lakeshore 370 ac resistance bridge manual
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lakeshore 370 ac resistance bridge manualPlease consider the Model 372.Its primary application is the measurement of resistive materials in cryogenic environments from 20 mK to 1 K.A unique, patented, matched impedance current source and active common mode reduction circuitry minimize noise and self-heatingUsed with Lake Shore. The Model 370 uses 4-lead AC measurement for the best possible accuracy with the lowest possible excitation current. AC coupling at each amplifier stage reduces offsets for higher gain and greater sensitivity than DC techniques allow. Phase sensitiveA low excitation frequency of 13.7 Hz reduces the effect of lead capacitance on measurement. These features, in conjunctionThe Model 370 current excitation source provides stable, reliable, low power excitation current. Twenty-one AC current levels from 3.16 pA to 31.6 mA RMS offer low noise with no significant DC component to contribute to sensor self-heating. Two operating modes provide excitation control options to meet user and application needs. Performance enhancement Innovative features enhance the performance of the Model 370 from excitation to output. Common mode voltage can come from many sources, including external noise coupling into the lead wires. The Model 370 provides a unique, patented, matched impedanceJust as voltage input terminals for a differential input have the same input impedance, the two current source output terminals of the Model 370 have the same source impedance. This matchedWith this strategy, the differential input remains truly differential for accurate resistance measurement. To further reduce the effect of common mode voltage, the Model 370 includes an active common mode reduction circuit. This circuit dynamically adjusts the current source output operation point to minimize common mode voltage at the measurement input. Active common mode reduction allows the Model 370 to operate in environments that would otherwise saturate the differential input amplifiers.
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Optocouplers isolate the analog front end of the Model 370 from digital circuitry and the instrument chassis. Optical isolation minimizes the effect of digital noise on the measurement and breaks ground loops. For applications where lead length is greater than 10 ft, or resistance is greater than 100 k?, the Model 370 also includes four separate driven guards that follow the voltage on each lead to provide the lowest possible voltage difference betweenDriven guards reduce the effect of cable capacitance and provide the best possible shielding. Driven guards are not available for scanned inputs. To accommodate conversion of changing output from phase sensitive detection to a stable signal, the Model 370 operates with a 200 ms minimum filter time constant. While this is adequate for measurement of small resistance values with large excitation,Linear filtering or averaging offers the best possible settling time, with selections from 1 s to 200 s. Excitation modes The Model 370 provides full scale resistance ranges from 2 m? to 2 M?. The selected resistance range is continuously displayed; excitation power dissipated in the resistor is also continuously calculated and displayed. The Model 370 includes both a current excitation mode and a voltage mode for resistor excitation. Current excitation is the instrument's primary mode of operation. In current mode, the Model 370 provides the appropriate voltage gain when the resistanceThe instrument changes gain when the resistance range is changed. In current mode, the actual excitation current is continuously displayed. In voltage mode, the Model 370 firmware simulates voltage excitation. In voltage mode, the instrument changes the current to maintain the voltage limit when the resistance range is changed. Because voltage is limited in voltage mode, excitation powerAs a result, voltage mode provides a convenient way to limit excitation power and resistor self-heating forIn voltage mode, the upper input voltage limit but not the actual voltage is continuously displayed. The autorange function increases or decreases the resistance range when measured resistance exceeds or fallsIn current mode, the autorange function modifies the voltage gain. In voltage mode, autorange modifies the current setting. Manual range selection provides the option of full user control. With manual range selection, the user selects excitation as well as resistance range; ranges do not change automatically. If input resistance exceeds the range, an overload message appears.Temperature conversion The temperature conversion function of the Model 370 converts measured resistance to temperature for calibrated resistance thermometers. Temperature as well as resistance values can be displayed; temperature and resistance values are also available forUp to twenty 200-point curves can be entered into nonvolatile memory via computer interface or theTemperature control The Model 370 provides all of the circuitry and software for digital proportional-integral-derivative (PID) control.A still heater function can also supply up to 1 W of power into a still heater load (nominal 100 ?) by way of one of the instrument's analog outputs to enhance temperature control. The best control stability is achieved using only one sensor, but the Model 370 can control temperature based on one of multiple scanned sensors. Because the Model 370 alternates control with scanned sensor readings and the alternating update rate is slower than operation based on a single sensor, control stability may degrade in some systems when multiple sensors are in use. Computer interfaces The Model 370 includes