User manual

19
Measured
Average
Power
Simulated
Average
Power
Measured
Resistance
Simulated
Resistance
Measured
Voltage
Simulated
Voltage
Voltage Source
86.5 mW
141 mW
176.6 Ω
176.85 Ω
3.909 V
5 V
DC signal source
10.8 mW
rms
17.7 mW
rms
176.6 Ω
176.85 Ω
1.381 Vrms
1.768 Vrms
AC signal source
Table 2.3. Observed Measurements Versus Simulated Measurements
Table 2.3 compares the simulated results versus some test measurements. As expected, the measured results match the
expected results. The measured voltage measurements are about 78% of the simulated voltage measurements and the
measured power calculations are about 61% of the simulated power calculations. Do your results match these test
results? In your opinion, what causes this voltage difference?
2.3 Interface Theory
Comparing your observed results with your simulated results, you can see that the two measurements are off by a
significant amount of voltage. If you just consider the case where you used a DC signal source, you provided 5 V to the
circuit but only measured 3.91 V across your circuit. Where did your missing volt go? Kirchhoff’s voltage law states that
the directed sum of the voltages around any closed circuit has to be zero; therefore the missing volt was most likely
dropped across some other element.
From your results, you know that 3.91 V is dropped across your measured 176.6 Ω in the resistor network. Since there is
no other resistive element in your resistor network that you have not accounted for with your measured resistance,
there must be another resistive element in between your resistor network and your signal source. In this case, you are
using the VirtualBench FGEN as your signal source. Figure 2.14 shows a portion of the FGEN specifications document.
Figure 2.14. VirtualBench FGEN Specifications
From Figure 2.13, you can see that the FGEN has several different waveforms, can update at a rate of 125 MS/s, and has
14 bits of resolution. However, for your circuit, the most important specification is the output impedance. The
VirtualBench FGEN has an output impedance of 50 Ω, meaning that any circuit powered by the FGEN would have to add
50 Ω to the total resistance. If you go back to your simulated circuit to model the FGEN correctly, you need to add a 50 Ω
resistor in series with the rest of the resistor network as shown in Figure 2.15.