Sensor Inputs

The Model 340 cryogenic temperature controller features two inputs with high-resolution 24-bit analog-to-digital converter and low noise circuit design, providing temperature readings with resolution as low as 0.1 mK at 4.2 K. Sensors are optically isolated from other instrument functions for quiet and repeatable sensor measurements.

Appropriate sensor excitation and input gain can be selected from the front panel. An autorange mode keeps the power in NTC resistors low to reduce self-heating as sensor resistance changes by many orders of magnitude. Automatic current reversal with rounded square wave excitation for NTC resistors eliminates the effect of thermal EMF.

Standard temperature response curves for silicon diodes, platinum RTDs, and many thermocouples are included. Up to twenty 200-point CalCurves™ for Lake Shore calibrated sensors or user curves can be loaded into non-volatile memory via a computer interface or the instrument front panel. CalCurves™ can be installed at the factory when purchased with a Model 340, or they can be field installed using the data card slot. A built-in SoftCal™1 algorithm can also be used to generate curves for silicon diodes and platinum RTDs, for storage as user curves.

1 The Lake Shore SoftCal™ algorithm for silicon diode and platinum RTD sensors is a good solution for applications that need more accuracy than a standard sensor curve but do not warrant traditional calibration. SoftCal™ uses the predictability of a standard curve to improve the accuracy of an individual sensor around a few known temperature reference points.

Temperature Control

The Model 340 offers two proportional-integral-derivative (PID) control loops. A PID control algorithm calculates control output based on temperature setpoint and feedback from the control sensor. Wide tuning parameters accommodate most cryogenic cooling systems and many small high-temperature ovens. Control output is generated by a high-resolution digital-to analog converter for smooth continuous control. The user can manually set the PID values or the autotuning feature of the Model 340 can automate the tuning process.

The main heater output for the Model 340 is a well-regulated variable DC current source. Heater output is optically isolated from other circuits to reduce interference and ground loops. Heater output can provide up to 100 W of variable DC power to control Loop 1. Features have been added to the Model 340 to minimize the possibility of overheating delicate sensors and wiring in cryostats. These features include setpoint temperature limit, heater current range limit, internal heater diagnostics, and a fuse in the heater output wiring. The Model 340 also has the ability to run a second independent control loop, intended to reduce the temperature gradients in one cooling system rather than to run two different cooling systems.

The setpoint ramp feature allows smooth, continuous changes in setpoint. This feature permits faster experiment cycles, since data can be taken as the system is changing in temperature. It can also be used to make a more predictable approach to a setpoint temperature. The zone feature can automatically change control parameter values for operation over a large temperature range. Values for ten different temperature zones can be loaded into the instrument, which will select the next appropriate zone value on setpoint change.

The Model 340 can run a set of instrument instructions called an internal program. Each program represents the temperature changes needed to conduct a user’s experiment. The setpoint can be changed or ramped up and down, and other controller parameters can be programmed. For simple experiments the internal program eliminates the need for computer control. It is also common for the internal program to be used along with the computer interface so the computer is not slowed down by temperature control overhead.

Several math features are included to improve usability and aid in setting up experiments. It is often useful to have reading filters and maximum and minimum calculations easily available on the front panel. The Model 340 cryogenic temperature controller also computes a linear equation on reading data to allow flexibility in how the display represents experimental inputs.

Interface

The Model 340 can be fully involved in computer-controlled experiments. It is equipped with IEEE-488 and RS-232C interfaces. Either interface can send settings to the Model 340 and collect reading data from it. Even the analog outputs, relays, and Digital I/O can be controlled by computer interface. The Model 340 has several features to make it more valuable as part of a larger measuring system. Two analog voltage outputs can be used to report a voltage that is proportional to the temperature of an input. The outputs can be controlled manually as a voltage source for any other application. Two relays can be used with the alarm setpoints in latching mode for error detection, or in nonlatching mode for simple on and off control. Digital I/O can be used with an external scanner or manually.

Configurable Display

The Model 340 includes a graphic LCD with fluorescent backlight display that is fully configurable and can display up to eight readings.

Heater output display

This shows a variation of the display with a large loop 1 heater output graphic bar where the PID parameters are not displayed, but the heater output is more prominent.

Reading display format

The user can display 1 to 8 readings from any of the available inputs. The units available are the sensor units of mV, V, W, kW, nF, or temperature units of °C or K. Results of the math feature can also be selected.

Input setup

The user can select the sensor type, and the controller will automatically select the sensor units, excitation, and range. If ‘special’ type is selected, the user can choose any available excitation and input range.

340 rear panel