Adaptive Schmitt trigger tames unruly signals

Adaptive Schmitt trigger tames unruly signals

Sometimes we are faced with the need to extract information from very noisy signals, for example, biomedical measurements. I needed to generate a square wave signal from the respiratory sensor, which differ greatly in amplitude and frequency, and, in addition, with a non-baseline. Presented here adaptive «super-Schmitt» with lots of customizable options perfect job of cleaning input.

Component values ​​shown in scheme selected for input ranges from 50 mV to 1 V peak-to-peak at a frequency of 0.07 Hz to 2 Hz.

U1 and U2 track the positive and negative peaks of the input signal, but, in contrast to conventional detectors are input to the resistors R1 and R2, providing for suppression of pulse interference level depending on the signal characteristics. These resistors R16, together with the rate of decline is determined by the output voltage. Fixed detectors and buffered chip U3 and U4 amplitude pulses are used to control the width of the loop gisterezica terminal of comparator U6 and the formation of its reference level switching.

Adjusting the hysteresis is operational transimpedance amplifier (OTU) U7, as used here is the chip LM3080, which today can be replaced by more modern instruments LM13700. The output current of the amplifier is actually equal to the product of the differential input voltage to a unidirectional bias current (input, indicated by the arrow). In this case, the saturation of the input (which is already at a voltage of 0.7 V) OTU behaves rather like a key switch polarity showing the bias current to the output current flowing -vtekayuschy or — depending on the sign of the difference between the input voltages. The voltage drop across R13 on the output current is added to the input voltage, or subtracted from, forming thus a hysteresis loop.

U5 acts as a differential current-voltage converter to control the input offset OTU, internally connected to the negative supply rail.

Finally, the output of comparator U6 compares the input signal is increased or decreased by the amount of hysteresis, or voltage in the middle of the LPF between the peaks of the signal (jumper in position «A»), either directly with a signal passed through a low pass filter (jumper in position » AT»). Choose the location depends on the characteristics of your signal. In my case it worked version of «A».

Trimmer resistor R4 set the initial width of the hysteresis loop and compensates for any offset of the amplifier. Diode D6 matches the input offset U7 with a range of output voltages used in the scheme of operational amplifiers. For other operational amplifiers and diode OTU can be excluded from the scheme. It is assumed that the power supply circuit are stable and do not have ripple.

The astute reader will ask why I did not do the obvious, and instead used the OTU circuit voltage-controlled current source in the U5. Alas, after 30 years, and I myself can not answer this question. Maybe that had some reason.

Can this circuit be something useful today? I will argue that, yes, despite the fact that with all the problems it can handle tasks perfectly microcontroller. Pick and choose.

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