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ADM1024 Datasheet(PDF) 11 Page - Analog Devices |
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ADM1024 Datasheet(HTML) 11 Page - Analog Devices |
11 / 28 page ADM1024 –11– REV. 0 A-TO-D CONVERTER These inputs are multiplexed into the on-chip, successive approximation, analog-to-digital converter. This has a resolution of eight bits. The basic input range is zero to 2.5 V, which is the input range of AIN1 and AIN2, but five of the inputs have built-in attenuators to allow measurement of 2.5 V, 5 V, 12 V and the processor core voltages VCCP1 and VCCP2, without any external components. To allow for the tolerance of these supply voltages, the A-to-D converter produces an output of 3/4 full-scale (decimal 192) for the nominal input voltage, and so has adequate headroom to cope with overvoltages. Table III shows the input ranges of the analog inputs and output codes of the A-to-D converter. When the ADC is running, it samples and converts an input every 748 µs, except for the external temperature (D1 and D2) inputs. These have special input signal conditioning and are averaged over 16 conversions to reduce noise, and a measure- ment on one of these inputs takes nominally 9.6 ms. INPUT CIRCUITS The internal structure for the analog inputs are shown in Figure 10. Each input circuit consists of an input protection diode, an attenuator, plus a capacitor to form a first-order low-pass filter which gives the input immunity to high frequency noise. 10pF 80k AIN1–AIN2 35pF 22.7k 122.2k +12V 25pF 55.2k 91.6k +5V 25pF 111.2k 36.7k +2.5VIN (SEE TEXT) 50pF 97.3k 42.7k +VCCP1/ VCCP2 MUX Figure 10. Structure of Analog Inputs 2.5 V INPUT PRECAUTIONS When using the 2.5 V input, the following precautions should be noted. There is a parasitic diode between Pin 18 and VCC due to the presence of a PMOS current source (which is used when Pin 18 is configured as a temperature input). This will become forward-biased if Pin 18 is more than 0.3 V above VCC. Therefore, VCC should never be powered off with a 2.5 V input connected. SETTING OTHER INPUT RANGES AIN1 and AIN2 can easily be scaled to voltages other than 2.5 V. If the input voltage range is zero to some positive voltage, all that is required is an input attenuator, as shown in Figure 11. R1 R2 AIN (1–2) VIN Figure 11. Scaling AIN(1–2) R R VFS 1 2 25 25 = (– . ) . Negative and bipolar input ranges can be accommodated by using a positive reference voltage to offset the input voltage range so it is always positive. To measure a negative input voltage, an attenuator can be used as shown in Figure 12. R1 R2 AIN (1–2) VIN +VOS Figure 12. Scaling and Offsetting AIN(1–2) for Negative Inputs R R V V FS OS 1 2 = || – This is a simple and cheap solution, but the following point should be noted. Since the input signal is offset but not inverted, the input range is transposed. An increase in the magnitude 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. The maximum negative voltage corresponds to zero output from the ADC. This means that the upper and lower limits will be transposed. Bipolar input ranges can easily be accommodated. By making R1 equal to R2 and VOS = 2.5 V, the input range is ±2.5 V. Other input ranges can be accommodated by adding a third resistor to set the positive full-scale input voltage. R2 +VOS R1 R3 AIN (1–2) VIN Figure 13. Scaling and Offsetting AIN(1–2) for Bipolar Inputs R R V R FS 1 22 = || – (R3 has no effect as the input voltage at the device Pin is zero when VIN = minus full-scale.) R R VFS 1 3 25 25 = + (– . ) . (R2 has no effect as the input voltage at the device pin is 2.5 V when VIN = plus full-scale). Offset voltages other than 2.5 V can be used, but the calculation becomes more complicated. TEMPERATURE MEASUREMENT SYSTEM Internal Temperature Measurement The ADM1024 contains an on-chip bandgap temperature sensor, whose output is digitized by the on-chip ADC. The temperature data is stored in the Temperature Value Register (address 27h) and the LSB from Bits 6 and 7 of the Temperature Configuration |
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