What are the Basic Differences Between CMOS and TTL Signals?Hardware: Multifunction DAQ (MIO)
What are some of the differences between CMOS and TTL signals and how do they compare?
Characteristics of CMOS logic:
1. Dissipates low power: The power dissipation is dependent on the power supply voltage, frequency, output load, and input rise time. At 1 MHz and 50 pF load, the power dissipation is typically 10 nW per gate.
2. Short propagation delays: Depending on the power supply, the propagation delays are usually around 25 nS to 50 nS.
3. Rise and fall times are controlled: The rise and falls are usually ramps instead of step functions, and they are 20 - 40% longer than the propagation delays.
4. Noise immunity approaches 50% or 45% of the full logic swing.
5. Levels of the logic signal will be essentially equal to the power supplied since the input impedance is so high.
Characteristics of TTL logic:
1. Power dissipation is usually 10 mW per gate.
2. Propagation delays are 10 nS when driving a 15 pF/400 ohm load.
3. Voltage levels range from 0 to Vcc where Vcc is typically 4.75V - 5.25V. Voltage range 0V - 0.8V creates logic level 0. Voltage range 2V - Vcc creates logic level 1.
CMOS compared to TTL:
1. CMOS components are typically more expensive than TTL equivalents. However, CMOS technology is usually less expensive on a system level due to CMOS chips being smaller and requiring less regulation.
2. CMOS circuits do not draw as much power as TTL circuits while at rest. However, CMOS power consumption increases faster with higher clock speeds than TTL does. Lower current draw requires less power supply distribution, therefore causing a simpler and cheaper design.
Resource: National Semiconductor CMOS Logic Databook and National Semiconductor LS/S/TTL Logic Databook
KnowledgeBase 331LET6I: How Do I Interface TTL Signals With CMOS Circuits?
KnowledgeBase 1QJC94NV: 3.3 Volt TTL from Digital Products
Developer Zone Article: High-Speed Digital I/O Logic Families
Report Date: 02/11/1998
Last Updated: 11/01/2013
Document ID: 16A9OC6O