Abstract:
A noise immunity circuit has a distributed electrical plane to which noise susceptible components are coupled. Also, in a close proximity are noise generating circuits that generate electrical noise onto the distributed electrical plane that affect the noise susceptible components. A coupling means is used to couple the noise susceptible circuits to the distributed electrical plane so that the noise of the distributed electrical plane is common to all points in the noise susceptible circuit. Also, inputs and power to the circuit are coupled to the distributed electrical plane so that the common noise is imposed upon them. Coupling preferably takes place through a variety of capacitors and resistors so that high frequency noise is coupled to the noise susceptible circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to electrical circuits susceptible to noise and, more specifically, to means for eliminating noise effects on susceptible circuits. 
     BACKGROUND OF THE INVENTION 
     Integrated circuits are increasingly using both digital and analog circuits on the same monolithic die. As the integration density and switching speeds of noisy digital circuits continue to increase with technological advancements so do the noise levels and the adverse effects of noise on susceptible circuits. Consequently, the integration of analog or other noise susceptible circuitry with high speed digital logic is becoming increasingly difficult. 
     Switching noise from logic circuits is coupled into noise susceptible circuits through the integrated circuit substrate and power lines. Switching noise can have a serious performance impact on noise susceptible circuits. Noise problems range from analog circuits running out of specification to more severe problems such as erratic circuit operation. 
     Clock distribution systems, the logic circuits they control and output drivers are a major source of the noise generated from integrated circuits. When logic circuits switch state while driving a capacitive load, a rapid capacitor charging current flow generates an undesirable noise disturbance that is coupled to the integrated circuit substrate. The noise is typically coupled either by parasitic capacitive coupling between noisy circuits and the substrate or through direct connection of a power supply of a noisy circuit to the substrate. The load capacitances are undesired, but inherent in any circuit. The problem is multiplied since integrated circuits typically contain hundreds to hundreds of thousands of logic circuit gates that, when switched simultaneously, create extreme voltage transients that may upset other circuits. 
     One method for separating analog and digital circuits is by using a higher resistivity integrated circuit substrate. A higher resistivity substrate helps to electrically partition the integrated circuit substrate into quiet and noisy areas, separated by a relatively high resistive path. One problem with higher resistivity substrates is that noise is never completely separated since noise can still couple to common supplies. The noise may affect the operation of sensitive circuits. Another problem with this technique is that the trend is for high density, lower cost processes that use a low resistivity substrate. A low resistivity substrate does not allow effective separation of digital and analog noise for substrate partitioning. Furthermore, most integrated circuit fabrication companies require that logic grounds connect electrically to the substrate for reliability and robustness thus increasing noise to the substrate. 
     Using a low resistivity integrated circuit substrate also presents noise problems. One technique to reduce digital noise is by isolating the substrate noise from as many analog circuit components as possible by using an N-well. An N-well, however, does not allow total isolation of the digital noise from the analog circuit since some noise will always be capacitively coupled to and through the N-well. Further, some components must lie directly on the substrate and cannot make use of an N-well. Because isolation differs according to component structure, this technique always results in differing amounts of noise coupled to different parts of the analog circuit. The variances will be seen as a differential signal by the circuit. Another method for reducing noise coupled into analog circuits using a low resistivity integrated circuit substrate is robustly connecting the substrate to a good external ground with a low impedance path. One problem with such a method is that extra pins must be provided on the integrated circuit to connect to the external ground. Another problem is that the high frequency components of the switching circuits make it impossible to obtain a low impedance path to external ground through inductive bond wires and package lead frame in a cost effective way and without using many ground pins. Typically, spare pins are either not available or may require increasing the package size to accommodate these extra pins. This method may reduce the effects of noise significantly, but for highly sensitive circuits may not provide adequate noise reduction. 
     Another method for reducing noise effects is using fully differential circuits. Fully differential circuits are good at rejecting noise that is common to their inputs. Several problems exist in the employment of fully differential circuits. One problem is that when laying out a circuit it is hard to match the noise induced on its differential signal lines. For example, a mismatch of just a few fempto-farads of capacitive coupling between differential signal lines to a noise signal can impair the performance. Another problem with differential circuits is that in automotive applications differential circuits are not a typical functional requirement since they are more complex, more area intensive and thus, more costly than single ended circuits. Yet another problem is that the ability to reject common mode noise in differential circuits decreases with increasing noise frequency. 
