very small voltages from a remote diode sensor. The following precautions should be taken: 1. Place the ADM1022 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. 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL 10MIL GND D+ D– GND Figure 13. 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 200 µV, and thermocouple voltages are about 3 µV/oC of temperature difference. Unless there are two thermocouples with a big temperature differential between them, thermocouple voltages should be much less than 200 µV. 5. Place 0.1 µF bypass and 1000 pF input filter capacitors close to the ADM1022. 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 a shielded twisted pair such as Belden #8451 microphone cable. Con- nect the twisted pair to D+ and D– and the shield to GND close to the ADM1022. Leave the remote end of the shield unconnected 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 C1 may be reduced or removed. In any case, the total shunt capaci- tance should not exceed 1000 pF. Cable resistance can also introduce errors. 1 Ω series resistance introduces about 0.5 °C error. |
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