Model 44C Cryogenic Temperature Controller

The Model 44C temperature controller is designed for use as a general purpose cryogenic instrument with extended operation into the ultra-low (>1K) temperature range. Additionally, it provides an interface to control external equipment including cryocooler and He3 refrigerator systems, offering a complete 'one box' solution in many applications.

The Model 44C provides four sensor inputs, each of which support wide range of temperature measurements by use of a ratiometric AC resistance bridge. This is extended into general purpose applications by supporting a DC mode for diode type sensors.

Three control loop outputs feature power levels as high as 50W and as low as 50mW in order to support both high and ultra-low temperature operation. All control modes are supported by all outputs.

The 44C front panel incorporates a high resolution graphics TFT type Liquid Crystal Display with an exceptionally wide viewing angle. With it's bright white LED backlight, complete instrument status can be seen at a glance, even from across the room.

The top level display is user configurable and lower level menus are organized into logical groups that provide complete information, in real-time, for most basic tasks.

Flexible Inputs: The Model 44C has four identical input channels, each of which can be easily configured to operate virtually any type of cryogenic thermometer.

A unique feature of the Model 44C is the use of a ratiometric AC resistance bridge to measure all types of temperature sensors. Excitation is a differential voltage-source followed by passive attenuators for minimum Johnson noise. Sensor voltage and excitation current are separately measured using dual matched amplifiers and dual analog-to-digital converters. This technique actively cancels gain, drift and low frequency noise measurement errors; thereby providing high accuracy even at extremely low excitation current.

The resistance bridge is used in a constant-voltage AC mode to provide robust support for the Negative Temperature Coefficient (NTC) sensors. These include
Ruthenium-oxide, Carbon-Glass, Cernox™, Carbon-Ceramic, Germanium and several others. Since they have a negative temperature coefficient, the use of a constant-voltage measurement will reduce, rather than increase, power dissipation in the sensor as temperature decreases. By maintaining the lowest possible power level, sensor self-heating is minimized and useful temperature range is greatly increased. An additional advantage of constant-voltage excitation is that NTC resistors lose sensitivity in the upper part of their range. By auto-ranging excitation current to maintain a constant voltage, sensitivity and noise immunity in that range is also improved.

The excitation source in the Model 44C is continuously variable so there are no steps in sensor self-heating. AC excitation used with resistor sensors is a 3.25Hz bipolar square wave. This effectively eliminates DC offset errors including the thermal EMF induced offsets that often occur in some cryogenic systems.

Ultra-Low temperature:

• <100µV Constant-Voltage excitation offers robust support for Ruthenium-Oxide or Cernox™ sensors to <50mK.
• Four input channels. Most He3 refrigerators require at least three.
• High precision control loop with PID control that is enhanced for operation below 1.0K.

Cryogen-Free systems:

• Proprietary cryocooler thermal signature removal offers the best control stability in the industry for cryocooler based systems.
• Four input channels. Cryogen-Free systems often require more than two.
• Especially effective in ultra-low temperature cryogen-free systems.

Superconducting Magnets:

• Constant-voltage sensor excitation extends the high and low temperature range of sensors normally used in high magnetic fields.
• Cryogen-Free magnet systems often require more than two inputs.