Abstract:
Techniques are disclosed for permitting low power operation of a signal processing circuit, such as a mixed signal processing circuit, by operating devices of the digital signal processing side at an energy-delay minimum. To permit this to occur, the negative logic supply rail of the digital signal processing circuit is operated at a negative potential. This negative potential is generated using a charge pump on an integrated circuit chip which can be also used to create a negative substrate potential. A positive logic supply rail can be generated using a DC to DC converter or voltage regulator. The potential of the positive logic supply rail can be negative, as long at it is more positive than the potential of the negative logic supply rail.

Description:
FIELD OF THE INVENTION 
     The invention relates to mixed signal processing and more particularly to an analog to digital converter having a negative bias generator providing a negative logic supply rail for powering digital processing circuitry. 
     BACKGROUND OF THE INVENTION 
     Mixed signal processing techniques are known in which an analog signal is converted to digital, processed in digital and then the output is provided in either analog or digital form. Techniques of this nature find frequent application in such diverse fields as audio circuits and industrial instrumentation. 
     Thermocouples are a type of sensor used in industrial instrumentation. They have two dissimilar metals, welded together, producing almost a dead ohmic short but with a strange, few millivolt temperature-dependent series potential. Thermocouples are sometimes placed in ovens with grounded inside surfaces. If one were to insulate the thermocouple from the inside of the oven, one would slow down its response to temperature transients. If one were required to bias the thermocouple up to a voltage, such as 1.5V, to allow analog signal circuitry such as analog to digital converters operating between 0V and 3V enough headroom to work accurately, and if one does not insulate the thermocouple, there would be a risk of sparks occurring from accidental contact between the thermocouple and the wall of the oven. 
     One would like to connect one side of the thermocouple to the same potential as the inside surface of the oven. This can be done by operating an ADC between a positive and negative potential (e.g. between −1.5V and +1.5V) and allowing the thermocouple to operate at zero V, the same potential as the grounded oven wall. However, one does not wish to buy an extra power supply in order to provide both +1.5V and −1.5V to an integrated circuit processing thermocouple signals. A single 3V supply is much cheaper. 
     Accordingly, it would be desirable if the problems associated with the prior art techniques could be overcome. 
     SUMMARY OF THE INVENTION 
     The problems of the prior art are overcome, in one embodiment of the invention by providing a negative bias generator to which analog, digital or both analog and digital processing may be referenced. 
     In another embodiment of the invention, the power consumed by digital processing circuitry in a mixed signal application is minimized by allowing the digital circuitry to operate referenced to a negative logic supply rail (for P substrate technologies) so that devices implementing the digital circuitry can operate at or near an energy or energy-delay minimum. 
     The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a thermocouple arrangement for measuring temperature. 
     FIG. 2 is a graph indicating the amount of noise induced by air turbulence associated with a thermocouple. 
     FIG. 3A is a block diagram showing a thermocouple arrangement in an oven enclosure. 
     FIG. 3B is a block diagram showing a thermocouple arrangement in an oven enclosure in which the thermocouple is biased to operate above ground potential. 
     FIG. 4 is a graph showing energy-delay relationships when scaling supply voltage to semiconductor circuits. 
     FIG. 5A is a graph illustrating a body effect in a MOS transistor. 
     FIG. 5B is a schematic diagram illustrating problems caused by body effect. 
     FIG. 6 is a block diagram of a thermocouple measuring arrangement in accordance with one aspect of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic diagram of a thermocouple arrangement for measuring temperature. In this arrangement, a pair of thermocouples  100 A and  100 B are connected in series. Thermocouple  100 A is a measuring thermocouple and thermocouple  100 B is a reference thermocouple. The reference thermocouple may be maintained at a reference temperature in an oven. In the arrangement shown in FIG. 1, the thermocouples  100 A and  100 B are connected to an integrated circuit chip  150  that, in this example, contains an analog to digital converter. The connection is made on an isothermal block  120 . As shown in FIG. 1, if the leads coming from the thermocouples  100 A and  100 B are made of iron, the connection on the isothermal block  120  may be to copper leads going to the ADC chip  150 . The connections shown at  130 A and  130 B, by virtue of being between dissimilar metals, introduce parasitic thermocouple voltages. Similarly, the coupling of copper wire to the ADC chip input pins introduce parasitic thermocouples  140 A and  140 B at the input to the ADC chip  150 . However, these parasitic thermocouples introduce substantially identical voltages that occur in common mode and can therefore be eliminated by input circuitry with high-common mode rejection. 
     Each of the thermocouples, both intended and parasitic, are very much temperature sensitive. 
     FIG. 2 is a graph indicating the amount of noise induced by air turbulence associated with a thermocouple. Variations in thermal resistance caused by uneven air flow, create differential lead temperatures, thereby causing thermoelectric voltage noise at the output of the reference. As shown in FIG. 2, the time period from 0 to 6 minutes represents the noise experienced from a thermocouple that had been shielded from ambient air with a small foam cup. The cup was then removed for the second half of the trace (6-12 minutes). The ambient in both cases was a lab environment with no excessive air turbulence from air conditioners, opening/closing doors, etc. Removing the foam cup increased the output noise by almost an order of magnitude in the 0.01 Hz to 1.0 Hz band. 
     Returning to FIG. 1, the thermosensitivity discussed in conjunction with FIG. 2 is one reason why precautions are taken when connecting thermocouples such as the use of the isothermal block  120  shown in FIG.  1 . 
