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AD524A Datasheet(PDF) 10 Page - Analog Devices |
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AD524A Datasheet(HTML) 10 Page - Analog Devices |
10 / 16 page AD524 REV. E –10– INPUT BIAS CURRENTS Input bias currents are those currents necessary to bias the input transistors of a dc amplifier. Bias currents are an additional source of input error and must be considered in a total error budget. The bias currents, when multiplied by the source resis- tance, appear as an offset voltage. What is of concern in calculat- ing bias current errors is the change in bias current with respect to signal voltage and temperature. Input offset current is the differ- ence between the two input bias currents. The effect of offset current is an input offset voltage whose magnitude is the offset current times the source impedance imbalance. –VS +VS AD524 LOAD TO POWER SUPPLY GROUND a. Transformer Coupled AD524 LOAD TO POWER SUPPLY GROUND –VS +VS b. Thermocouple AD524 LOAD TO POWER SUPPLY GROUND –VS +VS c. AC Coupled Figure 34. Indirect Ground Returns for Bias Currents Table I. Output Gain Resistor Values Output Nominal Gain R2 R1, R3 Gain 25 k Ω 2.26 k Ω 2.02 5 1.05 k Ω 2.05 k Ω 5.01 10 1 k Ω 4.42 k Ω 10.1 Although instrumentation amplifiers have differential inputs, there must be a return path for the bias currents. If this is not provided, those currents will charge stray capacitances, causing the output to drift uncontrollably or to saturate. Therefore, when amplifying “floating” input sources such as transformers and thermocouples, as well as ac-coupled sources, there must still be a dc path from each input to ground. COMMON-MODE REJECTION Common-mode rejection is a measure of the change in output voltage when both inputs are changed equal amounts. These specifications are usually given for a full-range input voltage change and a specified source imbalance. “Common-Mode Rejection Ratio” (CMRR) is a ratio expression while “Common- Mode Rejection” (CMR) is the logarithm of that ratio. For example, a CMRR of 10,000 corresponds to a CMR of 80 dB. In an instrumentation amplifier, ac common-mode rejection is only as good as the differential phase shift. Degradation of ac common-mode rejection is caused by unequal drops across differing track resistances and a differential phase shift due to varied stray capacitances or cable capacitances. In many appli- cations shielded cables are used to minimize noise. This tech- nique can create common mode rejection errors unless the shield is properly driven. Figures 35 and 36 shows active data guards that are configured to improve ac common mode rejec- tion by “bootstrapping” the capacitances of the input cabling, thus minimizing differential phase shift. VOUT REFERENCE AD524 –VS +VS 100 AD711 G = 100 RG2 +INPUT –INPUT Figure 35. Shield Driver, G ≥ 100 VOUT REFERENCE AD524 –VS +VS 100 AD712 RG2 +INPUT –INPUT –VS RG1 100 Figure 36. Differential Shield Driver GROUNDING Many data acquisition components have two or more ground pins that are not connected together within the device. These grounds must be tied together at one point, usually at the sys- tem power-supply ground. Ideally, a single solid ground would be desirable. However, since current flows through the ground wires and etch stripes of the circuit cards, and since these paths have resistance and inductance, hundreds of millivolts can be generated between the system ground point and the data |
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