A method of noise reduction in data acquisition systems from sensors

In this paper, we descibe a noise reduction technique for noise which appears at an instrumentation amplifier input. We use a square-wave signal with frequency f/sub c/=5 kHz modulated with the signal from the sensor (input signal). The modulated signal is applied at the instrumentation amplifier and then the amplified signal is modulated again with the same signal type (square-wave signal with frequency of 5 kHz). The noise and offset voltage which appears due to the amplifier are modulated only once by the final multiplier and translated to the harmonics of f/sub c/ while, by using a low-pass filter, we obtain a clear signal with no noise at the output. Unfortunately, in this process a residual offset voltage appears, but this is almost eliminated by using a bandpass filter with very exact tuning. The filters are implemented with common operational amplifiers using a second order active bandpass filter scheme. The circuit features a gain of 58 dB with a 1 kHz bandwidth and is capable of amplifying extremely small signals.