Lake Shore 370 User Manual
                                                                                                                                                                                        | |
Contents
| 
Lake Shore Model 370 AC Resistance Bridge User’s Manual2.4.5.1 Shielding and Grounding TheoryBegin any installation by separating noise sources from their targets. Sources of E-field and H-field noise can be justabout anything but the targets are most often signal leads and other wires and cables. H-field coupling can bedramatically reduced with an inch or less of separation. E-field noise requires more separation but is also more easilyshielded. Noise can also travel easily on the power lines. If clean power lines are not available for instrumentation placeline filters on noisy electrical or electronic devices. The line filter in the instrument can help but noise is always betterreduced at its source.The best defense against E-field noise is a conductive shield around the measurement. The more complete the shield thebetter. Ideally it should extend from the instrument measurement circuits through the cables and down to the measuredresistor. This ideal approach is impractical in all but a few cryogenic systems. The practical approach is to connect theshield from the measurement circuits, through the room temperature cables, to the conductive Dewar or vacuumchamber. This creates a faraday shield around the entire measurement circuit.The shield can only divert noise energy away from measurement leads if that energy has a place to go. Connecting theshield to measurement common (shield pin in the connector) is a good starting point but may not be the final answer. Themeasurement common is isolated from Earth ground and has, by design, a relatively low “mass”. Most noise sources arereferenced to Earth ground and, if they are large enough, can swing the entire measurement, shield and all. The shieldmay require a low impedance connection to Earth ground to effectively divert noise energy but connecting to Earthground can cause new problems.If the shield or any other part of the measurement circuit is connected to Earth ground it is necessary to prevent groundloops that couple line frequency noise into the measurement. A ground loop is created when measurement common istied to Earth ground at more than one point. The isolation of the instrument breaks the loop between the instrumentpower cord and the measurement common. No loop is formed if measurement common is tied to Earth ground at onlyone point.Loops are also the primary means of coupling H-field noise. Conductive shields do not reduce the effects of H-fields.Two ways to minimize H-field coupling are to reduce the loop area or break the loop (as described with ground loops).Loop area can be minimized by tightly twisting lead pairs, both voltage and current, within the shielded cables. This notonly reduces loop area but also guarantees that both leads are exposed to the same common mode effects which arerejected by the measurement input.2.4.5.2 ShieldsShields provide a low impedance path to measurement common to block E-field noise from getting to the signal leads.Shield pins are provided in the input connectors as attachment points for the shield conductors in lead cables. Shieldingindividual leads inside a cryostat is difficult because of limited space and the potential for heat leak. Cable shields areusually connected to the experimental Dewar to create a faraday shield around the measurement.2.4.5.3 Electrical IsolationThe precision analog front end is electrically isolated from digital circuitry and chassis of the instrument byoptocouplers. This can improve measurements in two ways. First, isolation makes it more difficult for digital noise fromcontrol circuits and computer interfaces to affect the measurement leads. Second, isolation breaks ground loops. It isoften desirable to extend shielding from the measurement leads to the experimental Dewar. If the instrument shield andthe Dewar both have low impedance to Earth ground a ground loop will be created. Isolation separates the instrumentmeasurement from Earth ground to break the loop.2.4.5.4 Driven GuardsFor installations where lead length is greater than 10 feet or measured resistance is greater than 100 kΩ, four separatedriven guards are provided. The driven guards follow the voltage on each lead to maintain the lowest possible voltagedifference between the lead and its environment. This technique reduces the effect of cable capacitance. Without theguards, cable capacitance and measured resistance act together as a low-pass filter and can attenuate the desiredmeasurement signal. Guards are not necessary when measured resistance is low or cable capacitance is low. The guardson the Model 370 can be turned on or off by the user as required by the application. Cables between the instrument andcryostat can be guarded even if it is impractical to continue guarding inside the cryostat. Driven guards are not availablefor scanned inputs but they are recommended for cables that run between the Model 370 and optional scanners.Theory of Operation 2-7 |
This manual is suitable for:
manualsdatabase
Your AI-powered manual search engine