I tried to build EMG sensor based on LT1167. I found typical application (figure 8) and try simulate it in LTSPICE.
AC Analysis looks ok but something weird happen in trans.
After first filter (Highpass - red wave) it looks ok but after second filter it gives me square wave. Why this is happened? Is suppose to just gain in this frequency. Maybe I don't understand what going on in circuit.
You may be seeing the output stage gain causing the output saturating and running into the rails.
There is a large gain (1+ 10k/100 = 101 V/V). I cannot see your attached vertical scale. The point is if the signal is large enough after the high pass filter (C1, R4), U2 (running off of +/-3V supplies) will limit the output swing.
Can you reduce the input signal, or the gain (reduce R5) to see if the problem goes away?
Amplitude was 0.1 V when the problem appeared. I changed to 0.001 and i get smooth sin at 1V. What about real circuit? Is it actually appeared square wave if gain will get signal higher than 3V?
Looks like reducing the input signal amplitude allows you to resolve the square wave at output problem.
In the real circuit you'll also run into voltage swing limitations if you exceed the part's voltage swing / power supply limitation and often times yes, it does resemble a square wave.
Thank you for resolve this issue