1、CHAPTER 3AMPLIFIERS FOR SIGNAL CONDITIONING,1. INTRODUCTIONThis section examines the critical parameters of amplifiers for use in precision signal conditioning applications. Offset voltages for precision IC op amps can be low as 10uV with corresponding temperature drifts of 0.1uV/oC. Open loop gains
2、 greater than 1 million are common, along with common mode and power supply rejection ratios of the same magnitude. Applying these precision amplifiers while maintaining the amplifier performance can present significant challenges to a design engineer, i.e., external passive component selection.,It
3、is important to understand that DC open-loop gain, offset voltage, and common mode rejection (CMR) alone should not be the only considerations in selecting precision amplifiers. The AC performance of the amplifier is also important, even at “low“ frequencies. For example, the open loop gain is 10 at
4、 100kHz, and 100,000 at 10Hz. Loss of open loop gain at the frequency of interest can introduce distortion, especially at audio frequencies. Loss of CMR at the line frequency or harmonics thereof can also introduce errors.,In addition, a wide selection of precision amplifiers are now available which
5、 operate on single supply voltages which complicates the design process even further because of the reduced signal swings and voltage input and output restrictions. Offset voltage and noise are now a more significant portion of the input signal .,AMPLIFIERS FOR SIGNAL CONDITIONING Input Offset Volta
6、ge 1,000,000, Unity Gain Bandwidth Product, fu 500kHz - 5MHz Always Check Open Loop Gain at Signal Frequency 1/f (0.1Hz to 10Hz) Noise100dB,2. PRECISION OP AMP CHARACTERISTICS 2.1 lnput Offset VoltageInput offset voltage error is usually one of the largest errorsources for precision amplifier circui
7、t designs. However, it isa systemic error and can usually be dealt with by using amanual offset null trim or by system calibration techniquesusing a microcontroller or microprocessor. Both solutionscarry a cost penalty, and todays precision op amps offerinitial offset voltages as low as 10uV for bip
8、olar devices, andfar less for chopper stabilized amplifiers.,Measuring input offset voltages of a few microvolts requires that the test circuit dose not introduce more error than the offset voltage itself.,The measurement is made at the amplifier output using an digital voltmeter.The offset referred
9、 to the input is calculated by dividing the output voltage by the noise gain.,As simple as it looks, this circuit may give inaccurate results. The largest potential source of error comes from parasitic thermocouple junctions formed where two different metals are joined. The thermocouple voltage form
10、ed by temperature difference between two junctions can range from 2V/oC to more than 40 V/oC. The accuracy of the measurement depend on the mechanical layout of the components and how they are placed. Keep in mind that the two connections of a component such as a resistor create two equal, but oppos
11、ite polarity thermoelectric voltages which cancel each other assuming both are at exactly the same temperature.,OP177/AD707 OFFSET ADJUSTMENT PINS,It is important to note that the offset drift of an op amp with temperature will vary with the setting of its offset adjustment. In most cases a bipolar
12、op amp will have minimum drift at minimum offset.,The offset adjustment pins should therefore be used only to adjust the op amps own offset, not to correct any system offset errors since this would be at the expense of increased temperature drift.,The drift penalty for a JFET input op amp is much wo
13、rse than for a bipolar input and is in the order of 4V/oC for each millivolt of nulled offset voltage. It is generally better to control the offset voltage by proper selection of devices and device grades. Single op amps in small packages do not generally have null capability, and offset adjustments
14、 must be done elsewhere the system when using these devices. 2.2 Input Offset Voltage and Input Bias Current ModelsThus far, we have considered only the op amp input offset voltage. However,input bias currents also contribute to offset error .,OP AMP TOTAL OFFSET VOLTAGE MODEL,For a precision op amp
15、 having a standard bipolar input stage using either PNPs or NPNs, the input bias currents are typically 50nA to 400nA and are well matched.By making R3 equal to the parallel combination of R1 and R2, their effect on the net RTI and RTO offset voltage is approximately canceled. 2.3 DC Open Loop Gain
16、NonlinearityIt is well understood that in order to maintain accuracy, a precision amplifiers DC open loop gain AVOL should be high. This can be seen by examining the equation for the closed loop gain :,If AVOL in the above equation is infinite, the closed loop gain is exactly equal to the noise gain
17、. However, for finite values of AVOL, there is a closed loop gain error given by the equation:,Notice from the equation that the percent gain error is directly proportional to the noise gain, therefore the effects of finite AVOL are less for low gain.,“IDEAL“ CLOSED LOOP GAIN = NOISE GAIN = NG,ACTUA
