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AD624SCHIPS Datasheet(PDF) 10 Page - Analog Devices |
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AD624SCHIPS Datasheet(HTML) 10 Page - Analog Devices |
10 / 15 page REV. C AD624 –10– GROUNDING Many data-acquisition components have two or more ground pins which 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 acqui- sition components. Separate ground returns should be provided to minimize the current flow in the path from the most sensitive 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 (see Figure 34). OUTPUT REFERENCE ANALOG GROUND* *IF INDEPENDENT, OTHERWISE RETURN AMPLIFIER REFERENCE TO MECCA AT ANALOG P.S. COMMON SIGNAL GROUND AD574A DIGITAL DATA OUTPUT + 1 F 0.1 F 1 F 1 F DIG COM 0.1 F 0.1 F 0.1 F AD624 SAMPLE AND HOLD AD583 ANALOG P.S. +15V C –15V +5V DIGITAL P.S. C Figure 34. Basic Grounding Practice Since the output voltage is developed with respect to the poten- tial on the reference terminal an instrumentation amplifier can solve many grounding problems. 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. AD624 V+ OUTPUT CURRENT BOOSTER V– VIN+ VIN– X1 RL (REF) (SENSE) Figure 35. AD624 Instrumentation Amplifier with Output Current Booster Typically, IC instrumentation amplifiers are rated for a full ±10 volt output swing into 2 kΩ. In some applications, how- ever, the need exists to drive more current into heavier loads. Figure 35 shows how a current booster may be connected “inside the loop” of an instrumentation amplifier to provide the required current without significantly degrading overall perfor- mance. The effects of nonlinearities, offset and gain inaccuracies of the buffer are reduced by the loop gain of the IA output amplifier. Offset 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, from ground, to be shared between signal and reference offset. AD624 VIN+ VIN– REF SENSE LOAD AD711 –VS +VS VOFFSET Figure 36. Use of Reference Terminal to Provide Output 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, including those caused by PC layouts or other connection techniques, which appears between the reference pin and ground will increase the gain of the noninverting signal path, thereby upsetting the common- mode rejection of the IA. Inadvertent thermocouple connections created in the sense and reference lines should also be avoided as they will directly affect the output offset voltage and output offset voltage drift. In the AD624 a reference source resistance will unbalance the CMR trim by the ratio of 10 k Ω/R REF. For example, if the refer- ence source impedance is 1 Ω, CMR will be reduced to 80 dB (10 k Ω/1 Ω = 80 dB). An operational amplifier may be used to provide that low impedance reference point as shown in Figure 36. The input offset voltage characteristics of that amplifier will add directly to the output offset voltage performance of the instrumentation amplifier. 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 37. AD624 +INPUT REF R1 +VX– SENSE LOAD AD711 A2 IL –INPUT 40.000 RG 1 + IL = = VX R1 VIN R1 Figure 37. Voltage-to-Current Converter |
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