Datasheet AD654 (Analog Devices) - 9

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