0.2 Operational amplifiers and signal conditioning  (Page 4/5)

The resultant signal should be an 800 mV sine wave with minimal offset.

Using the inverting gain circuit shown in Figure 8, amplify the output of the differential amplifier circuitby an additional factor of ten. This can be done with another op-amp and a 10 k ohm resistor and a 30 k ohm resistor. Theresultant output signal should have a mean of zero and amplitude of ~2.4 V. Adjust the potentiometer to remove any offset.

Part 7: low-pass filtering

A low pass filter can be used to attenuate high-frequency noise in an analog signal and to minimize the portion of the signal that will be aliased. Later in this lab, youwill see a demonstration of the damaging effects of aliasing.

7.1 build an active low-pass filter

• Build the first-order filter shown in Figure 9 with R1 = R2 = 10 k ohm and C2 = 0.1 micro F. First order filters are so-called becausetheir dynamics are modeled by first-order differential equations.
• Connect the output of the circuit to Channel 1 on your DAQ system.
• The filter’s time constant is equal to R2C2. Calculate values for the cut-off frequency and the time constant.

7.3 modifying existing vi to measure magnitude ratio

By measuring the magnitude of the input and output of a filter, you can determine the how much the filterattenuates the signal.

• Delete the Amplitude and Frequency constants from the Block Diagram.
• Hold Ctrl as you drag the Tone Measurements icon to make a copy.
• Place a Split Signals icon to the left of the Tone icons.
• Place a Divide function to the right of the Tone icons.
• Create a numeric indicator at the x/y output terminal of the Divide function.
• Rename the numeric indicator Magnitude Ratio. (The Magnitude Ratio represents the output amplitude divided by the inputamplitude.)
• Wire the Block Diagram as shown in Figure 10.
• Save the VI.

7.2 testing a filter

We expect that the filter will allow frequencies below the cut-off frequency to "pass", and willattenuate signals at higher frequencies. The Magnitude Ratio will be recorded as the output magnitude divided by the inputmagnitude.

• Connect the signal going into your filter to Channel 0 of module 1 of the SCXI.
• Connect the signal coming out of your filter to Channel 1 of module 1 of the SCXI.
• Input a 1 V sine wave with zero offset into the low-pass filter circuit.
• Starting with a frequency of about 10 Hz, slowly increase the frequency of the input signal. What happens to the outputsignal?
• Increase frequencies to 2 kHz.
• Using the table below, determine the magnitude ratio at several frequencies.

Table 1: Magnitude Ratio Data for First-Order Low-Pass Filter (Active)

• Using Excel, plot the following two sets of data on a single log-log chart.

• • Calculated magnitude ratio vs. frequency.
  • Measured magnitude ratio vs. frequency

Do the magnitude ratio and phase difference between the input and output behave as you would expect?