AD652
+V
S
AD652
1
20kΩ
SYNCHRONOUS
VOLTAGE-TO-
FREQUENCY
CONVERTER
5V
REFERENCE
16
15
14
13
2
3
4
C
INT
5
ANALOG
GND
R
L
5V
DIGITAL
GND
FREQ
OUT
V
IN
±5V
6
ONE
SHOT
20kΩ
1mA
Q
AND
D
CK
12
11
10
7
8
CLOCK
9
00798-010
–V
S
"D"
Q FLOP
C
OS
Figure 10. Bipolar Offset
SVFC CONNECTION FOR BIPOLAR INPUT VOLTAGES
A bipolar input voltage of ±5 V can be accommodated by
injecting a 250 µA current into Pin 5 (see Figure 10). A −5 V
signal provides a zero sum current at the integrator summing
junction, which results in a zero-output frequency; a +5 V signal
provides a 0.5 mA (full-scale) sum current, which results in the
full-scale output frequency.
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:
1.
2.
3.
4.
Use an external resistor for R
OS
and the internal 20 kΩ
resistor for R
IN
(as shown in Figure 10).
Use the internal 20 kΩ resistor as R
OS
and an external R
IN
.
Use two external resistors.
Use two internal resistors for R
IN
and R
OS
(available on
PLCC version only).
CASE 2
R
IN
∼
EXTERNAL
R
OS
∼
INTERNAL
V
REF
R
OS
V
IN
R
IN
IDEAL
TRANSFER
RELATION
F
ZERO
00798-011
F
OUT
V
IN
V
ZERO
–5V
+5V
Figure 11. Ideal Bipolar Input Transfer Curve over Temperature
F
OUT
CASE 1
R
IN
∼
INTERNAL
R
OS
∼
EXTERNAL
IDEAL
TEMPERATURE
PERTURBED
TRANSFER
V
IN
–5V
V
ZERO
F
OUT
IDEAL
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, V
ZERO
(the input voltage required to produce an output
frequency of 0 Hz) and F
ZERO
(the output frequency when
V
IN
= 0 V) changes as the transfer function is displaced parallel
to the voltage axis with temperature. In the second case, F
ZERO
remains constant, but V
ZERO
changes as the transfer function
rotates about F
ZERO
with temperature changes. In the third case,
with two external resistors, the V
ZERO
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.
F
ZERO
TEMPERATURE
PERTURBED
V
IN
V
ZERO
–5V
F
OUT
CASE 3
R
IN
∼
EXTERNAL
R
OS
∼
EXTERNAL
IDEAL
F
ZERO
TEMPERATURE
PERTURBED
V
IN
–5V
V
ZERO
Figure 12. Actual Bipolar Input Transfer over Temperature
Rev. C | Page 10 of 28
00798-012
AD [ ANALOG DEVICES ]
