Ride the logic thresholds

Roger Griswold, Maxim Integrated Products, Sunnyvale, CA -- EDN, 8/3/2000

Most logic devices have a 5V supply rail. However, you often need to know the upper and lower switching thresholds for a device operating at supply levels other than 5V. In some cases, it is also important to know the variation of these thresholds with supply voltage. A simple test circuit allows you to make these measurements (Figure 1). The basic idea is to create an oscillator by feeding an output signal back to the control input and then monitor the control input with an oscilloscope. Then, you can easily monitor the upper and lower transition points on the scope. To slow the frequency of oscillation and reduce the effects of propagation delay, it's sometimes desirable to add an RC network.

The circuit examines the threshold voltages at the control input of an analog switch, but the scheme applies to any simple logic device. Switching thresholds for the IC depend on the logic-supply voltage you apply to V_L. (The analog-switch channel COM-NO handles voltages of ±15V.) The data sheet specifies IN thresholds for the most common usage (V_L=5V), but the chip accommodates V_L from 4.5V to the positive rail (15V in this case). The circuit characterizes threshold voltages over the full operating range of V_L (Figure 2). At power-up, the switch is open. V+ charges C₁ through R₁ and R₂ until IN reaches its upper threshold, closing the switch. C₁ then discharges to ground (through R₂ and the switch) until IN reaches its lower threshold, thereby opening the switch and repeating the cycle.