Datasheet AMP04 (Analog Devices) - 7

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