Such “solid” output voltage levels will be true only for conditions of minimum loading. In practice, it may be found that a gate circuit will tolerate “high” signals of considerably less voltage and “low” signals of considerably greater voltage than those specified here.Ĭonversely, the extremely small output margins shown-guaranteeing output states for “high” and “low” signals to within 0.05 volts of the power supply “rails”-are optimistic. What is shown represents “worst-case” input signal performance, based on manufacturer’s specifications. ![]() The margins for acceptable “high” and “low” signals may be greater than what is shown in the previous illustrations. Shown here are the acceptable “high” and “low” states, for both input and output, of CMOS integrated circuits operating at 10 volts and 15 volts, respectively: Unlike TTL, which is restricted to a power supply voltage of 5 volts, CMOS may be powered by voltages as high as 15 volts (some CMOS circuits as high as 18 volts). Noise Margins at Higher Operating VoltagesĬMOS noise margins widen even further with higher operating voltages. In other words, CMOS circuits can tolerate over twice the amount of superimposed “noise” voltage on their input lines before signal interpretation errors will result. It should be obvious from these figures that CMOS gate circuits have far greater noise margins than TTL: 1.45 volts for CMOS low-level and high-level margins, versus a maximum of 0.7 volts for TTL. “Acceptable” output signal voltages (voltage levels guaranteed by the gate manufacturer over a specified range of load conditions) range from 0 volts to 0.05 volts for a “low” logic state, and 4.95 volts to 5 volts for a “high” logic state: For a CMOS gate operating at a power supply voltage of 5 volts, the acceptable input signal voltages range from 0 volts to 1.5 volts for a “low” logic state, and 3.5 volts to 5 volts for a “high” logic state. ![]() Simply put, the noise margin is the peak amount of spurious or “noise” voltage that may be superimposed on a weak gate output voltage signal before the receiving gate might interpret it wrongly:ĬMOS gate circuits have input and output signal specifications that are quite different from TTL. The difference between the tolerable output and input ranges is called the noise margin of the gate.įor TTL gates, the low-level noise margin is the difference between 0.8 volts and 0.5 volts (0.3 volts), while the high-level noise margin is the difference between 2.7 volts and 2 volts (0.7 volts). Such a signal would be considered uncertain, and no logic gate manufacturer would guarantee how their gate circuit would interpret such a signal.Īs you can see, the tolerable ranges for output signal levels are narrower than for input signal levels, to ensure that any TTL gate outputting a digital signal into the input of another TTL gate will transmit voltages acceptable to the receiving gate. If a voltage signal ranging between 0.8 volts and 2 volts were to be sent into the input of a TTL gate, there would be no certain response from the gate. ![]() “Acceptable” output signal voltages (voltage levels guaranteed by the gate manufacturer over a specified range of load conditions) range from 0 volts to 0.5 volts for a “low” logic state, and 2.7 volts to 5 volts for a “high” logic state: “Acceptable” input signal voltages range from 0 volts to 0.8 volts for a “low” logic state, and 2 volts to 5 volts for a “high” logic state. However, real TTL gate circuits cannot output such perfect voltage levels, and are designed to accept “high” and “low” signals deviating substantially from these ideal values. Ideally, a TTL “high” signal would be 5.00 volts exactly, and a TTL “low” signal 0.00 volts exactly. TTL gates operate on a nominal power supply voltage of 5 volts, +/- 0.25 volts. However, in reality, logic signal voltage levels rarely attain these perfect limits due to stray voltage drops in the transistor circuitry, and so we must understand the signal level limitations of gate circuits as they try to interpret signal voltages lying somewhere between full supply voltage and zero. In a perfect world, all logic circuit signals would exist at these extreme voltage limits, and never deviate from them (i.e., less than full voltage for a “high,” or more than zero voltage for a “low”). Logic gate circuits are designed to input and output only two types of signals: “high” (1) and “low” (0), as represented by a variable voltage: full power supply voltage for a “high” state and zero voltage for a “low” state.
0 Comments
Leave a Reply. |