Lake Shore Model 340 Temperature Controller User’s Manual
1-4
Introduction
Sensor B
Input
A
Input
B
Input
µP
Power Supply
Input
Heater
Digital
AC Line
386EX
µP
Code
Flash
Curve
Flash
SRAM
NOVRAM
Display
Keypad
IEEE-488
Serial
Analog Out 1
Analog Out 2
High Alarm Relay
Low Alarm Relay
Digital I/O
Memory Card
High Power
Current Source
D/A
Heater
A/D
Meter
Heater
Rear
Panel
Isolation
Isolation
Isolation
A/D
A/D
Rear
Panel
Front
Panel
Sensor A
340-1-3.eps
Figure 1-3. Model 340 Temperature Controller Block Diagram
1.3.2 Input
Isolation
The sensor inputs on the Model 340 are isolated from chassis and earth ground to minimize the amount of
earth ground reference noise on the sensor input leads and in the sensor measurement. The two sensor
inputs included in the Model 340 make differential measurements and share their isolated input ground. If an
option is added to the Model 340 it may have its own isolated ground. The heater output is also isolated from
earth ground and from the sensor inputs to minimize interaction between the heater output and any other
circuitry. Figure 1-3 illustrates the isolation boundaries.
1.3.3 Input Reading Capability
The Model 340 is well suited to measure temperature in addition to controlling. Sensor inputs are read
constantly with a high-resolution analog-to-digital converter (A/D) that is dedicated to that input. Data from the
ADC is converted to temperature using a temperature response curve for the sensor. Temperature readings
and raw sensor readings are always available for every input and can be displayed, sent over computer
interface or used for one of the many built in features of the Model 340. New readings are taken as often as
20 times a second, the update rate is slower for more complex readings. The display is updated more slowly
than data is read because the flicker of fast changes is distracting. Readings are available at full speed over
computer interface.