The thresholds voltages, and thus 21 7 1 the hysteresis, of the Schmitt trigger described can be set independently.
The circuit is based on opamp 1C1, which has been connected as an inverting Schmitt trigger. The two voltages at which the output of the circuit changes level are preset with P1 (high level = UH) and P2 (low levels = U1) Provided that UH is greater than UL
U1<Ur ->Uo = UH -> Ur = UH ;
U1> Ur -> Uo = UL ->Ur = UL.
These Two expressions are shown as a graph in Fig. 2. From this, it is clear that when the output is high, it will change to low only if the input has risen above UH, and will then change back to high again only if the input voltage has dropped below UL.
The changing state of the output results in the changing of the reference voltage at the non-inverting input of IC1. In the first instance, this is caused by the weak, but fast, positive feedback via R6 and R7. The hysteresis caused by this network amounts to only 20 or 30 mV, but that is more than enough to give the opamp good switching performance.
Once the opamp has changed state. Ur changes for the second time, because 1C2a-1C2d alter the junction R6-C7, from UH to UL or vice versa.
Capacitor C7 in conjunction with the on resistance of the analog switches slows the transfer process: the reference voltage changes smoothly from one value to the other in about 250 ns. This assumes that the speed at which the input signal changes is not too high. To enable signals with steep edges to be processed, there is a low-pass filter at the input (R5—C8).
The threshold voltages are set with the aid of a millivoltmeter connected successively to UH and UL. Presets P1 and P2 should preferably be multiturn types to enable the levels to be set accurately to within a millivolt.
The trigger may be used with unipolar and bipolar input signals. If only positive inputs need to be processed, the power Ipo supply for the trigger may be that asymmetrical. To in effect, the negative supply line is linked to ‘0’ and the supply (max 16 V) connected to + and 0. With a supply voltage of 16 V., the trigger draws a current of ≤30 mA.
The switching performance is tested with a triangular voltage (0-5 V) at the input. With that input and the thresholds set to 2 V and 4 V, the trigger switched accurately at frequencies up to 5 kHz.