     In addition to thermosensitivity, the arrangement shown in FIG. 1 is vulnerable to magnetically-coupled noise, such as that which may be coupled from current changes in a heating unit. ADC input circuits typically contain both series resistors and ESD protection diodes, and interference spikes can cause currents to flow in the diodes and disturb the apparent temperature reading. For example, a 0.7V interface spike can readily cause a 1 microamp transient current, which yields a 10 microvolt error signal if it flows through a series resistance of only 10 ohms. Such an error is roughly equivalent to that caused by turbulent air flow as shown in FIG.  2 . 
     FIG. 3A is a block diagram showing a thermocouple arrangement in an oven enclosure. When a thermocouple  100 B is placed in an oven enclosure  110 , and connected to an ADC  150 , one option would be to have one side of the thermocouple connected to the ADC negative supply. In this case, protection diodes inside the ADC will conduct if interference pickup causes the other side of the thermocouple to go below −0.7V. 
     FIG. 3B is a block diagram showing a thermocouple arrangement in an oven enclosure in which the thermocouple is biased to operate above ground potential. In this case, protection diodes will not conduct unless interference pickup exceeds 1.65V+0.7V=2.35V, a factor of 3 better than FIG.  3 A. This however creates a potential problem with sparking when the thermocouple is placed inside an oven enclosure which is grounded. If the thermocouple accidentally comes in contact with the oven enclosure, measurement or power-supply circuitry can be damaged. One can insulate the thermocouple from the oven wall; however, this would reduce the thermal responsiveness. 
     FIG. 4 is a graph showing energy-delay relationships when scaling supply voltage to semiconductor circuits such as CMOS devices. The graph shows normalized delay, energy and energy-delay product as a function of supply voltage. Delay is normalized to the delay value at the largest supply voltage (3 volts), and energy is normalized to energy value at the smallest supply voltage (0.6 volts). One can see that the delay values are very high as the voltage is reduced toward the 0.6 volt level. Similarly, energy consumption is very high as supply voltages increase to the 3.0 volt level. One can see from the drawing that the energy-delay product has a minimum in the general area of 1.0 volt. Therefore, it would be desirable to operate CMOS devices at about a 1 volt level in order to minimize the energy delay product, and thus minimize power consumption in the devices without significantly adversely impacting delay. For switching speeds normally found in ADCs, the delay may not be a significant factor, but operating at an energy minimum is very important. 
     FIG. 5A is a graph illustrating a body effect in a MOS transistor. The threshold voltage of a MOS transistor is not a constant; it varies slightly as a function of the voltage between the source terminal of the transistor and the silicon substrate. This variation is called the body effect, and it is illustrated in FIG.  5 A. The silicon substrate is usually connected to a circuit ground during packaging. However, a fixed bias voltage is sometimes applied between circuit ground and the substrate, and this fixed bias voltage must be taken into account in estimating the body effect. If the source-to-bulk (substrate) voltage V sb =0, then V th  is at its minimum value of approximately 0.2 VDD. As V sb  is increased, V th  increases. 
     FIG. 5B is a schematic diagram illustrating problems caused by body effect. The impact of body effect on a circuit, such as shown in FIG. 5B, is that the body effect raises the threshold of an N channel device by about 0.3 volts for every volt between the N-channel source and the substrate. It is also clear that a CMOS inverter will not work if the N channel cannot turn on. This situation means that if one were to operate the circuit shown in FIG. 5B with a −2.0 volt negative bias voltage, then the N-channel threshold would be raised from 0.7 volt, when the substrate was at 0 volt, to 1.3 volts when the substrate is at −2 volts. Under these circumstances, the inverter would need a VDD of at least 1.3 volts which is getting away from the energy delay optimum shown in FIG.  4 . 
     FIG. 6 is a block diagram of a thermocouple measuring arrangement in accordance with one aspect of the invention. Signal voltage across thermocouple  100  is applied to an analog to digital modulator  600 . In a preferred embodiment, this modulator is a delta-sigma modulator which produces a stream of 1-bit digital values on line  605  which represent the input signal. The sampling of the thermocouple signal is done with respect to V ref+  and V ref− , with V ref+  being generated by source  610  and V ref−  being signal ground. The 1-bit digital output  605  is applied to digital filter  630 . 
     Most of the power associated with the analog to digital converter chip shown in FIG. 6 is consumed in the digital filter. Therefore it is important that the digital filter  630  operate with the least power possible, or in other words, substantially at the energy delay minimum shown in FIG. 4. A charge pump  620 , connected between VDD and ground generates a negative bias voltage (NBV) with respect to which the digital filter and other circuitry can be operated. In the circuit shown, if the charge pump  620  were to generate a negative bias voltage of −1 volt, the potential across a digital filter would be substantially that required for the energy delay minimum shown in FIG.  4 . However, if other considerations dictated any different negative bias voltage, one could adjust the voltage across digital filter  630  to be substantially 1 volt either by voltage regulation (not shown) or by use of a DC to DC converter  635  (shown). The output of the digital filter can then be applied to level shifter  640  to produce the desired voltage swing for application to the output driver  650 . 
     A second advantage to the configuration shown in FIG. 6 is that the thermocouple  100  is referenced to V ref−  (signal ground). Thus, if thermocouple  100  were placed in an oven  110 , and the oven were grounded, then there would be no danger of sparking. Additionally, the thermocouple  100  would operate at the midrange of the delta-sigma modulator  600  where the modulator is very accurate. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims and their equivalents.