Categories
Ask Engineering

Low-pass (band-pass) filter for op amp circuit

I have a circuit that amplifies a one-shot inductive trigger pulse of about 1.3Khz (@780 micro sec period) from about 10-200mV to about 3-4V. Between C1 and R3 there is a 0.13uH inductor coil with 11 ohms resistance that is being triggered externally (magnetically) and eddy currents are created as a result. I did not know what symbol I should use for the inductor coil which is actually that IG1 sine-wave source in my circuit. So there is no ground there, just a coil between C1 and R3.

I am using the op amp as a non-inverting amp. The inverting amp (using the negative input of the amp) was too unstable. It basically acted like a comparator and with every tiny imbalance between the + and – it fired the Output. The circuit is single supply 3.3V, and only the op amp’s power pin is powered.

It works good, but seems to have problems with weaker signals. The oscope shows either a lower amplitude first square wave of the pulse-period, or a lower amplitude second square wave. From the 3-4V it goes to 1-2V. Weaker, not only means lower mV amplitude, but it seems that at frequencies lower/higher away the signal is being cut off…by the circuit’s inherent low/high-pass (band-pass)?

I used Texas Instrument’s online program to design an op amp circuit with the correct band-pass I need. It’s 500Hz-10Khz (a range of about 100 micro sec – 2000 mico sec periods). I used a single stage op amp, for now.

But, I hope to see were the problem is with my current circuit before I make this other one suggested by TI. The TI circuit is designed to cut-off at about -40DB by default as part of the TI design. Is that minimum recommended cut-off for band-pass filters?

Here is the TI-designed circuit:

And here is the one I built that seems to have intermittent cut-off(?) issues:

This is the oscope when reading a strong/good signal:

Reading a weaker signal. Seem like the first wave is spiked instead of being more like a square:

Reading a weaker signal. Seem like the first wave is spiked instead of being more like a square:

Leave a Reply

Your email address will not be published. Required fields are marked *