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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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SS8017TR датащи(PDF) 7 Page - Silicon Standard Corp. |
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SS8017TR датащи(HTML) 7 Page - Silicon Standard Corp. |
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7 / 16 page  www.SiliconStandard.com 7 of 16 A/D Conversion Sequence If a Start command is written (or generated automati- cally in the free-running auto-convert mode), both two channels are converted, and the results of both meas- urements are available after the end of conversion. A BUSY status bit in the status byte shows that the de- vice is actually performing a new conversion; however, even if the ADC is busy, the results of the previous conversion are always available. Remote-Diode Selection Temperature accuracy depends on having a good-quality, diode-connected small-signal transistor. Accuracy has been experimentally verified for all of the devices listed in Table 1. The 8017 can also directly measure the die temperature of CPUs and other inte- grated circuits having on-board temperature- sensing diodes. The transistor must be a small-signal type with a relatively high forward voltage; otherwise, the A/D input voltage range can be violated. The forward voltage must be greater than 0.25V at 10µA; check to ensure this is true at the highest expected temperature. The forward voltage must be less than 0.95V at 200A; check to ensure this is true at the lowest expected temperature. Large power transistors don't work at all. Also, ensure that the base resistance is less than 100 Ω. Tight specifications for forward current gain (+50 to +150, for example) indicate that the manufacturer has good process controls and that the devices have consistent VBE characteristics. Thermal Mass and Self-Heating Thermal mass can seriously degrade the 8017's effec- tive accuracy. The thermal time constant of the SSOP-16 package is about 140sec in still air. For the 8017 junction temperature to settle to within +1°C after a sudden +100°C change requires about five time con- stants or 12 minutes. The use of smaller packages for remote sensors, such as SOT23s, improves the situa- tion. Take care to account for thermal gradients be- tween the heat source and the sensor ,and ensure that stray air current across the sensor package does not interfere with measurement accuracy. Table 1. Remote-Sensor Transistor Manufacturers MANUFACTURER MODEL NUMBER Philips PMBS 3904 Motorola (USA) MMBT3904 National Semiconductor (USA) MMBT3904 Note:Transistors must be diode-connected (base short -ed to collector). ADC Noise Filtering The ADC is an integrating type with inherently good noise rejection, especially of low-frequency signals such as 60Hz/120Hz power-supply hum. Micro-power operation places constraints on high-frequency noise rejection; therefore, careful PC board layout and proper external noise filtering are required for high- accuracy remote measurements in electrically noisy environ- ments. High-frequency EMI is best filtered at DXP and DXN with an external 2200pF capacitor. This value can be increased to about 3300pF(max), including cable ca- pacitance. Higher capacitance than 3300pF introduces errors due to the rise time of the switched current source. Nearly all noise sources tested cause the ADC meas- urements to be higher than the actual temperature, typically by +1°C to 10°C, depending on the frequency and amplitude (see Typical Operating Characteristics). PC Board Layout Place the 8017 as close as practical to the remote diode. In a noisy environment, such as a computer motherboard, this distance can be 4 in. to 8 in. (typical) or more as long as the worst noise sources (such as CRTs, clock generators, memory buses, and ISA/PCI buses) are avoided. Do not route the DXP -DXN lines next to the deflection coils of a CRT. Also, do not route the traces across a fast memory bus, which can easily introduce +30°C error, even with good filtering, Otherwise, most noise sources are fairly benign. Route the DXP and DXN traces in parallel and in close proximity to each other, away from any high-voltage traces such as +12VDC. Leakage currents from PC board contamination must be dealt with carefully, since a 20M Ω leakage path from DXP to ground causes about +1°C error. Route the 2 pairs of DXP1-DXN and DXP2-DXN traces independently (Figure 2a). Connect the common DXN as close as possible to the DXN pin on IC (Figure 2a). Connect guard traces to GND on either side of the DXP-DXN traces (Figure 2b). With guard traces in place, routing near high-voltage traces is no longer an issue. Route through as few vias and crossunders as possible to minimize copper/solder thermocouple effects. When introducing a thermocouple, make sure that both the DXP and the DXN paths have matching thermocou- ples. In general, PC board- induced thermocouples are not a serious problem, A copper-solder thermocouple exhibits 3µV/°C, and it takes about 200µV of voltage error at DXP -DXN to cause a +1°C measurement error. SS8017 Rev.2.01 6/06/2003 |
Аналогичный номер детали - SS8017TR |
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Аналогичное описание - SS8017TR |
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