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AD524BE Scheda tecnica(PDF) 11 Page - Analog Devices |
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AD524BE Scheda tecnica(HTML) 11 Page - Analog Devices |
11 / 16 page AD524 REV. E –11– acquisition components. Separate ground returns should be provided to minimize the current flow in the path from the sensi- tive points to the system ground point. In this way supply currents and logic-gate return currents are not summed into the same return path as analog signals where they would cause measure- ment errors. Since the output voltage is developed with respect to the poten- tial on the reference terminal, an instrumentation amplifier can solve many grounding problems. 0.1 F 0.1 F DIGITAL P.S. +5V C –15V ANALOG P.S. 1 F DIG COM AD574A C +15V 6 OUTPUT REFERENCE *ANALOG GROUND AD524 DIGITAL DATA OUTPUT SIGNAL GROUND *IF INDEPENDENT; OTHERWISE RETURN AMPLIFIER REFERENCE TO MECCA AT ANALOG P.S. COMMON 1 F1 F 0.1 F 0.1 F AD583 SAMPLE AND HOLD Figure 37. Basic Grounding Practice SENSE TERMINAL The sense terminal is the feedback point for the instrument amplifier’s output amplifier. Normally it is connected to the instrument amplifier output. If heavy load currents are to be drawn through long leads, voltage drops due to current flowing through lead resistance can cause errors. The sense terminal can be wired to the instrument amplifier at the load, thus putting the IxR drops “inside the loop” and virtually eliminating this error source. V– V+ X1 AD524 OUTPUT CURRENT BOOSTER (REF) (SENSE) RL VIN+ VIN– Figure 38. AD524 Instrumentation Amplifier with Output Current Booster Typically, IC instrumentation amplifiers are rated for a full ±10 volt output swing into 2 k Ω. In some applications, however, the need exists to drive more current into heavier loads. Figure 38 shows how a high-current booster may be connected “inside the loop” of an instrumentation amplifier to provide the required current boost without significantly degrading overall perfor- mance. Nonlinearities, offset and gain inaccuracies of the buffer are minimized by the loop gain of the IA output amplifier. Off- set drift of the buffer is similarly reduced. REFERENCE TERMINAL The reference terminal may be used to offset the output by up to ±10 V. This is useful when the load is “floating” or does not share a ground with the rest of the system. It also provides a direct means of injecting a precise offset. It must be remem- bered that the total output swing is ±10 volts to be shared be- tween signal and reference offset. When the IA is of the three-amplifier configuration it is neces- sary that nearly zero impedance be presented to the reference terminal. Any significant resistance from the reference terminal to ground increases the gain of the noninverting signal path, thereby upset- ting the common-mode rejection of the IA. In the AD524 a reference source resistance will unbalance the CMR trim by the ratio of 20 k Ω/R REF. For example, if the refer- ence source impedance is 1 Ω, CMR will be reduced to 86 dB (20 k Ω/1 Ω = 86 dB). An operational amplifier may be used to provide that low impedance reference point as shown in Figure 39. The input offset voltage characteristics of that amplifier will add directly to the output offset voltage performance of the instrumentation amplifier. –VS +VS AD524 REF SENSE LOAD VIN+ VIN– VOFFSET AD711 Figure 39. Use of Reference Terminal to Provide Output Offset An instrumentation amplifier can be turned into a voltage-to- current converter by taking advantage of the sense and reference terminals as shown in Figure 40. AD524 REF SENSE LOAD AD711 +INPUT –INPUT R1 VX IL VX R1 IL = = = (1 + VIN R1 ) 40,000 RG A2 Figure 40. Voltage-to-Current Converter By establishing a reference at the “low” side of a current setting resistor, an output current may be defined as a function of input voltage, gain and the value of that resistor. Since only a small current is demanded at the input of the buffer amplifier A2, the forced current IL will largely flow through the load. Offset and drift specifications of A2 must be added to the output offset and drift specifications of the IA. |
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