AD652
+V
S
AD652
SYNCHRONOUS
VOLTAGE-TO-
FREQUENCY
CONVERTER
5V
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
REFERENCE
20kΩ
ANALOG
GND
C
INT
DIGITAL
GND
ONE
SHOT
R
L
5V
V
IN
FREQ
OUT
20kΩ
±5V
1mA
CLOCK
Q
CK
D
AND
C
"D"
FLOP
OS
–V
Q
S
Figure 10. Bipolar Offset
SVFC CONNECTION FOR BIPOLAR INPUT VOLTAGES
V
REF
A bipolar input voltage of ±± V can be accommodated by
injecting a 2±0 µA current into Pin ± (see Figure 10). A −± V
signal provides a zero sum current at the integrator summing
junction, which results in a zero-output frequency; a +± V signal
provides a 0.± mA (full-scale) sum current, which results in the
full-scale output frequency.
F
OUT
R
OS
R
IN
V
IN
IDEAL
TRANSFER
RELATION
F
ZERO
V
IN
–5V
+5V
Using an external resistor to inject the offset current has some
effect on the bipolar offset temperature coefficient. The ideal
transfer curve with bipolar inputs is shown in Figure 11. The
user actually has four options to use in injecting the bipolar
offset current into the inverting input of the op amp:
V
ZERO
Figure 11. Ideal Bipolar Input Transfer Curve over Temperature
F
OUT
CASE 1
R
R
∼
OS
INTERNAL
EXTERNAL
IN
∼
IDEAL
1. Use an external resistor for ROS and the internal 20 kΩ
resistor for RIN (as shown in Figure 10).
TEMPERATURE
PERTURBED
TRANSFER
F
ZERO
V
V
V
IN
IN
IN
2. Use the internal 20 kΩ resistor as ROS and an external RIN.
3. Use two external resistors.
–5V
–5V
–5V
V
ZERO
F
OUT
IDEAL
CASE 2
4. Use two internal resistors for RIN and ROS (available on
PLCC version only).
R
R
∼
OS
EXTERNAL
INTERNAL
IN
∼
TEMPERATURE
PERTURBED
F
ZERO
Option 4 provides the closest to the ideal transfer function as
diagrammed in Figure 11. Figure 12 shows the effects of the
transfer relation on the other three options. In the first case, the
slope of the transfer function is unchanged with temperature.
However, VZERO (the input voltage required to produce an output
frequency of 0 Hz) and FZERO (the output frequency when
VIN = 0 V) changes as the transfer function is displaced parallel
to the voltage axis with temperature. In the second case, FZERO
remains constant, but VZERO changes as the transfer function
rotates about FZERO with temperature changes. In the third case,
with two external resistors, the VZERO point remains invariant
while the slope and offset of the transfer function change with
temperature. If selecting this third option, the user should select
low drift, matched resistors.
V
ZERO
F
OUT
CASE 3
R
R
∼
OS
EXTERNAL
EXTERNAL
IN
∼
IDEAL
TEMPERATURE
PERTURBED
F
ZERO
V
ZERO
Figure 12. Actual Bipolar Input Transfer over Temperature
Rev. C | Page 10 of 28
ADI [ ADI ]