18、L CLOSED LOOP GAIN =,% CLOSED LOOP GAIN ERROR =, Assume AVOL = 2,000,000, NG = 1,000%GAIN ERROR 0.05% Assume AVOL Drops to 300,000%GAIN ERROR 0.33% CLOSED LOOP GAIN UNCERTAINTY= 0.33% - 0.05% = 0.28%,Changes in the output voltage level and the output loading are the most common causes of changes in
19、the open loop gain of op amps. Most op amps have fixed loads, so open loop gain changes with load are not generally important. However, the sensitivity of open loop gain to output signal level may increase for higher load currents.If temperature shift is the sole cause of the nonlinearity error, it
20、can be assumed that minimizing the output loading will help. To verify this, the nonlinearity is measured with no load and then compared to the loaded condition. An oscilloscope X-Y display test circuit for measuring DC open loop gain nonlinearity is shown below.,CIRCUIT MEASURES OPEN LOOP GAIN NONL
21、INEARITY,2.4 Op Amp Noise The three noise sources in an op amp circuit are the voltage noise of the op amp, the current noise of the op amp (there are two uncorrelated sources, one in each input), and the Johnson noise of the resistances in the circuit.,Op amp noise has two components - “white“ nois
22、e at medium frequencies and low frequency “1/f noise,whose spectral density is inversely proportional to the square root of the frequency.,INPUT VOLTAGE NOISE, nV/Hz,2.5 Common Mode Rejection and Power SupplyRejection If a signal is applied equally to both inputs of an op amp so that the differentia
23、l input voltage is unaffected, the output should not be affected. In practice, changes in common mode voltage will produce changes in the output. The common mode rejection ratio or CMRR is the ratio of the common mode gain to the differential mode gain of an op amp. For example, if a differential in
24、put change of Y volts will produce a change of 1V at the output, and a common mode change of X volts produces a similar change of 1V, then the CMRR is X/Y. it is normally expressed in dB, and typical LF values are between 70 and 120dB.,When expressed in dB, it is generally referred to as common mode
25、 rejection(CMR). At higher frequencies, CMR deteriorates.CMR=20log10CMRRCMRR produces a corresponding output offset voltage error in op amps configured in the non-inverting mode .,PSR=20log10PSRR,If the supply of an op amp changes, its output should not, but it will. The specification of power suppl
26、y rejection ratio or PSRR is defined similarly to the definition of CMRR. If a change of X volts in the supply produces the same output change as a differential input change of Y volts, then the PSRR on that supply is X/Y. When the ratio is expressed in dB, it is generally referred to as power suppl
27、y rejection, or PSR. The definition of PSRR assumes that both supplies are altered equally in opposite directions - otherwise the change will introduce a common mode change as well as a supply change, and the analysis becomes considerably more complex.,3. SINGLE SUPPLY OP AMPSOver the last several y
28、ears, single-supply operation has become an increasingly important requirement because of market requirements. Accuracy and precision requirements have forced IC manufacturers to meet the challenge of “doing more with less“ in their amplifier designs. SINGLE SUPPLY AMPLIFIERS Single Supply Offers: L
29、ower Power Battery Operated Portable Equipment, Design Tradeoffs: Reduced Signal Swing Increases Sensitivity to ErrorsCaused by Offset Voltage, Bias Current, Finite Open-Loop Gain, Noise, etc. Must Usually Share Noisy Digital Supply Rail-to-Rail Input and Output Needed to Increase Signal Swing Preci
30、sion Less than the best Dual Supply Op Ampsbut not Required for All Applications,In a single-supply application, the most immediate effect on the performance of an amplifier is the reduced input and output signal range. As a result of these lower input and output signal excursions, amplifier circuit
31、s become more sensitive to internal and external error sources. To keep battery current drain low, larger resistors are usually used around the op amp. Since the bias current flows through these larger resistors, they can generate offset errors equal to or greater than the amplifiers own offset volt
32、age.,4.INSTRUMENTATION AMPLIFIERS (IN-AMPS),An instrumentation amplifier is a closed-loop gain block which has a differential input and an output which is single-ended with respect to a reference terminal.,In order to be effective, an in-amp needs to be able to amplify microvolt-level signals, while
33、 simultaneously rejecting volts of common mode signal at its inputs. This requires that in-amps have very high common mode rejection (CMR): typical values of CMR are 70dB to over 100dB, with CMR usually improving at higher gains. Specifying CMR over frequency is more important than specifying its DC
