FIELD OF THE INVENTION

This invention relates to electronic circuits for analog-to-digital conversion and, more particularly, to an improved A/D converter circuit and technique.

BACKGROUND OF THE INVENTION

Various types of circuits exist in the art for the important task of analog-to-digital conversion, but existing approaches suffer one or more disadvantages. So-called "dual-slope" A/D converters, commonly used in digital multimeters and other applications, can achieve high resolution, but are very slow. [See, for example, F.H. Musa et al., "A CMOS Monolithic Three And A Half Digit A/D Converter", IEEE Int'l. Solid State Circuits Conf. , pp. 144- 145, 197b.j At the other extreme are A/D converters based on the so-called "flash" technique [see e.g. J.G. Peterson, "A Monolithic Video A/D converter", IEEE Journal of Solid State Circuits, SC-14, No. 6, 932-937, Dec. 1979] in which for an N-bit A/D converter the input signal is simultaneously compared with 2 -1 reference voltages using 2 -1 comparators to generate 2^-1 outputs. The flash A/D converter also contains a rather complex digital circuit for converting the 2 -1 outputs from the comparators to N-bits of binary information. Thus, complexity is an obvious drawback of this approach. An alternative to the flash A/D converter has been proposed which utilizes a folding or sawtooth characteristic to achieve high speed [see A. Arbel et al., "Fast ADC", IEEE Trans, on Nuc Sci., NS-22, 446, 1975]. However, this approach has not been widely accepted

because of the need for a complex circuit to obtain the folding characteristic.

It is among the objects of the present invention to provide an A/D converter which is fast and accurate, but not unduly complex.

SUMMARY OF THE INVENTION

The present invention is directed to an analog-to- digital converter circuit for receiving an analog input signal and producing a digital output having a plurality of binary bits representative of the input signal.

In the disclosed embodiments of the invention, a number of devices are utilized, each of which has a voltage versus current characteristic with a plurality of peaks, and negative resistance regions between. said peaks. In the illustrated embodiments, these devices are resonant tunneling diodes. For each bit to be produced, a pair of said devices are provided, each being coupled in series arrangement with a resistive means. Means are provided . for applying predetermined portions of the input signal to both of the series arrangements for each respective bit to be produced. Means are also provided for combining signals from both of the series arrangements for each respective bit to be produced. The outputs of the combining means respectively represent the produced binary bits.

In the preferred embodiment of the invention, the means for applying predetermined portions of the input signal comprises means for applying different fractional portions of the input signal to respective pairs of series arrangements. In this embodiment, the input signal comprises an input voltage. Means are provided for applying a voltage offset to the input signal. Also, the means for applying predetermined portions of the input signal includes a voltage divider.

In one form of the invention, the means for combining signals from said respective series arrangements comprises means for adding the voltages across the respective resistive means of each said pair of series arrangements. In another form of the invention, the means for combining signals from said respective series arrangements comprises means for subtracting the voltages across the respective

resistance means of each said pair of series arrangements.

The circuit of the present invention has a number of important advantages. The transition between states ("0" to "1", or vice versa) is very fast due to the high switching speed of the narrow negative resistance region of the RTD's I-V characteristic. Also, because of the judicious use of offsets, the quantization uncertainty is one-half the least significant bit size. Further, the number of RTD's necessary for sψ. A/D converter of N bits is only 2N, as compared to 2 -1 devices required for some of the A/D approaches described above.

Further features and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic diagram, partially in block form, of an analog-to-digital converter in accordance with an embodiment of the invention.

Fig. 2 shows the current- vs. voltage characteristic of a five quantum well RTD device.

Fig. 3a illustrates an RTD in series with a resistor, and Fig. 3b illustrates the I-V characteristic of an RTD with and without a series resistor.

Fig. 4a illustrates a circuit arrangement useful in understanding the invention, and Fig. 4b illustrates the behavior of the voltage across the resistor of the Fig. 4a circuit.

Fig. 5a illustrates another circuit arrangement useful in understanding the invention, and Fig. 5b illustrates the behavior of the voltage across the resistor of the Fig. 5a circuit.

Fig. 6a illustrates another circuit arrangement useful in understanding the invention, and Fig. 6b illustrates the behavior of the voltage across the resistor of the Fig. 6a circuit.

Fig. 7a illustrates another circuit arrangement useful in understanding the invention, and Fig. 7b illustrates the behavior of the voltage across the resistor of the Fig. 7a circuit.

Fig. 8a illustrates another circuit arrangement that is useful in understanding the invention, and Fig.s 8b-8e are waveforms which illustrate the behavior of the voltage across the resistor of Fig. 8a for different fractions of the input voltage.

Fig. 9a illustrates another circuit that is useful in understanding the invention, and Fig.s 9b-9e are waveforms which illustrate the behavior of the voltage across the resistor of Fig. 9a for different fractions of the input voltage.

Fig.s 10b—lOe illustrate square waves obtained by adding the waveforms of Fig.s 8b-8e to the waveforms of Fig.s 9b-9e, respectively.