IEEE-488.2 parallel and RS-232C serial interfaces.http://superbia.lgbt/flotaganis/1655146774 Both use the instrument chassis as ground, while measurement input is optically isolated from the chassis to minimize interface noise and ground loops. Both interfaces can accommodateAll instrument parameters, all available status information, and almost every instrument function, including analog voltage outputs and relays, can be accessedAnalog outputs With two analog voltage outputs and two relays, the Model 370 can perform functions that might otherwise require additional hardware and system complexity. Configured for use as resistance monitors, the analog voltage outputs provide a voltage proportionalAnalog outputs can be controlled manually from the front panel, by computer interface, and for some functions, by the internal operatingClosed loop control is not available for analog output functions. Configurable display The Model 370 includes an eight line by forty character vacuum fluorescent display. Input readings can be displayed in m?, ?, k?, M?, mK, or K. These are designed specifically to increase the Model 370 input capability from 1 to either 8 or up to 16 resistors without sacrificing measurement performance.The scanner also allows extension of the Model 370 shield to all resistor leads.Different preamplifiers in the 3716 and 3708 optimize them for different applications. The Model 370 supplies power and controlScanner operation is fully integrated so most of the Model 370 hardware and firmware features can be used with the scanner. Supported hardware features include matched impedance current source and ground isolation. The scanners provide four 25-pin D-subEach connector accommodates four inputs, with four signal and two shield lines for each input. Guarding is supported between the instrument and scanner only. Interface cables from the Model 370 to the scanner areSupported firmware features include temperature conversion, math functions, linear equations, and in some applications, temperature control. The Model 370 can store measurement range and temperature conversion data for each resistor. Appropriate parameterAs with any scanner, the resistance reading rate is slower during input changes, resulting in longer filter settling times and a longer sample periodThe Model 370 can be used with third party scanners; however, access to integrated features is lost. Low bias provides the lowest available resistor self-heating when excitation currents are in the range of 1 pA to 30 pA. It also provides the. Unused leads are connected to measurement common to reduce noise pickup, a persistent problem when measuring large resistances. Performance of the Model 3716 scanner is so nearly identical to the Model 370 that they share specifications for resistance range, accuracy, and resolution (noise). With DC bias current of 50 pA, however, it is not recommendedThese measurements require very low DC bias current to prevent measurement errors as a result of self heating. Specifications for resistance range, accuracy, and resolution (noise) are different than the standalone. No government Methods and apparatus disclosed herein may be subject to U.S. Patents existing or applied for. Lake Shore Cryotronics, Lake Shore shall Methods and apparatus disclosed herein may be subject to U.S. Patents existing or applied for. Lake Shore Cryotronics, Inc.Lake Shore shall not be liable for errors contained herein or for incidental or consequential damages in connection with furnishing, performance, or use of this material. All rights reserved. No portion of this manual may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical,. A Class A product is allowed to radiate more RF than a Class B product. Title Page Model 370 Front Panel. 1-1 Dilution Refrigerator Block Diagram. 2-2 He Cryostat Block Diagram. 2-3 Single-Stage ADR Block Diagram.2-4 Examples of PID Control. The Model 370 was designed and manufactured in the United States of America by Lake Shore Cryotronics, Inc. The Model 370 features include the following.The autorange function increases or decreases the resistance range when measured resistance exceeds or falls below the range in use. Low bias provides the lowest available resistor self-heating when excitation currents are in the range of 1 pA to 30 pA.Failure to comply with these precautions or with specific warnings elsewhere in this manual violates safety standards of design, manufacture, and intended instrument use. Lake Shore Cryotronics, Inc.With careful installation of a high quality bridge like the Model 370 it is not difficult to achieve 50 mK. Below that, each detail of the overall system has to be optimized to a low end of between 10 and 20 mK. So many variations of factory and home built units exist that it is unlikely that any two are identical. Figure 2-1 illustrates only the most basic features. Even at such relatively warm temperatures the benefits of a well designed resistance bridge must be considered when selecting instrumentation. The Model 370 has many features that can help automate and control these systems in temperature ranges where leaded resistance measurement is practical. Figure 2-3 illustrates a typical first stage of an ADR but other stages can be added. Current can also be scaled easily, which is necessary to achieve low excitation. The Model 370 has 21 current settings down to 3.16 pA. These currents have low noise and almost no DC component to self heat the measured resistor. Two excitation modes are available to best utilize the features of the current source in different applications. The Model 370 offers several features that help control noise and maintain signal integrity. The guards on the Model 370 can be turned on or off by the user as required by the