Data Acquisition

of the amplifier inputs. Such bias resistors normally consist of high impedance resistors,typically between 100 KΩ and 1 MΩ, connected between the HI and LO signal lines andAGND.5.4.1 Dynamic rangeOne of several considerations in determining the analog input requirements of an A/D boardis the range of voltages, which each channel is required to measure. The physical parametersto be measured, the type of sensor(s) used and how they are connected, determine the inputvoltage ranges required.The input range specifications quoted by board manufacturers of A/D boards refer to theminimum and maximum voltage levels that the A/D converter on the board can quantize.Typically, a selection of input ranges is provided, either unipolar (e.g. 0 to 10 V), for measuringpositive voltages only, or bipolar (e.g. –10 V to +10 V), for measuring both positiveand negative voltages. This allows the user to match the input signal range to that of the A/Dconverter, taking into account the resolution of the A/D converter and the gain required of theinput amplifier.When considering the input range, it is only the dynamic range of the input signal thatneeds to be taken into account. For example, consider a strain gauge setup in a Wheatstonebridge configuration. The input voltage to be read has a common mode component due to theexcitation of the bridge, while the small differential voltage changes, (of interest) are due tothe change in strain gauge resistance. The common mode voltages do not provide any usefulinformation and are greatly attenuated, (almost eliminated), by using differential inputs andinstrument amplifiers with high CMRR. Only the small differential voltage changes areamplified and converted by the A/D converter. The amplifier gain should therefore beselected so that the maximum differential voltage change expected at the input will beamplified to cover as much of the input range of the A/D converter as possible.As only one of the allowable range settings can be selected at any time, typically byjumpers on the board itself, care should be taken in matching the input signal requirementswhere more than one channel is sampled. The A/D converter input range selected mustaccurately measure the signal inputs from a number of channels, possibly different sensors,and therefore potentially different input voltage levels and signal ranges. The input rangeshould therefore cover each channel’s input range with as little overlap as possible, thusgiving the greatest number of data points and therefore the highest resolution and accuracy.It should be noted that the input ranges specified do not necessarily refer to the maximumor minimum voltage levels that can be applied at any single input, or to the maximumallowable common mode voltage, which can be applied, to a differential input. These arespecifications, more related to the input amplifier. If there are any doubts with regard to this,users should consult the board manufacturer.5.4.2 ResolutionThe resolution specification quoted by manufacturers of A/D boards refers to the resolutionof the A/D converter used on the board. It is usually expressed by the number of bits the A/Dconverter uses to represent the analog input voltage (i.e. n-bit) or as a fraction of themaximum number of discrete levels, which can be used to represent the analog signal (i.e.1/2 n ). The resolution implicitly defines the number of discrete ranges into which the full-scalevoltage (FSV) input range can be divided to approximate an analog input voltage. A 12-bit