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AMP04FP Datasheet(PDF) 7 Page - Analog Devices |
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AMP04FP Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page AMP04 REV. B –7– Programming the Gain The gain of the AMP04 is programmed by the user by selecting a single external resistor—RGAIN: Gain = 100 k Ω/RGAIN The output voltage is then defined as the differential input voltage times the gain. VOUT = (VIN+ – VIN–) × Gain In single supply systems, offsetting the ground is often desired for several reasons. Ground may be offset from zero to provide a quieter signal reference point, or to offset “zero” to allow a unipolar signal range to represent both positive and negative values. In noisy environments such as those having digital switching, switching power supplies or externally generated noise, ground may not be the ideal place to reference a signal in a high accu- racy system. Often, real world signals such as temperature or pressure may generate voltages that are represented by changes in polarity. In a single supply system the signal input cannot be allowed to go below ground, and therefore the signal must be offset to accom- modate this change in polarity. On the AMP04, a reference input pin is provided to allow offsetting of the input range. The gain equation is more accurately represented by including this reference input. VOUT = (VIN+ – VIN–) × Gain + VREF Grounding The most common problems encountered in high performance analog instrumentation and data acquisition system designs are found in the management of offset errors and ground noise. Primarily, the designer must consider temperature differentials and thermocouple effects due to dissimilar metals, IR volt- age drops, and the effects of stray capacitance. The problem is greatly compounded when high speed digital circuitry, such as that accompanying data conversion components, is brought into the proximity of the analog section. Considerable noise and error contributions such as fast-moving logic signals that easily propagate into sensitive analog lines, and the unavoidable noise common to digital supply lines must all be dealt with if the accu- racy of the carefully designed analog section is to be preserved. Besides the temperature drift errors encountered in the ampli- fier, thermal errors due to the supporting discrete components should be evaluated. The use of high quality, low-TC compo- nents where appropriate is encouraged. What is more important, large thermal gradients can create not only unexpected changes in component values, but also generate significant thermoelec- tric voltages due to the interface between dissimilar metals such as lead solder, copper wire, gold socket contacts, Kovar lead frames, etc. Thermocouple voltages developed at these junctions commonly exceed the TCVOS contribution of the AMP04. Component layout that takes into account the power dissipation at critical locations in the circuit and minimizes gradient effects and differential common-mode voltages by taking advantage of input symmetry will minimize many of these errors. High accuracy circuitry can experience considerable error con- tributions due to the coupling of stray voltages into sensitive areas, including high impedance amplifier inputs which benefit from such techniques as ground planes, guard rings, and shields. Careful circuit layout, including good grounding and signal routing practice to minimize stray coupling and ground loops is recommended. Leakage currents can be minimized by using high quality socket and circuit board materials, and by carefully cleaning and coating complete board assemblies. As mentioned above, the high speed transition noise found in logic circuitry is the sworn enemy of the analog circuit designer. Great care must be taken to maintain separation between them to minimize coupling. A major path for these error voltages will be found in the power supply lines. Low impedance, load related variations and noise levels that are completely acceptable in the high thresholds of the digital domain make the digital supply unusable in nearly all high performance analog applications. The user is encouraged to maintain separate power and ground between the analog and digital systems wherever possible, joining only at the supply itself if necessary, and to observe careful grounding layout and bypass capacitor scheduling in sensitive areas. Input Shield Drivers High impedance sources and long cable runs from remote trans- ducers in noisy industrial environments commonly experience significant amounts of noise coupled to the inputs. Both stray capacitance errors and noise coupling from external sources can be minimized by running the input signal through shielded cable. The cable shield is often grounded at the analog input common, however improved dynamic noise rejection and a reduction in effective cable capacitance is achieved by driving the shield with a buffer amplifier at a potential equal to the voltage seen at the input. Driven shields are easily realized with the AMP04. Examination of the simplified schematic shows that the potentials at the gain set resistor pins of the AMP04 follow the inputs precisely. As shown in Figure 5, shield drivers are easily realized by buffering the potential at these pins by a dual, single supply op amp such as the OP213. Alternatively, applica- tions with single-ended sources or that use twisted-pair cable could drive a single shield. To minimize error contributions due to this additional circuitry, all components and wiring should remain in proximity to the AMP04 and careful grounding and bypassing techniques should be observed. VOUT 1/2 OP213 1/2 OP213 Figure 5. Cable Shield Drivers |
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