Multism Live Reading

Using the free software Multism Live

Reading the attached Question 11 design the following circuit in Multiism Live:

And answer all questions A to F

Design brief
As stated above, the sensor signal contains frequencies from 0 Hz to 3 kHz. Its maximum open-circuit amplitude is 100 microvolts. Also (not stated above), the sensor has an internal resistance of 500 Ω, and the input resistance of the preamplifier must be 5 kΩ.

Over the frequency range of the signal, the frequency response of the preamplifier should be flat; that is, the gain the should be constant. (In meeting this requirement for a flat frequency response, you may wish to revisit Book 1, Chapter 2, pp. 14 to 32.) As stated above, it is good practice to roll off the gain outside the operating frequency range.

You should model the sensor as an alternating voltage source in series with a 500 Ω resistor to represent the sensor’s internal resistance.

The value of R2 in Figure 3.22 of Book 1 is not critical and in practice is often chosen to be the same as R1.

The maximum signal amplitude the ADC can accommodate is 1 mV. Your preamplifier should therefore provide suitable gain over the entire frequency range of the sensor output.

Constructing your design in Multisim Live
In Multisim Live, for the op amp use the ‘5 Terminal Opamp’ option as it is more realistic than the ‘3 Terminal Opamp’. Note that the ‘5 Terminal Opamp’ needs to be powered by a split DC power supply, as in Figure 4. In Figure 4, 5 V batteries are used but other sensible voltages can be used.

Described image
Multisim Live
Figure 4 Split power supply for the five-terminal op-amp in Multisim Live.
Long description
You should model the sensor in Multisim Live as shown in Figure 5, where the 500 Ω resistor represents the internal resistance of the sensor. The components and arrangement in Figure 5 replace Vs in Book 1 Figure 3.22.

Described image
Multisim Live
Figure 5 Modelling the sensor. The 500 Ω resistor represents its internal resistance.
Long description
Include in your circuit the voltage probes PR1 and PR2 shown in Figure 5. Voltage probe PR1 shows the voltage V1 in Figure 5, and voltage probe PR2 shows the signal voltage fed to the preamplifier. The voltage monitored by PR2 is the voltage Vin in Figure 3.22 of Book 1. In your design you should also include a voltage probe PR3 showing the output from the op-amp. That is, your PR3 should monitor the voltage shown in Book 1 Figure 3.22 as Vout. Because the output impedance of an op-amp is extremely low (typically less than 1 ohm) and the input impedance of an ADC is high (millions of ohms at 0 Hz), any loading on the op-amp’s output by the ADC can be neglected and PR3 therefore shows the signal voltage that is digitised by the ADC.

To get an image of your Multisim Live circuit, in Multisim Live go to the menu icon at the top left (the icon consists of a small three-by-three array of squares). Select ‘Export’, ‘Schematic Image’. For this sort of image, a PNG file is usually better than a JPG, but which you choose is not critical.