AD654
R1
R2
R3
+
AD589
–
OSC/
DRIVER
R4
1N4148
1 F
R
T
V
S
(10V TO 15V)
AD654
140
Q1
2N3906
R
S
R5
C
T
0.01 F
I
T
(10V) C
T
R6
220
CMOS
OUTPUT
TTL
OUTPUT
(1 LOAD)
AD592
1 A/k
f=
Figure 8. Two-Wire Temperature-to-Frequency Converter
TWO-WIRE TEMPERATURE-TO-FREQUENCY
CONVERSION
Figure 8 shows the AD654 in a two-wire temperature-to-frequency
conversion scheme. The twisted pair transmission line serves the
dual purpose of supplying power to the device and also carrying
frequency data in the form of current modulation.
The positive supply line is fed to the remote V/F through a
140
Ω
resistor. This resistor is selected such that the quiescent
current of the AD654 will cause less than one V
BE
to be dropped.
As the V/F oscillates, additional switched current is drawn through
R
L
when Pin 1 goes low. The peak level of this additional cur-
rent causes Q1 to saturate, and thus regenerates the AD654’s
output square wave at the collector. The supply voltage to the
AD654 then consists of a dc level, less the resistive line drop, plus a
one V
BE
p-p square wave at the output frequency of the AD654.
This ripple is reduced by the diode/capacitor combination.
To set up the receiver circuit for a given voltage, the R
S
and R
L
resistances are selected as shown in Table I. CMOS logic stages
can be driven directly from the collector of Q1, and a single TTL
load can be driven from the junction of R
S
and R6.
Table I.
values shown in Table II. Since temperature is the parameter of
interest, an NPO ceramic capacitor is used as the timing capaci-
tor for low V/F TC.
When scaling per K, resistors R1–R3 and the AD589 voltage
reference are not used. The AD592 produces a 1
µA/K
current
output which drives Pin 3 of the AD654. With the timing
capacitor of 0.01
µF
this produces an output frequency scaled to
10 Hz/K. When scaling per
°C
and
°F,
the AD589 and resistors
R1–R3 offset the drive current at Pin 3 by 273.2
µA
for scaling
per
°C
and 255.42
µA
for scaling per
°F.
This will result in fre-
quencies sealed at 10 Hz/°C and 5.55 Hz/°F, respectively.
OPTOISOLATOR COUPLING
A popular method of isolated signal coupling is via optoelec-
tronic isolators, or optocouplers. In this type of device, the signal is
coupled from an input LED to an output photo-transistor, with
light as the connecting medium. This technique allows dc to be
transmitted, is extremely useful in overcoming ground loop
problems between equipment, and is applicable over a wide
range of speeds and power.
Figure 9 shows a general purpose isolated V/F circuit using a
low cost 4N37 optoisolator. A +5 V power supply is assumed for
both the isolated (+5 V isolated) and local (+5 V local) supplies.
The input LED of the isolator is driven from the collector out-
put of the AD654, with a 9 mA current level established by R1
for high speed, as well as for a 100% current transfer ratio.
5V
(ISOLATED)
R1
390
4N37
OPTO-ISOLATOR
R3
270
74LS14
OSC/
DRIVER
R2
120
V
IN
(0V TO 1V)
R
T
1k
C
T
1000pF
Q1
2N3904
5V
(LOCAL)
+V
S
10 V
15 V
R
S
( )
270
680
Table II.
R
L
( )
1.8k
2.7k
(+V
S
) R1 ( ) R2 ( ) R3 ( ) R4 ( ) R5 ( )
K
10 V
15 V
–
–
6.49k
12.7k
6.49k
12.7k
–
–
4.02k
4.02k
4.42k
4.42k
–
–
1k
1k
1k
1k
100k
100k
95.3k
78.7k
154k
105k
127k
127k
F = 10 Hz/K
AD654
GRN
LED
°C
10 V
15 V
°F
10 V
15 V
22.6k F = 10 Hz/°C
36.5k
22.6k F = 5.55 Hz/°F
36.5k
V/F OUTPUT
FS = 100kHz
TTL
At the V/F end, the AD592C temperature transducer is inter-
faced with the AD654 in such a manner that the AD654 output
frequency is proportional to temperature. The output frequency
can be sealed and offset from K to
°C
or
°F
using the resistor
REV. B
–7–
ISOLATED
LOCAL
Figure 9. Optoisolator Interface
AD [ ANALOG DEVICES ]
