The Circuit Designer's Companion - diagramas.diagram...

78 The Circuit Designer’s Companionvoltage drop, for power efficiency reasons; a low value resistor of, say 10mΩ willdevelop 50mV when 5A is passed through it and this can be amplified by a precisiondifferential amplifier and used for monitoring. Bulk metal resistors are available in flatchip form or as solder-in wire with values down to 3mΩ and power ratings of 1–10W.There are some precautions to take with such low values. To achieve an acceptableaccuracy it is normally necessary to make a four-terminal or “Kelvin” connection, so thatthe current carrying tracks and the voltage sense tracks go separately to the pads (Figure3.4). An easy way to do this for through-hole components is to use opposite sides ordifferent layers of the PCB for the two purposes, or at least to connect to opposite sidesof the pad. Even when this is done, there is still some pad area and solder in series withthe actual resistor element, which could compromise accuracy and/or tempco. To getaround this, specify a component actually designed to have four terminals.VRIFigure 3.4 The Kelvin connectionThe low impedance, low level voltage sense is susceptible to magnetic fieldinterference, and to control this you should minimise the loop area between the senseresistor and the sense circuit input. When you are monitoring ac signals with a few mΩof resistance, the self-inductance of the resistor itself may become significant. Forinstance at, say, 400Hz (a common aerospace ac power frequency) the impedance of astray inductance of 100nH is 0.25mΩ, which could give a 5% error on a voltagemeasured across a 5mΩ sense resistor. This makes wirewound types unattractive andbulk metal or chip resistors are to be preferred.Finally, a metallic element with a high dissipation and a low sense voltage may giverise to thermoelectric errors. The junction between the element and its termination is athermocouple, generating a voltage across it proportional to temperature. So a senseresistor in fact includes two thermocouples back-to-back, one at each terminal. As longas the temperatures at each termination are the same, their errors cancel out. This meansthat you should aim for thermal symmetry in the layout, by offering similar heatsinkingto each terminal (via the PCB tracks, usually) and by keeping other heat sources distant.Very high valuesDifferent considerations apply to multi-megohm resistors. Here, the main problem isleakage due to contamination across the terminals. The highest values (up to 10 14ohms) are encapsulated in glass envelopes and it is necessary to avoid handling theglass, and to touch only the leads, to prevent finger grease from affecting the resistanceacross the terminals. Electrically, it is helpful to have a “guard” electrode around theresistor to act as an electrostatic shield and to reduce or null out the effects of leakagecurrent into the terminals. High value resistors have long time constants, and even asmall amount of self-capacitance can have a significant effect: for instance a 100GΩresistor with a self capacitance of only 1pF has a time constant of 0.1 seconds.