Using Single Supply Operational Amplifiers in Embedded …

1 2000 Microchip Technology 1AN682 INTRODUCTIONB eyond the primitive transistor, the Operational ampli-fier is the most basic building block for analog applica-tions. Fundamental functions such as gain, loadisolation, signal inversion, level shifting, adding and/orsubtracting signals are easily implemented with thisbuilding block. More complex circuits can also beimplemented, such as the instrumentation amplifier, acurrent to voltage converter, and filters, to name only afew. Regardless of the level of complexity of the opera-tional amplifier circuit, knowing the fundamental opera-tion and behavior of this building block will save aconsiderable amount of upfront design time. Formal classes on this subject can be very comprehen-sive and useful.

2 However, many times they fall short interms of experience or common sense. For instance, acommon mistake that is made when designing withoperational Amplifiers is to neglect to include thebypass capacitors in the circuit. Operational amplifiertheory often overlooks this practical detail. If the bypasscapacitor is missing, the amplifier circuit could oscillateat a frequency that theoretically doesn t make text book solutions are used, this is a difficult problemto application note is divided into three sections. Thefirst section will list fundamental amplifier applicationswith the design equations included. These amplifier cir-cuits where selected with Embedded system integra-tion in second section will use these fundamental circuitsto build useful amplifier functions in Embedded third section will identify the most common singlesupply Operational amplifier (op amp) circuit designmistakes.

3 This list of mistakes has been gathered overmany years of trouble shooting circuits with numerousdesigners in the industry. The most common design pit-falls can easily be avoided if the check list from thisshort tutorial is Operational AMPLIFIER CIRCUITSThe op amp is the analog building block that is analogousto the digital gate. By Using the op amp in the design, cir-cuits can be configured to modify the signal in the samefundamental way that the inverter, AND, and OR gatesdo in digital circuits. In this section, fundamental buildingblocks such as the voltage follower, non-inverting gainand inverting gain circuits will be discussed. This will befollowed by a rail splitter, difference amplifier, summingamplifier and current to voltage Follower AmplifierStarting with the most basic op amp circuit, the bufferamplifier (shown in Figure 1) is used to drive heavyloads, solve impedance matching problems, or isolatehigh power circuits from sensitive, precise circuitry.

4 Figure 1:Buffer Amplifier; also called a buffer amplifier, shown in Figure 1, can be imple-mented with any Single Supply , unity gain stable ampli-fier. In this circuit as with all amplifier circuits, the opamp must be bypassed with a capacitor. For Single sup-ply Amplifiers that operate in bandwidths from DC tomegahertz, a 1 F capacitor is usually a smaller bypass capacitor is required foramplifiers that have bandwidths up to the 10s of mega-hertz. In these cases a F capacitor would be appro-priate. If the op amp does not have a bypass capacitoror the wrong value is selected, it may oscillate. The analog gain of the circuit in Figure 1 is +1 that this circuit has a positive overall gain but thefeedback loop is tied from the output of the amplifier toAuthor:Bonnie Baker Microchip Technology = VIN**Bypass Capacitor, 1 FVDD27346 VOUTVINU s i n g S i n g l e S u p p l y O p e r a t i o n a l A m p l i f i e r s in Embedded SystemsAN682DS00682C-page 2 2000 Microchip Technology inverting input.

5 An all too common error is to assumethat an op amp circuit that has a positive gain requirespositive feedback. If positive feedback is used, theamplifier will most likely drive to either rail at the amplifier circuit will give good linear performanceacross the bandwidth of the amplifier. The only restric-tions on the signal will occur as a result of a violation ofthe input common-mode and output swing limits. Theselimitations will be discussed in the third section of thisapplication note ( Amplifier Design Pitfalls ).If this circuit is used to drive heavy loads, the amplifierthat is actually selected must be specified to providethe required output currents. Another application wherethis circuit may be used is to drive capacitive loads.

6 Notevery amplifier is capable of driving capacitors withoutbecoming unstable. If an amplifier can drive capacitiveloads, the product data sheet will highlight this , if an amplifier can t drive capacitive loads, theproduct data sheets will not explicitly say. Another use for the buffer amplifier is to solve imped-ance matching problems. This would be applicable in acircuit where the analog signal source has a relativelyhigh impedance as compared to the impedance of thefollowing circuitry. If this occurs, there will be a voltageloss with the signal as a consequence of the voltagedivider between the source s impedance and the fol-lowing circuitry s impedance. The buffer amplifier is aperfect solution to the problem.

7 The input impedance ofthe non-inverting input of an amplifier can be as high as1013 for CMOS Amplifiers . In addition, the outputimpedance of this amplifier configuration is usually lessthan 10 . Figure 2:Load isolation is achieved Using a another use of this configuration is to separate aheat source from sensitive precision circuitry, as shownin Figure 2. Imagine that the input circuitry to this bufferamplifier is amplifying a 100 V signal. This type ofamplification is difficult to do with any level of accuracyin the best of situations. This precision measurementcan easily be disrupted by changing the output currentdrive of the device that is doing the amplification increase in current drive will cause self heating ofthe chip which will induce an offset change.

8 An analogbuffer can be used to perform the function of drivingheavy loads while the front end circuitry can be used tomake precision Analog Signals The buffer solves a lot of analog signal problems, how-ever, there are instances in circuits where a signalneeds to be gained. Two fundamental types of amplifiercircuits can be used. With the first type, the signal is notinverted as shown in Figure 3. This type of circuit isuseful in Single supply1 amplifier applications wherenegative voltages are usually not 3: Operational amplifier configured in anon-inverting gain input signal to this circuit is presented to the highimpedance, non-inverting input of the op amp. The gainthat the amplifier circuit applies to the signal is equal to:Typical values for these resistors in Single Supply cir-cuits are above 2k for R2.

9 The resistor, R1, restric-tions are dependent on the amount of gain desiredversus the amount of amplifier noise and input offsetvoltage as specified in the product data sheet of the opamp. Once again, this circuit has some restrictions in terms ofthe input and output range. The non-inverting input isrestricted by the common-mode range of the output swing of the amplifier is also restricted asstated in the product data sheet of the individual typically, the larger signal at the output of the ampli-fier causes more signal clipping errors than the smallersignal at the input. If undesirable clipping occurs at theoutput of the amplifier, the gain should be reduced. R1R2 VINVOUT*BufferPrecision Amplifier*Bypass Capacitor, 1 F*VDDMCP601 +VDDR1 VINVOUT*Bypass Capacitor, 1 FR2*VDDMCP601 VOUT1R2R1-------+ =VINVOUT1R2R1-------+ = this discussion, Single Supply implies that the neg-ative Supply pin of the Operational amplifier is tied toground and the positive Supply pin is tied to +5V.

10 Alldiscussion in this application note can be extrapolatedto other Supply voltages where the Single Supply ex-ceeds 5V or dual supplies are used. 2000 Microchip Technology 3AN682An inverting amplifier configuration is shown inFigure 4. With this circuit, the signal at the input resis-tor, R1, is gained and inverted to the output of the ampli-fier. The gain equation for this circuit is:The ranges for R1 and R2 are the same as in thenon-inverting circuit shown in Figure 3. Figure 4: Operational amplifier configured in aninverting gain circuit. In Single Supply environments aVBIAS is required to insure the output stays Single Supply applications, this circuit can easily bemisused.