Fig. 11 is a graph showing the ideal transfer curve of the A/D converter hereof for a gray code.

Fig. 12 is a schematic diagram, partially in block form, of an analog-to-digital converter in accordance with another embodiment of the invention.

Fig. 13 is a schematic diagram, partially in block form, of an analog-to-digital converter in accordance with still another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to Fig. 1, there is shown an analog-to- digital converter in accordance with an embodiment of the invention. An input analog voltage, V^ _n , to be digitized, is applied across terminals 111 and 112. The terminal 111 is coupled to one end of a voltage divider which comprises resistors designated 4R, 2R, R,- and R. The terminal 112 is coupled to the positive side of a DC voltage source, V /2, q which is used as an offset voltage, as will be described hereinbelow. The DC voltage source is coupled between the terminal 112 and the other end of the voltage divider. Each position of the voltage divider is coupled, via a buffer, to the junction between a pair of resonant tunneling diodes (RTDs). In particular, the buffers are designated 121, 123, 125 and 127, and the buffer 121 couples to the junction between RTD 131 and RTD 132, the buffer 123 couples to the junction between RTD 133 and RTD 134, the buffer 125 couples to the junction between RTD 135 and RTD 136, and the buffer 127 couples to the junction between RTD 137 and RTD 138. The RTDs may be of any suitable type which has a voltage versus current characteristic with multiple peaks and negative resistance regions between the peaks, as _. described further hereinbelow. The buffers are used to drive the RTD circuits to furnish a suitable output impedance for the input voltages to the RTDs.

Each of the RTDs has an associated load resistor, as seen in the diagram, the load resistors each having a value R _L in the present embodiment. The load resistors ^R L3 R .5 and R _L 7 are coupled to a rail 101, and the load resistors R _L 2' ^R L4' ^R L6 ^anc ^ ^R L8 ^{are ou} P ^le d to a rail 102. In the present embodiment, the rail 101 is at a voltage ^in ₍ ax ₎ ^{+ VT} ' ¹ ' wit respect to the bottom of the voltage divider, and the rail 102 is at a voltage -VT1 with respect to the bottom of the voltage divider. The labelled voltage across each load resistor, as explained further hereinbelow,

is as indicated in the Figure.

Four summing amplifiers 180, 181, 182 and 183 are provided. Each of the summing amplifiers is operative to sum the voltage across the load resistors associated with an adjacent pair of RTDs. Thus, the summing amplifier 180 sums the voltages across load resistors R-^- and ^R _j. g the summing amplifier 181 sums the voltages across load resistors R _L 5 and _j _6 the summing amplifier 182 sums the voltages across load resistors R^g and R _j __4, and the summing amplifier 183 sums the voltages across load resistors R _L1 and _j. 2* The outputs of the summing amplifiers are respectively the least significant bit (output of summing amplifier 180 - Dg) to the most significant bit (output of summing amplifier 183 - D3) of the binary type of digital code (a gray code, in this case) generated by the circuit.

To better understand operation of the Fig. 1 embodiment, the following description is set forth in conjunction with Fig.s 2-11. Fig. 2 shows the current vs. voltage (I-V) characteristic of a five quantum well RTD device. The illustrated characteristic is seen to have five substantially symmetrical peaks in its I-V characteristic. As is known, by varying the thickness of the barrier and spacer layers, one can tailor the peak-to-peak voltage, the peak-to-valley ratio, and the peak current of the RTD. [For description of the fabrication and electrical characteristics of a multi- well InGaAs/InAlAs RTD, reference can be made, for example, to A. Lakhani et al., "Eleven-Bit Parity Generator With A Single, Vertically Integrated Resonant Tunnelling Device", Electron Lett., 24, 681-683 (1988), and R. Potter et al., "A Vertically integrated Resonant Tunnelling Device With Multiple Negative Differential Resistances", Device Research Conference, Boulder, CO (1988).]

Fig. 3b shows the I-V characteristic of an RTD with and without a series resistor. The characteristic marked "Without R" is the type of sawtooth first shown in Fig. 2.

When an RTD is connected in series with a resistor, as illustrated in Fig. 3a, the slope of the positive resistance region of the I-V curve becomes more gradual and this region becomes wider. The negative resistance region becomes steeper and narrower. By suitably choosing the resistance R of the Fig. 3b circuit (e.g. a resistance that is approximately equal to the negative resistance of the negative resistance regions of the RTD), the width of the negative resistance region can be reduced so that it is negligible compared to the width of the positive resistance region.

In the diagram of Fig. 4a, the voltage across the resistance R is designated V _Q , and in the accompanying curve of Fig. 4b, which shows the voltage Vg as a function of V^ _n , the voltage between successive peaks is designated V _g . As also seen in Fig. 4b, the voltage to the onset of the first symmetrical positive resistance portion of the sawtooth waveform is designated VTl, and V„q is equal to Vs__>/4.

