Question

Filter A beam vibrates at 100 Hz. The vibrations are measured with a strain gage and Wheatstone bridge. The output voltage of the sensor is Vout = 2V s€, with Vs = 5.00 V the supply voltage and € = 8L/L the strain. The strain is in the range of +5 x 10-4. In addition, you know that: there is noise on the input: fnoise = 40,000Hz with amplitude 0.100 mV. The output is sent to your NI USB-6008 DAQ board, which has a 12bit A/D converter with a range of £ 5V. All op-amps have a GBP (non-inverting) of 1.0 MHz. Propose a scheme to filter the noise out of the signal. Make sure your circuit enables you to take full advantage of the ADC.

Answer 1

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Note that the desired frequency is 100Hz and the noise frequency is 40KHz. Thus, there is a huge bandgap between these two figures, resulting in a comparitively easy filter design. The filter to be designed is an active low pass filter that would pass in frequencies of 100Hz, and cut out frequencies of 40KHz. Let the cut off frequency of this filter be 5KHz. The filter schematic is shown below:

Low Pass Filter Stage R3 Non inverting amplifier with GBP of 1MHz. low frequencies high frequencies Vin Vout ER1

Since the cut-off is 5KHz, and the GBP is 1Mhz, the max gain of the op amp could be 1M/5K = 200. Since the input is plus/minus 5 x 10-4 and the output is fed to an ADC with full scale voltage at 5V, the gain can be set to the max value of 200. Thus, we have

$1+\frac{R_2}{R_1} = 200$

Choose R2 = 200K and R1 = 1K.

Also

$\frac{1}{2\pi R_{3}C_1} = 5000$

Choose R3 = 1K and C1 = 0.3nF

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