These considerations, as well as those regarding rail-to-rail precision, have implications in many circuits, namely instrumentation amplifiers, which will be covered in the next sections THE TWO OP AMP INSTRUMENTATION AMPLIFIER TOPOLOGY There are several circuit topologies for instrumentation amplifier circuits suitable for single-supply applications. The two op amp configuration is often used in cost and space-sensitive applications, where tight matching of input offset voltage, input bias currents, and open- loop gain is important. Also, when compared to other apologies, the two op amp instrumentation amplifier circuit offers the lowest power consumption and low total drift for moderate- gain(G=10) applications. Obviously, it also has the merit of using a single dual op amp io Figure 1. 8 shows the topology of a two op amp instrumentation circuit which uses a 5th gain-getting resistor, RG. This additional gain-setting resistor is optional, and should be used in those applications where a fine gain trim is required Its effect will be included in this analys Circuit resistor values for this topology can be determined from Equations 1.1 through 1.3, where Rl= R4. To maintain low power consumption in single-supply applications, values for R should be no less than 10kohms R1=R4= Eq.1.1 R R2=R3= Eq.12 2R G Eq.1.3 0.06G where G equals the desired circuit gain. Note that in those applications where fine gain trimming is not required, Eq 1.2 reduces to R2=R3= Eq.1.4 A nodal analysis of the topology will illustrate the behavior of the circuit's noda voltages and the amplifier output currents as functions of the applied common mode input voltage(V CM), the applied differential(signal) voltage (VIN, and the output reference voltage (VREF). These expressions are summarized in Equations 1.5 through 1.8, Eq. 1.12, and in Eq 1 13 for positive, input differential voltages. Due to the structure of the topology, expressions for voltages and currents are similar in form and magnitude for negative, input differential voltages
11 These considerations, as well as those regarding rail-to-rail precision, have implications in many circuits, namely instrumentation amplifiers, which will be covered in the next sections. THE TWO OP AMP INSTRUMENTATION AMPLIFIER TOPOLOGY There are several circuit topologies for instrumentation amplifier circuits suitable for single-supply applications. The two op amp configuration is often used in costand space-sensitive applications, where tight matching of input offset voltage, input bias currents, and open-loop gain is important. Also, when compared to other topologies, the two op amp instrumentation amplifier circuit offers the lowest power consumption and low total drift for moderate-gain (G=10) applications. Obviously, it also has the merit of using a single dual op amp IC. Figure 1.8 shows the topology of a two op amp instrumentation circuit which uses a 5th gain-setting resistor, RG. This additional gain-setting resistor is optional, and should be used in those applications where a fine gain trim is required. Its effect will be included in this analysis. Circuit resistor values for this topology can be determined from Equations 1.1 through 1.3, where R1 = R4. To maintain low power consumption in single-supply applications, values for R should be no less than 10kohms: R1 = R4 = R Eq. 1.1 R2 = R3 = R 0.9G –1 Eq. 1.2 RG = 2R 0.06G Eq. 1.3 where G equals the desired circuit gain. Note that in those applications where fine gain trimming is not required, Eq. 1.2 reduces to: R2 = R3 = R G–1 Eq. 1.4 A nodal analysis of the topology will illustrate the behavior of the circuit’s nodal voltages and the amplifier output currents as functions of the applied commonmode input voltage (VCM), the applied differential (signal) voltage (VIN), and the output reference voltage (VREF). These expressions are summarized in Equations 1.5 through 1.8, Eq. 1.12, and in Eq. 1.13 for positive, input differential voltages. Due to the structure of the topology, expressions for voltages and currents are similar in form and magnitude for negative, input differential voltages
From the figure, expressions for the four nodal voltages A, B, C, and Vout as well as the output stage currents of A1 (oA1) and A2 (oA2)have been developed. Note that the direction of the amplifier output currents, IOAI and loA2, is defined to be and loA2 are positive entities, their direction is into the device; thus, their ouput y into the amplifiers output stage. For example, if the nodal analysis shows that lo stages are sinking current. If the analysis shows that they are negative quantities their direction is opposite to that shown; therefore, their output stages are sourcing current Resistors RPi and Rpe at the inputs to the circuit are optional input current limiting resistors used to protect the amplifier input stages against input overvolta 1 kohm to prevent the unwanted effects of additional resistor noise and biag an Although any reasonable value can be used, these resistors should be less tI current-generated offset voltages. For protection against a specific level of overvoltage, the interested reader should consult the section on overvoltage effects on integrated circuits, found in Section 7 of this book THE TWO OP AMP INSTRUMENTATION AMPLIFIER TOPOLOGY IN SINGLE-SUPPLY APPLICATIONS WHERHI BR&阳R Figure 1.8 Using half-circuit concepts and the principle of superposition, the input signal voltage, VIN, on the non-inverting input of al is set to zero. Since the input signal plied to the non-inverting terminal of a2 an expression for the nodal voltage at the inverting terminal of al is given by eq 1.5 Eq.1.5 An expression for the output voltage of Al(node B)shows that it is dependent on all three externally applied voltages(VIN, VCM, and VREF), and is illustrated in Eq 1.6: V
12 From the figure, expressions for the four nodal voltages A, B, C, and VOUT as well as the output stage currents of A1 (IOA1) and A2 (IOA2) have been developed. Note that the direction of the amplifier output currents, IOA1 and IOA2, is defined to be into the amplifier’s output stage. For example, if the nodal analysis shows that IOA1 and IOA2 are positive entities, their direction is into the device; thus, their output stages are sinking current. If the analysis shows that they are negative quantities, their direction is opposite to that shown; therefore, their output stages are sourcing current. Resistors RP1 and RP2 at the inputs to the circuit are optional input current limiting resistors used to protect the amplifier input stages against input overvoltage. Although any reasonable value can be used, these resistors should be less than 1kohm to prevent the unwanted effects of additional resistor noise and bias current-generated offset voltages. For protection against a specific level of overvoltage, the interested reader should consult the section on overvoltage effects on integrated circuits, found in Section 7 of this book. THE TWO OP AMP INSTRUMENTATION AMPLIFIER TOPOLOGY IN SINGLE-SUPPLY APPLICATIONS Figure 1.8 Using half-circuit concepts and the principle of superposition, the input signal voltage, VIN- ,on the non-inverting input of A1 is set to zero. Since the input signal, VIN+ , is applied to the non-inverting terminal of A2, an expression for the nodal voltage at the inverting terminal of A1 is given by Eq. 1.5: VA =V CM Eq. 1.5 An expression for the output voltage of A1 (node B) shows that it is dependent on all three externally applied voltages (VIN, VCM, and VREF), and is illustrated in Eq. 1.6: VB –V IN + R2 RG + VCM 1+ R2 R1 = – VREF æ è ç ö ø ÷ æ è ç ö ø ÷ æ è ç ö ø ( ) ÷ R R 2 1 Eq. 1.6