application. Cables between the instrument and cryostat can be guarded even if it is impractical to continue guarding inside the cryostat. The two current source output terminals have the same source impedance, similar to the voltage input terminals of a differential input that have the same input impedance. Most analog circuit noise is distributed over a broad frequency band.Warm up is an exaggerated form of temperature drift because the temperature change inside the enclosure is larger and faster than would be experienced in a laboratory setting. Low excitation frequency was chosen for the Model 370 to minimize this effect but capacitance can still contribute to measurement error when resistance is high.Begin by cooling the resistor to the desired temperature and measure its value using current excitation and select a current in the middle of the available choices. Signal-To-Noise The Model 370 can read any of the 16 scanner channels, one at a time. Channel change can be done manually or with the autoscan feature. The instrument stores input setup information for each channel and will use the appropriate setup for each channel when selected. Most curves are generated from an individual sensor calibration because few sensors follow a standard curve at low temperatures. The load must be equipped with a temperature sensor for feedback and a resistive heater to add heating power into the system. There is an obvious exception in sensor location and a compromise is suggested in Paragraph 2.10.3. Manual output can be used for open loop control, meaning feedback is ignored and the heater output stays at the users manual setting. This is a good way to put constant heating power into a load when needed. Place the Model 370 in closed loop PID control mode, then turn integral, derivative and manual output settings off. Enter a setpoint above the cooling systems lowest temperature. The derivative setting is entered into the Model 370 in seconds and a good starting point is one fourth the integral setting in seconds. Instruments themselves may be shipped as several parts. The items included with the Model 370 and optional scanners are listed below. Contact Lake Shore immediately if there is a shortage of parts or accessories. See Figure 3-1. Readers are referred to paragraphs that contain installation instructions and connector pin-outs for each feature. Definition of the Model 3716 Scanner is given in Paragraph 3.5. If a fuse ever fails, it is important to replace it with the value and type indicated on the rear panel for the line voltage setting. The letter T on the fuse rating indicates that the instrument requires a time-delay or slow-blow fuse. In the off state the guard pins default to signal common but do not substitute for shields. The problem is that most test Dewars are connected to Earth ground either directly or through the many vacuum and transfer lines running in and out. In the unusual circumstance that one measurement lead is shorted to the cable shield (measurement common) the Model 370 can still make resistance measurements but the active CMR feature must be turned off. It is an extension of the isolated measurement circuitry of the Model 370 so its installation should follow the guidelines described in Paragraph 3.4 as well as this paragraph. These are identical to the connectors used on the resistance input of the Model 370. The cables included with the scanner are adequate for most installations. Connector pin definition on the scanner is identical to the input connector definition given in Paragraph 3.4.1 in case a cable must be made for a special application. The scanner control cable included with the scanner is adequate for most installations. A connector pin definition is provided in case a cable must be made for a special application. Both signal outputs and their common connections are part of the isolated measurement circuitry and should be treated like part of the resistance measurement. The output should never be connected to an external power source. There are additional firmware features in the Model 370 to help prevent damage to the load. The heater will automatically turn itself off if an open circuit condition is detected. The instrument will not allow the control setpoint to be larger than the setpoint temperature limit stored with the temperature response curve. The control heater output can be used to program a voltage programmable DC power supply of almost any size. The relay is in its normal state when it is Off and in its active state when it is On. Most of the instrument setup parameter values are retained when power is off with a few exceptions. If a Model 3716 Scanner is attached to the Model 370 the channel indicators take the place of the display update indicator. The group of keys closest to the display act directly on the resistance measurement, the center group contains temperature control features and the far right group combines instrument setup and data entry. Changes “entered” before Escape is pressed are kept. DISPLAY SETUP The display setup sequence for the Model 370 may seem complicated at first but the steps are necessary to provide flexibility to the operator. 