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AD652KP Datasheet(PDF) 9 Page - Analog Devices |
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AD652KP Datasheet(HTML) 9 Page - Analog Devices |
9 / 16 page AD652 REV. B –9– DIGITAL GROUND Digital Ground can be at any potential between –VS and (+VS –4 volts). This can be very useful in a system with derived grounds rather than stiff supplies. For example, in a small iso- lated power circuit, often only a single supply is generated and the “ground” is set by a divider tap. Such a ground cannot handle the large currents associated with digital signals. With the AD652 SVFC, it is possible to connect the DIG GND to –VS for a solid logic reference, as shown in Figure 14. Figure 14. Digital GND at –VS SINGLE SUPPLY OPERATION In addition to the Digital Ground being connected to –VS, it is also possible to connect Analog Ground to –VS of the AD652. Hence, the device is truly operating from a single supply voltage that can range from +12 V to +36 V. This is shown in Figure 15 for a positive voltage input and Figure 16 for a negative voltage input. In Figure 15, the comparator reference is used as a derived ground, and the input voltage is referred to this point as well as the op amp common mode (Pin 6 is tied to Pin 16). Since the input signal source must drive 0.5 mA of full-scale signal cur- rent into Pin 7, it must also draw the exact same current from the input reference potential. This current will thus be provided by the 5 V reference. Figure 15. Single Supply Positive Voltage Input In the single supply operation mode, an external resistor, RPULLUP, is necessary between the power supply, + VS, and the 5 V reference output. This resistor should be selected such that a current of approximately 500 µA flows during operation. For example, with a power supply voltage of +15 V, a 20 k Ω resistor would be selected ((15 V–5 V)/500 µA = 20 kΩ). Figure 16 shows the negative voltage input configuration for use of the AD652 in the single supply mode. In this mode the signal source is driving the “+” input of the op amp which requires only 20 nA (typical), rather than the 0.5 mA required in the positive input voltage configuration. The voltage at Pin 6 may go as low as 4 volts above ground (–VS Pin 8). Since the input reference is 5.0 volts above ground, this leaves a 1 V window for the input signal. In order to drive the integrating capacitor with a 0.5 mA full-scale current, it is necessary to provide an external 2 k Ω resistor. This results in a 2 kΩ resistor and a 1 V input range. The external 2 k Ω resistor should be a low TC metal-film type for lowest drift degradation. Figure 16. Single Supply Negative Voltage Input FREQUENCY-TO-VOLTAGE CONVERTER The AD652 SVFC also works as a frequency-to-voltage converter. Figure 17 shows the connection diagram for F/V conversion. In this case the “–” input of the comparator is fed the input pulses. Either comparator input may be used so that an input pulse of either polarity may be applied to the F/V. Figure 17. Frequency-to-Voltage Converter |
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