     Another method for improving noise immunity is to sample when the noise has decayed sufficiently. Typical systems sample data just before a new clock edge and after sufficient quieting from the previous clock edge occurs. The problem with such a method is that for faster clock frequencies, typically more than 4 MHz (considered low by today&#39;s standards), the substrate noise does not have time to sufficiently quiet during sampling and as technology advances clock frequencies will continue to increase. 
     Each of the methods described above reduces noise to a certain extent. Conventional thinking, however, is that separation or filtering of noise are the best methods to reduce the effects of noise. It is evident an improved method for eliminating the effects of noise would solve a long felt need. 
     SUMMARY OF THE INVENTION 
     The object of the invention is to enable analog or other noise susceptible circuits that share a common distributed electrical plane with digital or other noise generating circuits to achieve excellent noise immunity from noise induced on the plane. The present invention is a complete paradigm shift from prior art noise immunity solving methods. 
     The present invention has a distributed electrical plane and a reference electrical portion non-ideally coupled to the distributed electrical plane. Noise generating components are coupled to the distributed electrical plane and induce noise onto said distributed electrical plane. Noise susceptible components are also coupled to the distributed electrical plane. A coupling means for coupling the noise susceptible circuits to the distributed electrical plane more evenly distributes noise to the noise susceptible circuits so that the noise is common and indistinguishable throughout the noise susceptible circuits and the noise susceptible circuits operate with immunity to the noise. 
     By coupling noise from the distributed electrical plane (e.g., substrate of an integrated circuit) into the analog circuitry of an integrated circuit and coupling noise from the substrate into the analog inputs and outputs of the integrated circuit, the analog circuit acts as if no noise is present, since all the noise is common throughout the analog circuit. 
     One advantage of the present invention is substrate noise immunity is superior to other known methods of noise reduction. Another advantage is that noise reduction remains effective even at high noise frequencies where other techniques loose their ability to work and without the need for more costly integrated circuit packaging. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects and advantages of the present invention will be apparent to those skilled in the art upon reading the following description with reference to the accompanying drawings, in which: 
     FIG. 1 is a block schematic of an integrated circuit; 
     FIG. 2 is a schematic of a simple analog summing amplifier; 
     FIG. 3 is a wave form plot of the certain outputs of the schematic of FIG. 2; 
     FIG. 4 is a schematic summing amplifier of FIG. 2 having noise reduction according to the present invention; 
     FIG. 5 is a wave form plot of certain outputs in the schematic of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an integrated circuit  10  is comprised of various digital circuits  12 , output drivers  14  and analog circuits  16 . Preferably integrated circuit  10  is comprised of a single substrate, a clock source  18  and a clock distribution circuit  20  to control the timing of the switching of digital circuits  12 . Various functions may be performed by integrated circuit  10  that involve processing and outputting information. Digital input circuits  22  receive information from external sources for processing by integrated circuit  10 . Output drivers  14  have digital outputs  24  that are used to control various functions outside integrated circuit  10 . Analog circuits  10  have analog inputs  26  for receiving analog signals for processing by the integrated circuit  10 . Analog outputs  28  supply various analog signals outside integrated circuit  10 . 
     Clock source  18  and clock distribution circuit  20  are a major source of interference in integrated circuits and particularly to sensitive analog circuits. The noise generated by clock source  18 , clock distribution circuits  20  and digital circuits  12  are significant since they each drive parasitic capacitive loads with part of the capacitance connecting to the substrate. Another source of noise is that output drivers  14  typically drive significant capacitive loads. Typically interference in the form of current spikes is translated into voltage transients as the current spikes flow through parasitic inductances and resistances within the integrated circuit bond wires and package lead frame used to connect the internal integrated circuit to the external package. 
     The present invention is described with respect to an integrated circuit. However, this technique may also be applied to a printed circuit boards, ceramic hybrids or other electronic assemblies having one or more conductors if one of the layers has some common switching noise imposed upon it. 
     Referring now to FIG. 2, a simple summing amplifier is shown as an example of an analog circuit. Various other analog circuits may be used. Other noise producing circuitry is located proximate to the analog portion causing some adverse effect on the analog circuit. This circuit does not contain the noise reducing characteristics of the present invention. 
     The purpose of the summing amplifier is to buffer and invert an externally applied signal and pass it on to the next stage in the integrated circuit. The summing amplifier has an amplifier M 1  having a gate G, a source S and a drain D. Drain D is connected to a supply voltage V supply  through a resistor R 1 . Gate G is connected to drain D through resistor R 2 . Output of amplifier M 1  is signal RxOut that originates from drain D. Source S is connected to substrate ground  30 . 