4.4.1 Display Setup and Units Selection Display setup begins with choosing the number of reading locations on the display. The first display setup screen appears as a prompt for the number of reading locations.The brightness setting changes the entire VF display but does not affect the LED annunciators to the right of the display. The instrument assigns an excitation current to each resistance range that keeps the measurement voltage is as large as possible without exceeding the selected excitation voltage. It is still sometimes necessary to turn excitation off. Excitation should be turned off any time the current source leads are open circuited and then attached to a cold resistor with the instrument turned on. If a scanner is installed, the first input setup screen appears as a prompt for the channel number. All of the parameter changes that follow will act on the selected channel. If no scanner is installed, the sequence will begin below with change pause. If a curve is selected for a channel, the temperature coefficient (used for temperature control) is taken from that curve and the temperature coefficient parameter below is ignored. The monitor voltages are often used with the reference output. All channels can be selected for parameters under the Input Setup key by selecting the channel designator “All”. Because the Model 370 uses a linear average the measurement bandwidth is not a simple function of frequency. In closed loop PID mode the control parameter values are set manually as described in Paragraph 2.10. Once appropriate parameters are found they can be stored in the instrument and recalled with the zone feature. These are considered setup parameters because they are normally set when configuring the control loop and not used often during operation. Control parameters that change often are assigned their own keys. Setpoint units will default to ohms if no curve is selected for the control channel. The Model 370 can be configured to display the output range as its full scale current and the output as a percent of that current. There may be some rare circumstances when the heater output is not connected to a resistive load and bipolar control is desired. Chapter 2 of this manual describes the principals of closed loop (feedback) control including some tuning suggestions. Operation of the parameter is different during the two control modes. During open loop control the instrument uses the manual output value to calculate the heater output on a given heater range. The ramp can produce a smooth, linear transition in temperature rather than the step response normally associated with closed loop PID control. Firmware in the Model 370 manages automatic switching between the control channel and scan channel and will maintain the control heater output when the control channel is not active. Scaled readings can be displayed or sent to the analog voltage output as a control signal.Alarm setpoints and most other alarm parameters can be set independently for each channel. If the Model 3716 Scanner is installed, the alarm operates for the active scan channel only. If a scanner is installed, the alarm setup screen appears as a prompt for the channel number. All of the parameter changes that follow will act on the selected channel. Alarm Setup Channel Select With Alarm: Non-latching Use the s or t key to select latching or non-latching. Press Enter to accept the new selection. If latching is selected, the deadband screen will be skipped. The relays are most commonly associated with the high and low alarms, but they can also be controlled manually and used for other purposes. Both relays default to their normal state when instrument power is turned off. Paragraph 3.8.3 describes how to estimate the full-scale current sourced to a resistive heater. When the keypad is locked most parameter values may be viewed but only a few can be changed from the front panel. The temperature control heater can be turned off but not changed in range and latched alarms can be reset when the keypad is locked. Each location can hold from 2 to 200 data pairs (breakpoints) including a value in ohms or log ohms and a corresponding temperature value in Kelvin. The curves are stored in nonvolatile memory and retained when power is turned off. The Model 8000 is included with the calibrated sensor and can be loaded by the user. Lake Shore offers a free curve loading program that is compatible with Model 8000 CalCurves. The Model 370 does not automatically assign the new curve to a channel even if there is only one. This is a good way to make small changes to an existing curve. Curve copy may also be necessary if the user needs the same curve with two different temperature limits. The user should configure the zones using zone 1 for the lowest temperature and zone 10 for the highest temperature. Zone boundaries are always specified in Kelvin (K) and the upper limit of each zone acts as the lower limit of the next zone. Zone Setup Zone Enter A Value For Integral (I): 20.000 Use the data entry keys to enter an integral value between 1 and 10,000 or zero for off. Press Enter to accept the new selection and continue to the next setting screen. This setting is only used if the relay is set to zone mode. When in zone tuning mode, the instrument will update the control settings each time the setpoint is changed to a new zone. If the settings are changed manually, the controller will use the new setting while it is in the same zone and update to the zone table settings when the setpoint is changed to a value outside that zone. Cable lengths are limited to 2 meters for each device and 20 meters for the entire bus. The Model 370 can drive bus with up to 10 loads. Common commands all begin with an asterisk. They generally relate to “bus” and “instrument” status and identification. Common query commands end with a question mark (?). Model 370 common commands are detailed in Paragraph 6.3 and summarized in Table 6-6. The Model 370 will produce a service request only if bit 6 of the Service Request Enable Register is set. It indicates when the Model 370 has completed all selected pending operations. It is not related to the QOPC.A Pentium 90 or higher is recommended, running Windows 95 or better. It assumes your IEEE-488 (GPIB) card is installed and operating correctly (refer to Paragraph 6.1.5.1). Label2 Name