34、 value. It is important to note that a CMR specification for DC inputs alone is not sufficient in most practical applications.,4.1 Instrumentation Amplifier ConfigurationsLow-frequency CMR of op amps, connected as subtractors as shown below.,A mismatch of only 0.1% in the resistor ratios will reduce
35、 the DC CMR to approximately 66dB. Another problem with the simple op amp subtractor is that the input impedances are relatively low and are unbalanced between the two sides.,The simple subtractor circuit described above lacks the performance required for precision applications. An in-amp architectu
36、re which overcomes some of the weaknesses of the subtractor circuit uses two op amps .,VREF,This circuit is typically referred to as the two op amp in-amp. Dual IC op amps are used in most cases for good matching. The input impedance is high, permitting the impedance of the signal sources to be high
37、 and unbalanced. There is an implicit advantage to this configuration due to the gain executed on the signal. This raises the CMR in proportion. While thin film resistors fabricated on silicon have an initial tolerance of up to +20%, laser trimming during production allows the ratio error between th
38、e resistors to be reduced to 0.01% (100ppm). Furthermore, the tracking between the temperature coefficients of the thin film resistors is inherently low and is typically less than 3ppm/oC (0.0003%/ oC).,When dual supplies are used, VREF is normally connected directly to ground. In single supply appl
39、ications, VREF is usually connected to a low impedance voltage source equal to one-half the supply voltage. The gain from VREF to node “A“ is R1/R2, and the gain from node “A“ to the output is R2/R1. One major disadvantage of this design is that common mode voltage input range must be traded off aga
40、inst gain. The amplifier A1 must amplify the signal at V1 by 1+R1/R2. If R1 R2, A1 will saturate if the common mode signal is too high, leaving no headroom to amplify the wanted differential signal. For high gains (R1 R2), there is correspondingly more headroom at node “A“ allowing larger common mod
41、e input voltages.,The AC common mode rejection of this configuration is generally poor because the signal from V1 to VOUT has the additional phase shift of A1. In addition, the two amplifiers are operating at different closed-loop gains (and thus at different bandwidths). The use of a small trim cap
42、acitor “C“ as shown in the diagram can improve the AC CMR somewhat.A low gain (G = 2) single supply two op amp in-amp configuration results when RG is not used. The input common mode and differential signals must be limited to values which prevent saturation of either A1 or A2. Their upper and lower
43、 output limits are designated VOH and VOL, respectively. Using the equations shown in the diagram, the voltage at V1 must fall between 1.3V and 3.7V to prevent A1 from saturating.,Notice that VREF is connected to the average of VOH and VOL (2.5V). This allows for bipolar differential input signals w
44、ith VOUT referenced to +2.5V.,VREF=,SINGLE SUPPLY RESTRICTIONS: VS = +5V, G = 2,V2,V1,SINGLE SUPPLY RESTRICTIONS: Vs = +5V, G = 100,VREF=,Note that the voltage at V1 can now swing between 0.124V and 4.876V.,V2,V1,The above discussion shows that regardless of gain, the basic two op amp in-amp does no
45、t allow for zero-volt common mode input voltages when operated on a single supply. This limitation can be overcome using the circuit shown below which is implemented in the AD627 in-amp. The PNP transistors not only provide gain but also level shift the input signal positive by about 0.5V, thereby a
46、llowing the common mode input voltage to go to 0.1V below the negative supply rail. The AD627 in-amp delivers rail-to-rail output swing and operates over a wide supply voltage range (+2.7V to18V). Without RG, the external gain setting resistor, the in-amp gain is 5. Gains up to 1000 can be set with
47、a single external resistor.,AD627 IN-AMP KEY SPECIFICATIONS Wide Supply Range : +2.7V to 18V Input Voltage Range: -Vs - 0.1V to +Vs - 1V 85A Supply Current Gain Range: 5 to 1000 75 V Maximum Input Offset Voltage (AD627B) 10ppm/oC Maximum Offset Voltage TC (AD627B) 10ppm Gain Nonlinearity 85dB CMR 60
48、Hz, 1k Source Imbalance (G = 5) 3V p-p 0.1Hz to 10Hz Input Voltage Noise (G = 5),For true balanced high impedance inputs, three op amps may be connected to form the in-amp shown below.,This circuit is typically referred to as the three op amp in-amp. The gain of the amplifier is set by the resistor,
49、 RG, which may be internal, external, or (software or pin-strap) programmable. In this configuration, CMR depends upon the ratio matching of R3/R2 to R3/R2.The classic three op amp configuration has been used in a number of monolithic IC instrumentation amplifiers. Besides offering excellent matching between the three internal op amps, thin film laser trimmed resistors provide excellent ratio matching and gain accuracy at much lower cost than using discrete op amps and resistor networks,
