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AD652JP
(Rev.:RevB)

Monolithic Synchronous Voltage-to-Frequency Converter

Analog Devices 

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.

AD652

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 kΩ resistor. This results in a 2 kΩ resistor and a 1 V

input range. The external 2 kΩ resistor should be a low TC

metal-film type for lowest drift degradation.

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 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 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 kΩ resistor

would be selected ((15 V–5 V)/500 µA = 20 kΩ).

REV. B

–9–

Figure 17. Frequency-to-Voltage Converter

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