Logic Family Noise Margin



 Slew Rate Definition and waveform
Digital Logic Slew Rate and Switching Diagram

The graphic above indicates the the difference between VOH and VOL, in general.
The graph also provides an equation for determining IC slew rate and gives an indication of the logic speed [using the voltages below].
The table below provides data for Noise Margin for each of the logic families listed.





Noise Margin Calculation
Logic Noise Margin is the difference between what the driver IC outputs as a valid logic voltage and what the receiver IC expects to see as a valid logic voltage. There are two different types of noise margin, one for a logic high value [1] and one for a logic low value [0]. For a valid logic high, the worst case noise margin for the circuit is the minimum high level voltage which may be output from the driver; minus, the minimum high level voltage which may be seen at the receiver IC. For a valid logic low, the worst case noise margin for the circuit is the maximum low level voltage which may be output from the driver; minus, the maximum low level voltage which may be seen at the receiver IC. The equations for noise margins are provided below, use the minimum of maximum numbers as described above.

Noise Margin Output high = VOH [driving device] - VIH [receiving device]
Noise Margin Output low = VIL [receiving device] - VOL [driving device]
The higher the numbers the better, with negative numbers indicating in-operability [no Noise Margin].
Use Minimum numbers for output High, and maximum numbers for Output Low to calculate Noise Margin.

Noise Margin
-- VOH VIH Margin VIL VOL Margin
TTL [5volt] 2.4v 2.0v 400mV 0.8v 0.5v 300mV
FCT [5volt] 2.5v 2.0v 500mV 0.8v 0.5v 300mV
BTL [5 volt] 2.1v 1.62v 480mV 1.47v 1.1v 370mV
GTL [5 volt] 1.5v 1.05v 450mV 0.95v 0.55v 400mV
CMOS [5 volt] 4.9v 3.85v 1050mV 1.35v 0.1 1340mV
LVTTL [3volt] 2.4v 2.0v 400mV 0.8v 0.4v 400mV
LVCMOS [3 volt] 2.8v 2.0v 800mV 0.8v 0.2v 600mV
CMOS [2.5v] 2.0v 1.7v 300mV 0.7v 0.4v 300mV
CMOS [1.8v] 1.35v 1.1v 250mV 0.66v 0.45v 210mV

The Noise Margins listed in the table are IC to IC within the same logic family. If you mix logic families the Noise Margin will need to be re-calculated using the noise margin equation listed above. The table also shows that the noise margin does not really get smaller as the voltage is reduced or as the logic family changes. Also note that the noise margin in CMOS is higher than the noise margin in TTL for 5 volt logic levels.

So ..... Why do I care about noise margin? How does noise margin effect my design? What changes effect noise margin?

Digital logic devices switch between two [and only two] states, high [1] and low [0]. The higher the noise margin, the greater the difference between what is considered a valid high or a valid low ~ with out going into an undefined region [a voltage not considered high or low]. So a valid high [for a particular digital logic family] may be between 2.4 volts and 2.0 volts. For one IC to communicate with another IC, a logic high may reside between 2.4 volts and 2.0 volts. Producing a maximum valid voltage or noise margin of 400mV. If you have ground bounce issues, power voltage droop issues, trace reflection issues, trace coupling issues or another problem which will reduce the input receiver IC voltage as a posed to what the driver IC outputs will reduce the noise margin ~ which is why a higher noise margin is better. If a design problem results in a negative noise margin then the receiver will see neither a valid logic high or low, so what does the receiver see? I don't know, what ever it wants ~ which means it's output will switch to what ever it has to [an invalid level].





Terms -

VCC: The voltage applied to the power pin(s). In most cases the voltage the device needs to operate at.
VIH: [Voltage Input High] The minimum positive voltage applied to the input which will be accepted by the device as a logic high.
VIL: [Voltage Input Low] The maximum positive voltage applied to the input which will be accepted by the device as a logic low.
VOL: [Voltage Output Low] The maximum positive voltage from an output which the device considers will be accepted as the maximum positive low level.
VOH: [Voltage Output High] The maximum positive voltage from an output which the device considers will be accepted as the minimum positive high level.
VT: [Threshold Voltage] The voltage applied to a device which is "transition-Operated", which cause the device to switch. May also be listed as a '+' or '-' value.


A graph for Standard voltage devices resides on the Logic Voltage Threshold page.
An additional chart of Low Voltage [LV] logic threshold levels is provided on the Low Voltage Threshold Interface Level page.
Another chart of Interface bus threshold levels is provided on the Interface Threshold Voltage Level page.


Description of TTL, ECL and CMOS Glue Logic Families

Related pages on this site:
Standard Logic Voltage Thresholds Interface Logic Thresholds Glue Logic Speed x Power Chart How to Termination Traces Ground/Power Planes

Back to the Logic Design Page

Definition for logic family terms:
TTL: Transistor Transistor Logic
ECL: Emitter Coupled Logic
CMOS: Complementary Metal-Oxide Semiconductor
BICMOS: Bipolar Complementary Metal Oxide Semiconductor
IC: Integrated Circuit




ECL Noise Margin;
Noise margin for ECL components is not covered here, but there are some general comments.
The noise margin changes depending on what ECL family is driving what ECL family [mixing types lowers the margin]
The noise margin for 10K ECL driving 100K is much better than 100K ECL driving 10K ECL.
In addition to changes due to interfacing, noise margins will also change based on operating temperature.
In fact the noise margin might increase or decrease over 100 mV over the operating temperature range.


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Modified 1/17/12
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