User manual TEXAS INSTRUMENTS SLOA058

[. . . ] Application Report SLOA058­ November 2000 A Single-Supply Op-Amp Circuit Collection Bruce Carter Op-Amp Applications, High Performance Linear Products One of the biggest problems for designers of op-amp circuitry arises when the circuit must be operated from a single supply, rather than ±15 V. This application note provides working circuit examples. 1 2 3 Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. 1 Split Supply vs Single Supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. 2 Virtual Ground. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] There are some limitations of a simulated inductor: · · · One end of the inductor is connected to virtual ground. The simulated inductor cannot be made with high Q, due to the series resistor R1. It does not have the same energy storage as a real inductor. The collapse of the magnetic field in a real inductor causes large voltage spikes of opposite polarity. The simulated inductor is limited to the voltage swing of the op amp, so the flyback pulse is limited to the voltage swing. 2. 6 Instrumentation Amplifiers Instrumentation amplifiers are used whenever dc gain is needed on a low-level signal that would be loaded by conventional differential-amplifier topologies. Instrumentation amplifiers take advantage of the high input impedance of noninverting op-amp inputs. The basic instrumentation amplifier topology is shown in Figure 10. 10 A Single-Supply Op-Amp Circuit Collection SLOA058 +Vcc Vin- + R5 R1 R2 +Vcc ASSUMES Vin- AND Vin+ REFERENCED TO Vcc/2 R1 = R3 (matched) R2 = R4 (matched) R5 = R6 Gain = R2/R1 (1 + 2R5/R7) R7 + +Vcc R6 Vin+ + R4 R3 Vout Vcc/2 Figure 10. Basic Instrumentation-Amplifier Circuit This circuit, and the other instrumentation amplifier topologies presented here, assume that the inputs are already referenced to half-supply. This is the case with strain gauges that are operated from Vcc. The basic disadvantage of this circuit is that it requires matched resistors; otherwise, it would suffer from poor CMRR (see for example, Op Amps for Everyone[3]). The circuit in Figure 10 can be simplified by eliminating three resistors, as shown in Figure 11. +Vcc Vin- + - R1 R2 +Vcc ASSUMES Vin- AND Vin+ REFERENCED TO Vcc/2 R1 = R3 (matched) R2 = R4 (matched) Gain = R2/R1 + +Vcc Vout Vin+ + R3 R4 Vcc/2 Figure 11. Modified Instrumentation-Amplifier Circuit A Single-Supply Op-Amp Circuit Collection 11 SLOA058 Here, the gain is easier to calculate, but a disadvantage is that now two resistors must be changed instead of one, and they must be matched resistors. Another disadvantage is that the first stage(s) cannot be used for gain. An instrumentation amplifier can also be made from two op amps; this is shown in Figure 12. R1 R2 +Vcc R3 R4 +Vcc ASSUMES Vin- AND Vin+ REFERENCED TO Vcc/2 R1 = R4 (matched) R2 = R3 (matched) Gain = 1 + R1/R2 Vcc/2 + + Vout Vin - Vin+ Figure 12. Instrumentation Circuit With Only Two Op Amps However, this topology is not recommended because the first op amp is operated at less than unity gain, so it may be unstable. Furthermore, the signal from Vin- has more propagation delay than Vin+. 3 Filter Circuits This section is devoted to op-amp active filters. In many cases, it is necessary to block dc voltage from the virtual ground of the op-amp stage by adding a capacitor to the input of the circuit. This capacitor forms a high-pass filter with the input so, in a sense, all these circuits have a high-pass characteristic. The designer must insure that the input capacitor is at least 100 times the value of the other capacitors in the circuit, so that the high-pass characteristic does not come into play at the frequencies of interest in the circuit. For filter circuits with gain, 1000 times might be better. If the input voltage already contains a Vcc/2 offset, the capacitor can be omitted. These circuits will have a half-supply dc offset at their output. [. . . ] Akerberg-Mossberg Low-Pass Filter 22 A Single-Supply Op-Amp Circuit Collection SLOA058 R5 HIGH PASS R2=R3=R4=R5=R C2=C3=C Fo=1/(2pRC) R6 = R/2 Butterworth R6 > R/2 Chebyshev R6 < R/2 Bessel Unity Gain: C1=C, R1=R Other Gain: R1/R AND C1/C Vcc/2 Vin C2 Vcc/2 +Vcc R2 R1 R6 + +Vcc Vcc/2 R3 R4 + C1 Vcc/2 + Vout +Vcc C3 Figure 28. Akerberg-Mossberg High-Pass Filter BAND PASS R2 = R3 = R4 = R5 = R C1 = C2 = C Fo = 1/(2pRC) Unity Gain: R1 = R6 Other Gain: ­ R6/R1 R1, R6 also control Q low values, low Q high values, high Q Vcc/2 Cin Vin R1 C1 R5 +Vcc R2 R6 + +Vcc Vcc/2 R3 +Vcc C2 + R4 Vcc/2 + Vout Figure 29. Akerberg-Mossberg Band-Pass Filter A Single-Supply Op-Amp Circuit Collection 23 SLOA058 NOTCH R1=R2=R3=R4=R5=R6=R C1 = C2 = C3 = C Fo = 1/(2 RC) R/2 < R7 < 2 x R R7 controls Q low value, low Q high value, high Q C1 +Vcc Vcc/2 R3 Vcc/2 +Vcc R2 R6 R7 + +Vcc C2 R5 + R1 Vcc/2 Cin Vin C3 R4 Vcc/2 + Vout Figure 30. Akerberg-Mossberg Notch Filter 3. 2. 6 BiQuad Biquad is a well know topology (Figure 31). [. . . ]