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MAX6642 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX6642 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 14 page range. We have observed variations in remote tempera- ture readings of less than ±2°C with a variety of dis- crete transistors. Still, it is good design practice to verify good consistency of temperature readings with several discrete transistors from any manufacturer under consideration. ADC Noise Filtering The integrating ADC used has good noise rejection for low-frequency signals such as 60Hz/120Hz power-sup- ply hum. In noisy environments, high-frequency noise reduction is needed for high-accuracy remote mea- surements. The noise can be reduced with careful PCB layout and proper external noise filtering. High-frequency EMI is best filtered at DXP with an external 2200pF capacitor. Larger capacitor values can be used for added filtering, but do not exceed 3300pF because excessive capacitance can introduce errors due to the rise time of the switched current source. Nearly all noise sources tested cause the temperature conversion results to be higher than the actual temper- ature, typically by +1°C to +10°C, depending on the frequency and amplitude (see the Typical Operating Characteristics). PCB Layout Follow these guidelines to reduce the measurement error of the temperature sensors: 1) Connect the thermal-sense diode to the MAX6642 using two traces—one between DXP and the anode, the other between the MAX6642’s GND and the cathode. Do not connect the cathode to GND at the sense diode. 2) Place the MAX6642 as close as is practical to the remote thermal diode. In noisy environments, such as a computer motherboard, this distance can be 4in to 8in (typ). This length can be increased if the worst noise sources are avoided. Noise sources include CRTs, clock generators, memory buses, and ISA/PCI buses. 3) Do not route the thermal diode lines next to the deflection coils of a CRT. Also, do not route the traces across fast digital signals, which can easily introduce a 30°C error, even with good filtering. 4) Route the thermal diode traces in parallel and in close proximity to each other, away from any higher voltage traces, such as +12VDC. Leakage currents from PCB contamination must be dealt with careful- ly since a 20M Ω leakage path from DXP to ground causes about +1°C error. If high-voltage traces are unavoidable, connect guard traces to GND on either side of the DXP trace (Figure 4). 5) Route through as few vias and crossunders as pos- sible to minimize copper/solder thermocouple effects. 6) When introducing a thermocouple, make sure that both the thermal diode paths have matching ther- mocouples. A copper-solder thermocouple exhibits 3µV/°C, and it takes about 200µV of voltage error at DXP to cause a +1°C measurement error. Adding a few thermocouples causes a negligible error. 7) Use wide traces. Narrow traces are more inductive and tend to pick up radiated noise. The 10-mil widths and spacing recommended in Figure 4 are not absolutely necessary, as they offer only a minor improvement in leakage and noise over narrow traces. Use wider traces when practical. 8) Add a 47 Ω resistor in series with VCC for best noise filtering (see the Typical Operating Circuit). 9) Copper cannot be used as an EMI shield; only fer- rous materials such as steel work well. Placing a copper ground plane between the DXP-DXN traces and traces carrying high-frequency noise signals does not help reduce EMI. Twisted-Pair and Shielded Cables Use a twisted-pair cable to connect the remote sensor for remote-sensor distances longer than 8in or in very noisy environments. Twisted-pair cable lengths can be between 6ft and 12ft before noise introduces excessive errors. For longer distances, the best solution is a shielded twisted pair like that used for audio micro- phones. For example, Belden #8451 works well for dis- tances up to 100ft in a noisy environment. At the device, connect the twisted pair to DXP and GND and the shield to GND. Leave the shield unconnected at the remote diode. For very long cable runs, the cable’s parasitic capaci- tance often provides noise filtering, so the 2200pF capacitor can often be removed or reduced in value. ±1°C, SMBus-Compatible Remote/Local Temperature Sensor with Overtemperature Alarm ______________________________________________________________________________________ 11 MINIMUM 10 mils 10 mils 10 mils 10 mils THERMAL DIODE CATHODE/GND THERMAL DIODE ANODE/DXP GND GND Figure 4. Recommended DXP PC Traces |
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