     External connections to integrated circuit  10  are represented by inductances L 1 , L 2  and L 3 . The inductances represent the parasitic inductances of integrated circuit package and bond wires. 
     External signal V signal  is connected to gate G through an external resistor RS and through a parasitic inductance L 1 . V signal  is referenced to external ground  32 . Resistor RS helps determine the voltage gain of the buffer and protect integrated circuit  10  from large V signal  values. 
     The integrated circuit has an input power supply with a power supply voltage V supply  Voltage V supply  is connected to drain D through parasitic inductance L 2  and resistor R 1 . 
     Substrate ground  30  is coupled to external ground  32  by parasitic inductance L 3 . A reference point RxIn is located at gate G. Another reference point VddInt is located between resistor R 1  and parasitic inductance L 2 . External ground  32  is a reference portion that may be a ground plane, a node of a circuit or other common electrical connection. 
     Referring now to FIG. 3, a wave form plot is shown of signals VddInt, RxOut, RxIn and V signal . Each of the signals except V signal  has noise imposed upon it from digital switching activity on the integrated circuit as coupled by the substrate. The noise is periodic in nature which is common for clock noise. V signal  does not have the noise imposed upon it since V signal  is referenced to external ground  32 . All other signals are referenced to substrate ground. Consequently, problems arise when processing signals are not commonly referenced. Since it is not practical to effectively have the same voltages on the integrated circuit internal ground as on the external ground due to inductances and noise, once signals enter the integrated circuit, they should be referenced to the substrate that is commonly the integrated circuit internal ground. 
     Referring now to FIG. 4, the summing amplifier circuit is shown with noise coupling according to the present invention. Each of the common components to FIG. 2 have been primed and will not be discussed further. For noise reduction capacitors C sub1 , C sub2 , C sub3  and resistor R 3  have been added. Capacitor C sub1  is connected to the gate G′. Resistor R 3  is connected between resistor R 1 ′ and parasitic inductance L 2 ′. Capacitor C sub2  is connected between R 1 ′ and R 3 . Capacitor C sub3  is connected to output RxOut. Each capacitor C sub1 , C sub2  and C sub3  is also connected to substrate ground  30 . 
     Capacitor C sub1  is connected to gate G′ and coupled to the input signal V′ signal  so that gate G′ will have the noise from substrate reference  30  imposed upon it. External resistor RS′ with C sub1  forms a dual purpose filter that allows the relatively low V signal  frequencies to enter M 1 ′ through resistor R 1 ′ while at the same time coupling high frequency substrate noise to the gate of M 1 ′. C sub2  and R 3  have been added to help couple high frequency substrate noise to the supply voltage V′ supply  before being input into drain D of M 1 ′. R 3  also helps to isolate the substrate noise from the integrated circuit&#39;s supply pin. The best results have been obtained when each input of integrated circuit  10  have some coupling to the substrate. It is also preferred to couple as many circuit nodes and circuit components as convenient to the substrate to achieve the best possible noise distribution. Much of the previously undesired inherent parasitic capacitive coupling that was crippling in the prior art is now advantageous in the present invention. 
     The exact capacitances are not critical. It is, however, preferable to provide more capacitance. The values of the capacitors for this example range from 2.5 picofarads to about 10 picofarads. Each of the capacitances do not need to be the same. 
     Subsequent analog stages to this circuit should also use theses techniques for noise immunity. In many cases the analog signal will be processed into a digital signal. Once digitized there is no further need to utilize the present invention. 
     In cases where it is not appropriate to couple substrate noise directly onto an analog input pin, noise immunity may still be obtained by applying the present invention to subsequent analog stages driven from the input pin. 
     For analog output pins that drive only input pins of the same integrated circuit, i.e., through some external components, coupling noise to the output pins will aid in noise immunity. 
     For analog output pins that drive external circuits, noise can be filtered out externally using conventional low pass filtering since noise is typically a nuisance. 
     Referring now to FIG. 5, wave form plots of VddInt′, RxOut′, RxIn′ and V signal ′ are shown with respect to the substrate ground. The noise present on the wave forms of FIG. 3 have been essentially eliminated by the present invention. 
     As would be evident to one skilled in the art, several modifications of the invention may be made while still being within the scope of the appended claims. For example, the previous examples used a P-substrate where the substrate was the common plane. If the substrate is an N-substrate, the substrate would be the power plane.