AD652KP datasheet(14/28 Pages) AD

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AD652

Rev. C | Page 14 of 28

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.

Thus, the device is truly operating from a single-supply voltage

that can range from 12 V to 36 V. This is shown in Figure 21 for

a positive voltage input, and in Figure 20 for a negative voltage

input.

In Figure 21, the comparator reference is used as a derived

ground; the input voltage is referred to this point as well as to

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

current into Pin 7, it must also draw the exact same current

from the input reference potential. This current is therefore

provided by the 5 V reference.

In single-supply operation, 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 is selected

((15 V–5 V)/500 µA = 20 kΩ).

Figure 20 shows the negative voltage input configuration for

using the AD652 in single-supply mode. In this mode, the signal

source is driving the + input of the op amp, which requires only

20 nA (typical) compared to the 0.5 mA required in the positive

input voltage configuration. The voltage at Pin 6 may go as low

as 4 V above ground (−VS Pin 8). Since the input reference is 5.0

V above ground, this leaves a 1 V window for the input signal.

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.

+VS

–

ONE

SHOT

AND

"D"

FLOP

QCK

1mA

20k

Ω

AD652

SYNCHRONOUS

VOLTAGE-TO-

FREQUENCY

CONVERTER

REFERENCE

CLOCK

ANALOG

GND

DIGITAL

GND

COS

CINT

SIGNAL

SOURCE

1V FULL SCALE

INPUT

REFERENCE

RPULLUP

Figure 20. Single-Supply Mode Negative Voltage Input

+VS

–

ONE

SHOT

AND

"D"

FLOP

QCK

1mA

20k

Ω

AD652

SYNCHRONOUS

VOLTAGE-TO-

FREQUENCY

CONVERTER

REFERENCE

CLOCK

ANALOG

GND

DIGITAL

GND

FREQ

OUT

COS

CINT

SIGNAL

SOURCE

ANALOG

GND

ISIGNAL

0.5mA

FULL SCALE

INPUT

REFERENCE

RPULLUP

ISIGNAL

Figure 21. Single-Supply Mode Positive Voltage Input

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