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Schmitt Inverter

Introduction

The Schmitt Inverter is a digital device that acts like a NOT gate - a logic 1 at the input results in a logic 0 at the output and vice versa. However, the schmitt Inverter behaves differently to a simple NOT gate because the output changes state very cleanly - it is either logic 0 or logic 1 and never anything else - and the input voltage needed to make the output change state is different depending on whether the input voltage is rising or falling. These features make the schmitt inverter an ideal interface between analogue circuits (such as potential dividers) and digital circuits (such as counters).

All of the standard logic gates are available as Schmitt input logic gates - the output responds to changes in the input voltage differently depending on whether the input voltage is rising or falling.

Schmitt trigger circuits - very much like Schmitt Inverters - can also be made from Op-Amps.

Overview

SchmittThe Schmitt Inverter often comes as 14 pin IC with six inverters on one chip. The inverters can be used completely independently of each other. The hex Schmitt Inverter has the same pin layout as a hex NOT gate and can be used as a direct replacement, especially when the inverter has an analogue or slowly changing input.


SchmittA Schmitt Inverter acts like a NOT gate. The transfer characteristics show that when Vin is LOW (zero), Vout is HIGH and when Vin is HIGH (logic 1), Vout is LOW. Vout changes from HIGH to LOW when Vin is approximately 3 Volts (for a CMOS IC using a 5 V power supply). Vout changes from LOW to HIGH when Vin is approximately 2 Volts.


Schmitt Inverter compared to NOT gate

It is important to understand the difference between a NOT gate or simple Inverter and the Schmitt Inverter.

The transfer characteristics for a typical NOT gate show that there is a region, shaded, where the output voltages changes slowly from one logic level to the other.

Section A: Vin is LOW enough that Vout is definitely Logic 1, HIGH

Section B: Vin is between Logic 1 and Logic 0 and Vout is neither HIGH nor LOW but in an in-between state. In this region, circuits connected to the Inverter could interpret Vout as HIGH or LOW leading to unreliable and unpredictable system behaviour

Section C: Vin is HIGH enough that Vout is definitely Logic 0, LOW

If the input voltage to a logic circuit, not just an inverter circuit, is either analogue or changes slowly from one logic state to another, Schmitt input gates should be used to avoid errors and unexpected system behaviour.


Dealing with slowly changing signals

Consider a system that takes a slowly rising input voltage and at some particular value (the threshold voltage) the system output goes from Logic 1 to Logic 0 and remains at Logic 0. The system might be a comparator, a logic circuit or a simple NOT gate. An application of this system might be a trigger for a monostable that opens the curtains when the slowly rising light level reaches a certain brightness in the morning. The light level, and hence the input voltage, is generally increasing but fluctuates, maybe due to clouds or shadows.


Slowly changing inputThe Input voltage reaches the threshold voltage and the Output goes from Logic 1 to Logic 0 as required

The Input now falls back below the threshold voltage and the Output returns to Logic 1 for a short time before going back to Logic 0. This is not what is required. Instead of a single falling edge Output signal, there are now four falling edges on the example shown


Consider a similar system that takes a slowly rising Input voltage and produces a falling edge when a certain threshold voltage is reached. This time the system uses a Schmitt Inverter.

Slowly changing inputThe Input voltage reaches the upper threshold voltage and the Output falls from HIGH to LOW. A single clean falling edge is produced as required

As the Input changes and falls again, it does not reach the lower threshold voltage. The Output remains LOW and no further falling edges are produced. The system works as expected producing a single clear transition in the Output voltage from HIGH to LOW the first time the Input signal becomes high enough. Subsequent (small) changes in the Input voltage have no effect