lblCommand Caption Command Label3 Name lblResponse Caption Response. Input from the user is shown in bold and terminators are added by the program.RS-232C is a standard of the Electronics Industries Association (EIA) that describes one of the most common interfaces between computers and electronic equipment. Asynchronous timing is used for the individual bit data within a character. This timing requires start and stop bits as part of each character so the transmitter and receiver can resynchronized between each character. The query format is. Query mnemonics are often the same as commands with the addition of a question mark. Parameter data is often unnecessary when sending queries. A Pentium 90 or higher is recommended, running Windows 95 or better, with a serial interface. It uses the COM1 communications port at 9600 Baud. Label2 Name lblCommand Caption Command Label3 Name lblResponse Caption Response. Input from the user is shown in bold and terminators are added by the program.A summary of all the commands is provided in Table 6-6. All the commands are detailed in Paragraph 6.3.1, which is presented in alphabetical order. When the temperature control output (heater output) is used to control a resistive heater, this parameter Remarks: should always be set to unipolar. The Example: setpoint is in Kelvin with a 30 second setpoint change pause time. The heater output is displayed in current. Example: IEEE Interface Mode Query MODE.Returns the most recent power calculation for the specified channel. MODELS The list of Model 370 and optional scanner model numbers is provided as follows. Model Description Of Models Standard AC Resistance Bridge. Refer to the Lake Shore Temperature Measurement and Control Catalog for details. A list of accessories for the Model 370 follows: Model. Rack Mounting Kit for Model 370 AC Resistance Bridge. Hardware to attach one Model 370 to RM-1 a 483 mm (19-inch) rack mount cabinet. IMI-7031 Varnish (formerly GE 7031 Varnish) (1 pint can). IMI-7031 Insulating Varnish and Adhesive possesses electrical and bonding properties which, when combined with its chemical resistance and good saturating properties, make it an excellent material for cryogenic temperatures. When contacting Lake Shore directly, please specify the name of a department if do not know the name of an individual. It requires two good fuses of the same rating to operate safely. WARNING: To avoid potentially lethal shocks, turn off controller and disconnect it from AC power before performing these procedures. TROUBLESHOOTING It is likely through the course of installation and usage that the Model 370 will meet conditions that cannot yield a valid resistance reading. These conditions can usually be traced to environmental noise or misapplied measurement parameters. If the combination of signal and differential noise components cause the output of this stage to exceed 10 V peak, the differential overload error (VDIF OVL) is displayed. Access to these points is provided through the MONITOR output BNC connector on the back of the instrument. To select the desired monitoring point, refer to Paragraph 4.7.4 (Monitor Output Voltages). Refer to Paragraph 7.2. Use the following procedure to replace any of these ICs. Wear shock-proof wrist straps (resistor limited to The Model 370 can drive a bus of up to 10 devices. A connector extender is required to use the IEEE-488 Interface and Relay Terminal Block at the same time. In contrast, the Model 3716 scanner is calibrated with a single trim-pot that is accessible from the scanner front panel. Some of the voltages and currents are so small that they cannot be measured with conventional calibration equipment. After the basic calibration technique is described, a method is described in Paragraph 8.12.2.6 for using the Model 370 and known resistances to perform a calibration. 8.12.2.1 Voltage Measurement Zero Offset To calibrate voltage zero offset perform the following sequence of operations. Allow the instrument to warm up for at least 1 hour. When a scanner is used, its pre-amplifier replaces the pre-amplifier on the Model 370. A compensation factor must be entered into the Model 370 so it can accurately subtract the gain of its pre-amplifier when a scanner is used. Disconnect the scanner if present and attach a resistive load during warm up. Select the default frequency of 13.7 Hz. Coefficient 632 mV Zero CALZ 2,12 10 nA 200 mV Zero CALZ 2,11 3.16 nA. These outputs can be calibrated independently of each other and their calibration is independent of the measurement input. A DC voltmeter with accuracy of 1 mV on a 10 V range and cable to connect to the analog output are required. The heater output is calibrated with individual zero offset and gain coefficients for each range.Another term for the IEEE-488 bus.A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the Earth, or to some conducting body of relatively large extent that serves in place of the Earth. Rhodium-iron is a spin fluctuation alloy which has a significant temperature coefficient of resistance below 20 K where most metals rapidly lose sensitivity. An electromotive force arising from a difference in temperature at two points along a circuit, as in the Seebeck effect. tolerance. The range between allowable maximum and minimum values. When the energies and velocities of the molecules in a body are increased, the temperature is increased whether the body is a solid, liquid, or gas. Although not explosive, there are a number of safety considerations to keep in mind in the handling of LHe and LN C2.0 PROPERTIES. During this transfer, it is important that all safety precautions written on the storage Dewar and recommended by the manufacturer be followed. Please try enabling it if you encounter problems.