Datasheet AD654 (Analog Devices) - 8

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