Electronic Design

Programmable Schmitt Triggers Are Digitally Controlled

The circuit in Figure 1 is a single-supply version of a programmable Schmitt trigger, or a comparator with hysteresis. A comparator, A1, and a digitally programmable potentiometer (DPP1) are used for the function. The lower (VLL) and upper (VUL) limits of the hysteresis characteristic are a function of the relative setting of the potentiometer's wiper, given as:

VLL = (1 − p) 2.5 V

and

VUL = 2.5 V + (p) 2.5 V

where p is a dimensionless number from 0 to 1, representing the potentiometer wiper's position from one end of the pot (0) to the other end (1).

The characteristic's lower limit can be programmed from 0 to 2.5 V, while the upper limit can be programmed from 2.5 to 5 V. The circuit's transfer characteristic (VO versus VS) illustrates the hysteresis curve, and the lower and upper limits (Fig. 2). These limits are complementary, meaning that their values sum to 5 V. So, they're not in-dependently programmable.

By adding steering diodes D1 and D2, plus a second potentiometer, the lower and upper limits of the characteristic can be independently programmed (Fig. 3). For this circuit:

VLL = (1 − p2) 2.5 V

and

VUL = 2.5 V + (p1) 2.5 V.

The CAT5114 32-tap DPPs let the lower limit be programmed from 0 to 2.5 V in 81-mV increments. The upper limit can be independently programmed from 2.5 to 5 V with the same resolution.

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