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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [. . . ] If a dc voltage is suddenly applied to the inverting input through resistor R1, the op amp ignores the sudden load because the change is also coupled directly to the noninverting input via C1. As C1 charges through R2, the voltage across R2 falls, so the op-amp draws current from the input through R1. This continues as the capacitor charges, and eventually the op-amp has an input and output close to virtual ground (Vcc/2). When C1 is fully charged, resistor R1 limits the current flow, and this appears as a series resistance within the simulated inductor. Real inductors generally have much less resistance than the simulated variety. 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. 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. 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. [. . . ] High-Pass Fliege Filter +Vcc Cin Vin R1 + C1 R2 C2 Vcc/2 +Vcc R3 Vout BAND PASS Gain fixed at 2 R1 controls Q low R1 => low Q high R1 => high Q R1 should be > R/5 R2 = R3 = R C1 = C2 = C R4 = R5, not critical Fo = 1/(2pRC) + R4 R5 Vcc/2 Figure 25. Band-Pass Fliege Filter A Single-Supply Op-Amp Circuit Collection 21 SLOA058 +Vcc Cin C1 + R1 R2 R3 C2 Vcc/2 R4 R6 +Vcc Vout NOTCH Vin R3 = R4 = R5 = R6 = R C1 = C2 = C Fo = 1/(2pRC) R1 = R2 = R*10/2 No control over Q Gain fixed at 1 + R5 Figure 26. Notch Fliege Filter 3. 2. 5 Akerberg-Mossberg Filter This is the easiest of the three-op-amp topologies to use (Figures 27­30). It is easy to change the gain, style of low-pass and high-pass filter, and the Q of band-pass and notch filters. [. . . ]