Noise Margin Definition

Transcription

1 Noise Margin Definition (from JEDEC Dictionary) Noise margin: The maximum voltage amplitude of extraneous signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage level. NOTE The term "input", as used here, refers to logic input terminals, power supply terminals, or ground reference terminals. How do we apply this? Noise Margin 1 -David M. Zar -1/25/2006

2 What is it good for? A B OR Allows digital circuits to propagate signals through any number of elements without error F A B F A B F L L H L H H H L H H H L Based on notes: FUR What does this mean? L =? 5V H =? 2.4 V IH V OH V MH = V OHMIN - V IHMIN V IHMIN 2V 2.0 V OHMIN 2.4V V ILMAX 0.8V V OLMAX 0.4V V OLMAX Max Output V V IL V OL V ML = V ILMAX - V OLMAX V ILMAX Max Input V, That is L for all Noise Margin 2 -David M. Zar -1/25/2006

3 Implicit Agreement or Contract For all (bounded, ignore random/real noise) conditions ( T, V dd / V cc, load, age, other inputs, ), truth tables satisfied if voltages in range. V OH V MH V IH V OL V IL V OH Repeat V ML What about V IHMAX, V ILMIN? Usually just damage but can effect output. Transfer Function V dd V O o V dd V I Vo V OHMIN V OLMAX V ILMAX V IHMIN V dd V I V dd /2 Noise Margin 3 -David M. Zar -1/25/2006

4 Specification Points? We can choose them, in general. Typical electronics textbook (Sedra & Smith) says: Choose specification points at gain = -1. This maximizes V MH + V ML. For symmetric transfer characteristic it is best, but may not be best for non-symmetric. e.g.: Could have V MH = 2V, V ML = -1V, Vsum = 1V. Usually want to maximize the smaller (unless we want something else (impedance?)). 3 1 V<=V MH <=V OLMAX V<=V ML 4 5 V OHMIN + _ A _ + <=V ILMAX B >=V OHMIN >=V IHMIN 2 Noise Margin 4 -David M. Zar -1/25/2006

5 MAX Square V OA V IB B A V ML V MH 4 V ILMAX V MH 3 V OLMAX V ML V IA 2 V IHMIN V OB 1 V OHMIN V<=V MH <=V OLMAX V<=V ML 4 <=V ILMAX 5 V OHMIN + _ A _ + B >=V OHMIN >=V IHMIN 2 Maximum square gives max smallest V M (& Equal); If symmetric it is at unity gain points. We need Gain > 1 and overlapping region (Can get by level shifting). Noise Margin 5 -David M. Zar -1/25/2006

6 Typical NM Definition? Most texts use (wrong): V OH Normal Operating point, not W.C. V OL Gives Larger noise margin def. Does not allow noise at every node. No design rule can be derived. (Here noise = 0 at all but one node? NO!) So what s a useful measure of effectiveness? Noise Margin 6 -David M. Zar -1/25/2006

7 Noise Immunity Our text has different orientation/emphasis: noise immunity. VM/Vswing VM is directed toward simplest gates. Lots of logic, short connections. Text is oriented to smaller number of longer connections where noise is greater and more components (FETS) can be used. Also oriented toward design (choose Vswing) Noise Margin 7 -David M. Zar -1/25/2006

8 Quiz Which is better? A B C VMH = 1V VMH = 0.5 VMH = 0.4 VML = 0.8V VML = 0.5 VML = 0.3 Naturally it depends!: what is max signal swing (VOH-VOL), what is source of noise. Figure of merit is VM/signal-swing (larger is better, 0.5 is best possible) So, A is obviously wins if the noise sources are the same in each case, but that is not always the case.» If the logic that produces the characteristics of A is noisy and causes more noise in the system than the logic that B uses» What if signal swing is 5V for A and 1.2V for C? 1/5 <.4/1.2! Noise Margin 8 -David M. Zar -1/25/2006

9 Other Thoughts Max input High, Min input Low: damage, incorrect operation? Generally expect, monotonic transfer characteristic, positive resistance, not always true Output and input currents must be considered, V M may apply only over some range of loads (essentially infinite for CMOS, 10 for 7400) Non-inverting logic elements V M applies to a system of logic elements, not to a single element, although we refer to the V M of a logic element Most noise is internally generated, not external so V M /signal-swing noise immunity, not absolute value of V M is almost always most important. Large asymmetry in transfer characteristic can be bad, large noise generation with small noise margin AC noise margin is typically greater than DC V IHmin for a set of logic elements? Largest V IHmin in the set! Clever circuits sometimes have clever failure modes TTL(74,74L,74H,74S,74LS,74ALS,74AS)/RTL/DTL/CMOS/ECL/PECL /CML/GTL/GTL+/BTL/HSTL/SSTL/LVDS/ Noise Margin 9 -David M. Zar -1/25/2006

10 Fundamental Issues If noise is less than noise margin, digital signals can propagate through an infinite number of elements and remain error free» Assuming fault-free components» Very good approximation at present, will not hold as voltages decrease and random noise becomes more important, not true at present in some highspeed systems, or in magnetic recording. A major goal of signal-integrity or high-speed design or digital systems engineering is to maintain noise amplitude less than noise margin during critical times (sampling times)» Minimize noise (or: small enough)» Maximize signal (or: large enough)» All with acceptable (or optimum) power dissipation, delay/speed, component count, design time,» Some systems aim for acceptable error rate, rather than error free» At the fundamental level of operation, digital circuits are analog, and must be analyzed as such. Digital is a convenient model or abstraction» Be careful with textbook definitions: spec points, slope= -1 Noise Margin 10 - David M. Zar - 1/25/2006