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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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ADM9240ARU Fiches technique(PDF) 10 Page - Analog Devices |
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ADM9240ARU Fiches technique(HTML) 10 Page - Analog Devices |
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10 / 22 page  ADM9240 –10– REV. 0 SETTING OTHER INPUT RANGES If any of the inputs is unused, and there is a requirement for monitoring another power supply such as –12 V, the input range of the unused input can easily be scaled and offset to accommo- date this. For example, if only one processor core voltage is to be monitored, the unused VCCP input can be used to monitor another supply voltage. If the voltage to be monitored is positive, it is simply a matter of using an input with a lower full scale than the voltage to be measured and adding an external input attenuator, but bear in mind that the input resistance ( ≈140 kΩ) of the on-chip attenua- tor will load the external attenuator. This can be accounted for in the calculation, but the values of the on-chip attenuator resis- tors are not precise and vary with temperature. Therefore, the external attenuator should have a much lower output resistance to minimize the loading. If this is not acceptable, a buffer ampli- fier can be used. If the input voltage range is negative, it must first be converted to a positive voltage. The simplest way to do this is simply to attenuate and offset the voltage, as shown in Figure 4, which shows the +VCCP2 input scaled to measure a –12 V input. Using the values shown, the input range is zero to –13.5 V, which will accommodate a +12.5% tolerance on the nominal value. R1 2.7k R2 1k –13.2V TO 0V IN +VCCP2 140k R3 39k 0V TO 3.6V +5V Figure 4. Scaling VCCP2 to –12 V (+10%) The resistor ratios are calculated as follows: R1/R2 = |V–|(max)/V+ (to give zero volts at the input for the most negative value of V–. R2 has no effect under this condition as the voltage across it is zero) and: (V+ – VFS)/VFS = R2/RP = (R1 and R2 in Parallel) (to give a voltage VFS at the input when V– is zero, where VFS is the normal full-scale voltage of the input used). This is a simple and cheap solution, but the following points should be noted. 1. Since the input signal is not inverted, an increase in the mag- nitude of the –12 V supply (going more negative), will cause the input voltage to fall and give a lower output code from the ADC. Conversely, a decrease in the magnitude of the –12 V supply will cause the ADC code to increase. This means that the upper and lower limits will be transposed. 2. Since the offset voltage is derived from the +5 V supply, variations in this supply will affect the ADC code. It is therefore a good idea to read the value of the +5 V sup- ply and adjust the limits for the –12 V supply accordingly. The 5 V supply is attenuated by a factor RP/(R2+RP), where RP is the parallel combination of R1 and R3. An increase in the 5 V supply increases the ADC input by the DV × R P/ (R2+RP), while a decrease in the 5 V supply correspondingly decreases the input to the ADC. 3. The on-chip input attenuators will load the external attenua- tor, as mentioned earlier. This technique can be applied to any other unused input. By suitable choice of V+ and the input resistors, a variety of nega- tive and/or bipolar input ranges can be obtained. TEMPERATURE MEASUREMENT SYSTEM The ADM9240 contains an on-chip bandgap temperature sen- sor. The on-chip ADC performs 9-bit conversions on the output of this sensor and outputs the temperature data in 9-bit twos complement format, but only the eight most significant bits are used for temperature limit comparison. The full 9-bit tempera- ture data can be obtained by reading the 8 MSBs from the Tem- perature Value Register (Address 27h) and the LSB from Bit 7 of the Temperature Configuration Register (Address 4Bh). The format of the temperature data is shown in Table II. Theo- retically, the temperature sensor and ADC can measure tem- peratures from –128 °C to +127°C with a resolution of 0.5°C, although temperatures below –40 °C and above +125°C are outside the operating temperature range of the device. Table II. Temperature Data Format Temperature Digital Output –128 °C 1 0000 0000 –125 °C 1 0000 0110 –100 °C 1 0011 1000 –75 °C 1 0110 1010 –50 °C 1 1001 1100 –25 °C 1 1100 1110 –0.5 °C 1 1111 1111 0 °C 0 0000 0000 +0.5 °C 0 0000 0001 +10 °C 0 0001 0100 +25 °C 0 0011 0010 +50 °C 0 0110 0100 +75 °C 0 1001 0110 +100 °C 0 1100 1000 +125 °C 0 1111 1010 +127 °C 0 1111 1111 LIMIT VALUES Limit values for analog measurements are stored in the appro- priate limit registers. In the case of voltage measurements, high and low limits can be stored so that an interrupt request will be generated if the measured value goes above or below acceptable values. In the case of temperature, a Hot Temperature Limit can be programmed, and a Hot Temperature Hysteresis Limit, which will usually be some degrees lower. This can be useful as it allows the system to be shut down when the hot limit is ex- ceeded, and automatically restarted when it has cooled down to a safe temperature. |
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