Consider next the modification of the Fig. 4a circuit to obtain the circuit of Fig. 5a; i.e. addition of the DC voltage VTl + V- / _{max )} i ⁿ the configuration shown. Fig. 5b

~- illustrates V _Q " (the voltage across the resistor R in Fig.

5a) as a function of ^ _n , and it is seen that the curve of

Fig. 5b is the reverse of that shown in Fig. 4b. When the circuit of Fig. 4a is offset by VTl + V /2, as shown in Fig. όa, the curve of Fig. 4b is offset by a DC voltage VTl +

V /2. If V^ _π ranges from zero to a maximum voltage ι ^V in ₍ max ₎ ' then the V _Q ⁴ - - V^ _n curve becomes Fig. 6b. In similar manner, the curve of Fig. 7b is obtained from the circuit of Fig. 7a. When V^ _n and V /2 of Fig. 6a are divided by 2^ as shown n Fig. 8a, one gets the curves of

Fig. 8b through Fig. 8e (for L = 0,1,2 and 3, respectively).

[It can be noted that the curve of Fig. 8b (i.e., for L = 0 is the same as that of Fig. 6b.] Similarly, for the values of L = 0,1,2 and 3 in Fig. 9a, one gets the curves of Fig.s

9b, 9c, 9d and 9e, respectively. [The curve of Fig. 9b (i.e., for L = 0) is the same as that of Fig. 7b.]

When, the curves of Fig.s 8b and 9b are summed (e.g. by summing the voltages across the resistors of Figs 8a and 9a), one obtains the square waveform of Fig. 10b. Similarly, summing the curves of Fig.s 8c and 9c results in the square waveform of Fig. 10c, summing the curves of Fig.s 8d and 9d results in the square waveform of Fig. lOd, and summing the curves of Fig.s 8e and 9e results in the square waveform of Fig. lOe.

The circuit of Fig. 1 will now be more readily understood. The input voltage to be digitized is applied as V^ _n , the voltage V /2 is the voltage offset described above, and the voltage divider serves to divide the offset input voltage into the appropriate fractions for application to the respective pairs of series arrangements of RTDs and resistors. The voltages across the resistors of each associated pair of series arrangements are added by summing amplifiers 180, 181, 182 and 183.

Referring again to Fig.s 10b through lOe, it is seen that in the present embodiment the outputs of the summing amplifiers of Fig. 1, represented as the output bits D ₃ (Fig lOe - most significant bit), ₂ (Fig. lOd - second most significant oit), D3 (Fig. 10c - third most significant bit), and D^ (Fig. 10b - least significant bit), produce a gray code digital representation of the analog input signal, V^ _n , as shown in the table of Fig. lϋf. The ideal transfer code for this Embodiment A/D converter, which produces gray code, is shown in Fig. 11. It will be understood, however, that any desired kind of binary code can be obtained by selecting the appropriate offsets, bias voltages, and voltage divide .values.

Referring again to the waveforms of Fig.s lOb-lOe, it is seen that because of the insertion of offset voltage V /2 the waveform of Fig. 10b is shifted by one-half the least

significant bit (LSB) so that the width of "0000" is only half the width of the other codes. This reduces the ideal maximum quantization uncertainty from 1 LSB to 1/2 LSB. The transition from a "0" to a "1" state, or vice versa, is very fast due to the high switching speed of the narrow negative resistance region of the. RTD's I-V characteristic.

As seen above, the embodiment of Fig. 1 was produced using square waves obtained by adding appropriately offset sawtooth waves, one of which has rising ramps after each negative resistance region, and the other of which has falling ramps after each negative resistance region. The square waves can alternatively be obtained by subtracting offset sawtooth waves which both have rising ramps or by subtracting sawtooth waves which both have falling ramps. The embodiment of Fig. 12 uses subtraction of sawtooth waveforms with rising ramps. The input signal V^ _n , offset voltage V /2, voltage divider, and RTDs are like their counterparts in the Fig. 1 embodiment. In the Fig. 12 embodiment, however, all the load resistors are coupled to the rail 102, with the resistors ^R L3' ^R L5 ^{and R} L7 ^be;Ln 9 coupled to rail 102 via a DC voltage V /2. Difference amplifier 1280 subtracts the voltage across R _L from the voltage across R _L g to obtain bit D . Similarly, difference amplifiers 1281, 1282, and 1283 derive bits D-^ , D ₂ , and D3, respectively, by subtracting voltages as indicated in Fig. 12,

The embodiment of Fig. 13 uses subtraction of sawtooth waveforms with falling ramps. The circuit is similar to that of Fig. 12 (with like reference numerals for like components), except that the resistors are all coupled to rail 101, as shown.

The invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. For example, as noted above, while the illustrated embodiments produce gray code, it will be

αnderstood that any desired kind of binary code can be obtained by selecting the appropriate offsets, bias voltages, and voltage divider values. Also, while RTDs are found to have I-V characteristics that are particularly suitable for use in the invention, other devices having the specified characteristics may also be employed.