AD652
SINGLE-SUPPLY OPERATION
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).
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.
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.
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
+V
S
R
PULLUP
AD652
SYNCHRONOUS
VOLTAGE-TO-
FREQUENCY
CONVERTER
5V
REFERENCE
1
2
3
4
5
6
7
8
16
INPUT
15
14
13
12
11
10
9
REFERENCE
ANALOG
GND
+
–
C
INT
2kΩ
DIGITAL
GND
ONE
SHOT
5V
R
L
SIGNAL
20kΩ
SOURCE
1mA
CLOCK
1V FULL SCALE
Q
CK
"D"
D
AND
C
OS
Q FLOP
Figure 20. Single-Supply Mode Negative Voltage Input
+V
S
R
PULLUP
AD652
SYNCHRONOUS
VOLTAGE-TO-
FREQUENCY
CONVERTER
5V
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
REFERENCE
I
SIGNAL
INPUT
REFERENCE
ANALOG
GND
C
INT
DIGITAL
GND
ONE
SHOT
R
L
5V
–
+
FREQ
OUT
20kΩ
1mA
CLOCK
C
Q
CK
I
SIGNAL
0.5mA
FULL SCALE
SIGNAL
SOURCE
D
AND
"D"
FLOP
OS
Q
ANALOG
GND
Figure 21. Single-Supply Mode Positive Voltage Input
Rev. C | Page ±4 of 28
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