108.3 Summary of Common Instruments and Their Lumped-Parameter Models ent Description, Model, Manufacturer Input Impedance, Zi Output Impedance, Z Function generator, FG501A, Tektronix 50 Multimeter, DM501A. Tektronix 10 MQ2(Volts mode) Oscilloscope, 54601A, Hewlett Packard I MQ213 PF Output Impedance Series Impedances Input Impedance SOURCE MODEL FIGURE 108.4 Simplified output and input models for instrument elements. Output Impedan Output Impedance -Zoo Zi SOURCE MODEL LOAD MODEL P AMP INPUT )P AMP OUTPUT IOX PROBE LOAD MODEL FIGURE 108.5 Model of representative instrumentation system. Each variable would be substituted as required. For example, Vs= 1.0sin2r1000t for a 0.707 Vrms, I kHz sine wave; Zo= 50 ]2 for the FG501A; Zi= 1 kQ2 for an op amp configured as an inverting amplifier with Ri= 1 kQ and gain of 10; AcVi=-100sin2r1000t; Zo= 1 Q2 for low current utput; Zi=9 MQ2lI1 4 PF for a compensated 10X probe; and with Zi= 1 MQ2ll13 PF for the input model of the HP54601A oscilloscope. (op amp)circuit(corresponds to the device under test in Fig. 1083)that was driven by a function generator for the source, and that measured the response with an oscilloscope connected to the output of the op amp using a 10X probe In this application, the impedance of the interconnect between the source and op amp can be neglected since the frequencies are low and the input impedance of the op amp is much greater than that of the cable. The circuit model of the compensated 10x probe contains a very high series impedance (9 MQ2lI1 14 pF)relative to the oscilloscope(1 MQ2 l13 pF)so it cannot be ignored. The models can be used to determine the frequency response of the complete system which describes the magnitude and phase response of the system to sinusoidal, steady-state inputs. This can reveal the contribution of each element to the overall response and helps indicate which elements produce the dominant response The graphical results of the frequency response analysis is termed a Bode plot. If each of n elements has an individual transfer function, H Go), i=1 to N, then a composite transfer function can be found for the total ystem, Tio), which is generally not the simple product of each transfer function, H, Go).H, Go).... HGo) due to loading effects between elements. The use of a circuit simulation program such as PSpice(MicroSim Corp )simplifies the investigation into instrument behavior. A library of subcircuit models can be developed for each instrumentation and interconnect element to support measurement system loading effects analy For example, a PSpice subcircuit definition for the HP54601A oscilloscope is SUBCKT HP54601A 1 2 Cin 1 2 13p in 1 2 1MEG ENDS e 2000 by CRC Press LLC© 2000 by CRC Press LLC (op amp) circuit (corresponds to the device under test in Fig. 108.3) that was driven by a function generator for the source, and that measured the response with an oscilloscope connected to the output of the op amp using a 10X probe. In this application, the impedance of the interconnect between the source and op amp can be neglected since the frequencies are low and the input impedance of the op amp is much greater than that of the cable. The circuit model of the compensated 10X probe contains a very high series impedance (9 MW||1.14 pF) relative to the oscilloscope (1 MW||13 pF) so it cannot be ignored. The models can be used to determine the frequency response of the complete system which describes the magnitude and phase response of the system to sinusoidal, steady-state inputs. This can reveal the contribution of each element to the overall response and helps indicate which elements produce the dominant response. The graphical results of the frequency response analysis is termed a Bode plot. If each of N elements has an individual transfer function, Hi (jw), i = 1 to N, then a composite transfer function can be found for the total system, T(jw), which is generally not the simple product of each transfer function, H1(jw)·H2(jw)·…·HN(jw) due to loading effects between elements. The use of a circuit simulation program such as PSpice (MicroSim Corp.) simplifies the investigation into instrument behavior. A library of subcircuit models can be developed for each instrumentation and interconnect element to support measurement system loading effects analysis. For example, a PSpice subcircuit definition for the HP54601A oscilloscope is: .SUBCKT HP54601A 1 2 Cin 1 2 13p Rin 1 2 1MEG .ENDS TABLE 108.3 Summary of Common Instruments and Their Lumped-Parameter Models Instrument Description, Model, Manufacturer Input Impedance, Zi Output Impedance, Zo Function generator, FG501A, Tektronix 50 W Multimeter, DM501A, Tektronix 10 MW (Volts mode) Oscilloscope, 54601A, Hewlett Packard 1 MW||13 pF FIGURE 108.4 Simplified output and input models for instrument elements. FIGURE 108.5 Model of representative instrumentation system. Each variable would be substituted as required. For example, Vs = 1.0sin2p1000t for a 0.707 Vrms, 1 kHz sine wave; Zo = 50 W for the FG501A; Zi = 1 kW for an op amp configured as an inverting amplifier with Ri = 1 kW; and gain of 10; AcVi = -10.0sin2p1000t; Zo = 1 W for low current output; Zi = 9 MW||1.4 pF for a compensated 10X probe; and with Zi = 1 MW||13 pF for the input model of the HP54601A oscilloscope