SECTION 1 SINGLE-SUPPLY AMPLIFIERS - Analog Devices

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For minimal impact on V OH and V OL due to amplifier output load currents, circuitresistor values should be greater than 10kohm in single-supply applications. Thus,Equations 1.22, 1.14, and 1.15 can all be used to design accurate and repeatablethree op amp instrumentation amplifier circuits with single-supply/rail-to-railoperational amplifiers.A COMPOSITE, SINGLE-SUPPLY INSTRUMENTATIONAMPLIFIER [3]As it has been shown throughout this chapter, operation of high performance linearcircuits from a single, low-voltage supply (5V or less) is a common requirement.While there are many precision single supply operational amplifiers (some rail-rail),such as the OP213, the OP291, and the OP284, and some good single-supplyinstrumentation amplifiers, such as the AMP04 and the AD626 (both covered later),the highest performance instrumentation amplifiers are still specified for dualsupplyoperation.One way to achieve both high precision and single-supply operation takesadvantage of the fact that several popular transducers (e.g. strain gauges) providean output signal centered around the (approximate) mid-point of the supply voltage(or the reference voltage), where the inputs of the signal conditioning amplifierneed not operate near “ground” or the positive supply voltage.Under these conditions, a dual-supply instrumentation amplifier referenced to thesupply mid-point followed by a “rail-to-rail” operational amplifier gain stage providesvery high DC precision. Figure 1.12 illustrates one such high-performanceinstrumentation amplifier operating on a single, +5V supply. This circuit uses anAD620 low-cost precision instrumentation amplifier for the input stage, and anAD822 JFET-input dual rail-to-rail output operational amplifier for the output stage.A PRECISION SINGLE-SUPPLY INSTRUMENTATIONAMPLIFIER WITH RAIL-TO-RAIL OUTPUTFigure 1.1222
In this circuit, R1 and R2 form a voltage divider which splits the supply voltage inhalf to +2.5V, with fine adjustment provided by a trimming potentiometer, P1. Thisvoltage is applied to the input of an AD822 which buffers it and provides a lowimpedancesource needed to drive the AD620’s output reference port. The AD620’sREFERENCE input has a 10kohm input resistance and an input signal current ofup to 200µA. The other half of the AD822 is connected as a gain-of-3 inverter, sothat it can output ±2.5V, “rail-to-rail,” with only ±0.83V required of the AD620. Thisoutput voltage level of the AD620 is well within the AD620’s capability, thusensuring high linearity for the “dual-supply” front end. Note that the final outputvoltage must be measured with respect to the +2.5V reference, and not to GND.The general gain expression for this composite instrumentation amplifier is theproduct of the AD620 and the inverting amplifier gains:GAIN⎛ 49. 4kΩ ⎞= +RF⎜ ⎟ ⎛ ⎝ RG ⎠⎝ ⎜⎞1 ⎟RI ⎠Eq. 1.28For this example, an overall gain of 10 is realized with R G = 21.5kohm (closeststandard value). The table (Figure 1.13) summarizes various R G /gain values.In this application, the total input voltage applied to the inputs of the AD620 can beup to +3.5V with no loss in precision. For example, at an overall circuit gain of 10,the common-mode input voltage range spans 2.25V to 3.25V, allowing room for the±0.25V full-scale differential input voltage required to drive the output ±2.5V aboutV REF .The inverting configuration was chosen for the output buffer to facilitate systemoutput offset voltage adjustment by summing currents into the buffer’s feedbacksumming node. These offset currents can be provided by an external DAC, or froma resistor connected to a reference voltage.The AD822 rail-to-rail output stage exhibits a very clean transient response (notshown) and a small-signal bandwidth over 100kHz for gain configurations up to 300.Figure 1.13 summarizes the performance of this composite instrumentationamplifier. To reduce the effects of unwanted noise pickup, a capacitor isrecommended across A2’s feedback resistance to limit the circuit bandwidth to thefrequencies of interest. Also, to prevent the effects of input-stage rectification, anoptional 1kHz filter is recommended at the inputs of the AD620.23
Page 1 and 2: SECTION 1SINGLE-SUPPLY AMPLIFIERSRa
Page 3 and 4: typically have open-loop gains betw
Page 5 and 6: from the many applications where co
Page 7 and 8: RAIL-TO-RAIL INPUT STAGE TOPOLOGYFi
Page 9 and 10: OP AMP OUTPUT STAGES USINGCOMPLEMEN
Page 11 and 12: These considerations, as well as th
Page 13 and 14: Since the input signal, V IN+ ,as w
Page 15 and 16: Circuit Ga in = V OH(MIN) - V OL(MA
Page 17 and 18: PERFORMANCE SUMMARY OF AD822 IN-AMP
Page 19 and 20: the current through the gain settin
Page 21: I OA3 =−(V IN +) ⎛ R13 + R5 RG
Page 25 and 26: the PNP input transistors are force
Page 27 and 28: Of course, the amplified bridge out
Page 29 and 30: the circuit has an adjustment range
Page 31: REFERENCES1. Systems Application Gu

SECTION 1 SINGLE-SUPPLY AMPLIFIERS - Analog Devices

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