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AD654JNZ Arkusz danych(PDF) 9 Page - Analog Devices |
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AD654JNZ Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 13 page AD654 –8– REV. At the receiver side, the output transistor is operated in the photo-transistor mode; that is with the base lead (Pin 6) open. This allows the highest possible output current. For reasonable speed in this mode, it is imperative that the load impedance be as low as possible. This is provided by the single transistor stage current-to-voltage converter, which has a dynamic load imped- ance of less than 10 ohms and interfaces with TTL at the output. USING A STAND-ALONE FREQUENCY COUNTER/LED DISPLAY DRIVER FOR VOLTMETER APPLICATIONS Figure 10 shows the AD654 used with a stand-alone frequency counter/LED display driver. With CT = 1000 pF and RT = 1 k Ω the AD654 produces an FS frequency of 100 kHz when VIN = +1 V. This signal is fed into the ICM7226A, a universal counter system that drives common anode LEDs. With the FUNCTION pin tied to D1 through a 10 k Ω resistor the ICM7226A counts the frequency of the signal at AIN. This count period is selected by the user and can be 10 ms, 100 ms, 1s, or 10 seconds, as shown on Pin 21. The longer the period selected, the more resolution the count will have. The ICM7226A then displays the frequency on the LEDs, driving them directly as shown. Refreshing of the LEDs is handled automatically by the ICM7226. The entire circuit op- erates on a single +5 V supply and gives a meter with 3, 4, or 5 digit resolution. 40 39 38 37 36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 8 7 6 5 1 2 3 4 VIN (0V TO 1V) 5V 5V 1k 1000pF 500 825 1k AD654 DI PIN 30 10k 30k 5V 5V 10MHz CRYSTAL 22M 39pF 39pF 5V 5V 10k 5V D1 (10ms) D2 (100ms) D3 (1s) D4 (10s) 4 8 8 D.P. g e d c b a f LED OVERFLOW INDICATOR D8 D7 D6 D5 D4 D3 D2 D1 AIN HOLD NC OSL JN OSL OUT NC D1 D2 D3 D4 D5 V+ D6 D7 D8 RANGE ICM7226A FUNCTION dp e g a GND d b c f + – NC = NO CONNECT Figure 10. AD654 With Stand-Alone Frequency Counter/ LED Display Driver Longer count periods not only result in the count having more resolution, they also serve as an integration of noisy analog signals. For example, a normal-mode 60 Hz sine wave riding on the input of the AD654 will result in the output frequency increasing on the positive half of the sine wave and decreasing on the negative half of the sine wave. This effect is cancelled by selecting a count period equal to an integral number of noise signal periods. A 100 ms count period is effective because it not only has an inte- gral number of 60 Hz cycles (6), it also has an integral number of 50 Hz cycles (5). This is also true of the 1 second and 10 sec- ond count period. AD654-BASED ANALOG-TO-DIGITAL CONVERSION USING A SINGLE CHIP MICROCOMPUTER The AD654 can serve as an analog-to-digital converter when used with a single component microcomputer that has an inter- val timer/event counter such as the 8048. Figure 11 shows the AD654, with a full-scale input voltage of +1 V and a full-scale output frequency of 100 kHz, connected to the timer/counter input Pin T1 of the 8048. Such a system can also operate on a single +5 V supply. The 8748 counter is negative edge triggered; after the STRT CNT instruction is executed subsequent high to low transitions on T1 increment the counter. The maximum rate at which the counter may be incremented is once per three instruction cycles; using a 6 MHz crystal, this corresponds to once every 7.5 µs, or a maximum frequency of 133 kHz. Because the counter overflows every 256 counts (8 bits), the timer interrupt is enabled. Each overflow then causes a jump to a subroutine where a register is incremented. After the STOP TCNT instruction is executed, the number of overflows that have occurred will be the number in this register. The number in this register multiplied by 256 plus the number in the counter will be the total number of negative edges counted during the count period. The count period is handled simply by decrementing a register the number of times necessary to correspond to the desired count time. After the register has been decremented the required number of times the STOP TCNT instruction is executed. The total number of negative edges counted during the count period is proportional to the input voltage. For example, if a 1 V full-scale input voltage produces a 100 kHz signal and the count period is 100 ms, then the total count will be 10,000. Scaling from this maximum is then used to determine the input voltage, i.e., a count of 5000 corresponds to an input voltage of 0.5 V. As with the ICM7226, longer count times result in counts hav- ing more resolution; and they result in the integration of noisy analog signals. C |
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