Lake Shore 370 User Manual
                                                                                                                                                                                        | |
Contents
| 
Lake Shore Model 370 AC Resistance Bridge User’s Manual2.4 MODEL 370 THEORY OF OPERATION2.4.1 Why is the Model 370 called a Bridge?Historically precision resistance measurements were made using one of several bridge configurations that are acombination of known reference resistors and the unknown being measured. Normally in a bridge one of the knownresistors is trimmed until a null appears at the measurement point of the circuit and at that time the unknown resistancecan be calculated. The technique is preferred because it relies only on resistance ratios and a null measurement, whicheliminates many of the errors normally associated with amplifiers, voltage references and current sources. As electroniccircuits improve fewer and fewer applications required the complexity of a bridge measurement.Some applications benefit so much from the bridge configuration that they may never be replaced. One example is straingauge measurements that integrate both measurement and temperature compensation into the bridge. Anotherapplication, often seen in physics laboratories, is low-noise resistance measurement where the reference resistors andunknown are all cooled to reduce thermal noise.The Model 370 does not use a bridge configuration for resistance measurement. Its measurement technique is describedin the next paragraph. Side-by-side testing demonstrated that the precision circuits in the Model 370 would not benefitfrom a bridge configuration unless the reference resistors were cooled. This path was not chosen because installation ismore complicated than most applications require. The name “Bridge” was retained to reflect the instrument's heritage ofprecision resistance measurements.2.4.2 Resistance MeasurementThe Model 370 uses a four lead, AC measurement technique to achieve the best possible accuracy with the lowestpossible excitation. Similar circuits are used extensively in small signal measurement because they are flexible and canextract small signal from relatively noisy environments.The Model 370 uses current excitation because controlled current is very stable and predictable. Current can also bescaled 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 areavailable to best utilize the features of the current source in different applications. One lower current setting, 1 pA, isavailable but not specified because leakage and bias currents in the measurement circuits can be a large percentage of theexcitation current. The 1 pA current setting should be used for relative measurements only.Precision resistance measurements use a four lead method to eliminate the effect of nominal lead resistance. Excitationcurrent is driven on one pair of leads while the resulting signal voltage is measured on the second pair. Measurementerrors from lead resistance are proportional to current flowing in the voltage leads. Instrument voltage inputs areexpected to have high impedance to prevent the unwanted current from flowing. The Model 370 has very high inputimpedance to both normal mode voltages (created by current source excitation) and common mode voltages (created byinduced noise on the leads).Offset voltages occur in every electronic circuit both in signal amplifiers and in lead wire connections that create thermalEMF voltages. Offset voltages affect accuracy more when the excitation is small because they are a larger percentage ofthe measured voltage. AC measurements allow AC filters that greatly reduce amplifier offsets at each stage, allowinghigher amplifier gain and greater sensitivity than DC techniques. The AC technique is also naturally bipolar allowing anyremaining thermal EMF or offset voltages to be averaged out of the resistance measurement. A relatively low frequencyof excitation, 13.7 Hz, was chosen to reduce the effects of lead capacitance on the measurement. In a typical cryostatlead heat sink requirements and limited space make it difficult to reduce capacitance enough to operate at significantlyhigher frequency.Environmental noise can limit the resolution of low excitation measurement. Well designed filtering is necessary toreclaim the desired signal from the noise. The Model 370 takes advantage of the AC filtering technique called phasesensitive detection that is used in Lock-In Amplifiers. This technique has no DC equivalent. Even with phase sensitivedetection it is still necessary to shield these leads from environmental noise. The Model 370 facilitates appropriateshielding techniques in addition to offering more active noise reduction circuits.Theory of Operation 2-5 |
This manual is suitable for:
manualsdatabase
Your AI-powered manual search engine