Differential Quantizer Reference Resistor Ladder for Use With an Analog-to-Digital Converter

1. Technical Field

This invention relates to analog-to-digital converters for converting an analog input signal to a plurality of digital signals and, more particularly, to a reference resistor network for converters employing a differential analog input signal.

2. Discussion

Analog-to-digital converters are used in a wide variety of applications for converting an analog signal to a plurality of digital signals. A typical converter may utilize flash quantizer comparator banks for quantizing the analog input signal. It is sometimes preferable to employ a differential signal path to improve system performance. Generally, a differential analog signal path implementation requires comparators each having four inputs. These inputs include positive and negative reference inputs and positive and negative signal inputs.

It is desirable to obtain an analog-to-digital converter for receiving a differential analog signal which requires comparators having only two inputs; the desired comparator inputs being the positive and negative signal inputs of the differential analog signal.

In accordance with the preferred embodiment of the present invention, a differential resistive spreading network is employed to spread the differential analog input signal amongst the flash quantizer comparator banks of an analog-to-digital converter. The invention advantageously utilizes two equal and independent resistor ladders which spread the positive and negative inputs of the analog differential input signal. Each resistor ladder has a plurality of serially connected resistors of equal value and further includes a reference current source for maintaining a uniform current therein. The converter comparators compare signals tapped from both resistor ladders. The comparators are then triggered in an incremental manner as the analog differential signal changes. As a result, the comparators in the quantizer comparator banks require only two input signals instead of four, thereby easing the comparator design.

The various advantages of the present invention will become apparent to those skilled in the art by reading the following specification and by reference to the drawings in which:

FIG. 1 is a block diagram which illustrates a typical four-input comparator as used in the prior art;

FIG. 2 is a circuit diagram which illustrates the differential resistive spreading ladder in accordance with the present invention for a two-bit flash quantizer example.

Turning now to FIG. 1, a typical four-input comparator 10 for a differential quantizer is illustrated therein. Generally, the differential analog signal path implementation usually requires that comparators of this sort having four inputs be employed. In all, such a comparator requires both positive and negative reference inputs 12 and 13 and positive and negative signal inputs 14 and 15.

FIG. 2 illustrates the technique of the present invention, which provides a differential reference resistor network 22 for a two-bit flash quantizer example. The reference resistor network 22 spreads a differential analog input signal in such a manner that requires comparators C1, C2, and C3 having only two inputs. The circuit configuration 20 is adapted to receive a differential analog signal and convert the signal to a digital output code. In general, the circuit includes positive and negative input terminals 30 and 40, buffer amplifiers 32 and 42, a resistive spreading network 22, a plurality of comparators C1, C2, and C3 and a binary encoder 26 for encoding and providing an N-bit binary output code 28.

The differential analog input signal includes both positive and negative signal portions. As such, separate input terminals 30 and 40 are provided to receive the differential analog signal. The negative input terminal 30 is connected to a first buffer amplifier 32 and is adapted to receive the negative portion of the differential input signal. Connected to the output of the first buffer amplifier 32 is a first resistive spreading ladder 34 which includes two resistors R1 and R2 of equal value for spreading the differential analog input signal and providing a first series of comparison signals therein. Coupled to the output of the first resistive ladder 34 is a reference current source 36 for maintaining a steady uniform current. A level shifted replica of the negative portion of the analog signal is generated at each node of the first resistive spreading ladder 34. The amount of level shift is equal to the sum of the voltages developed across the resistors R1 and R2 by the reference current source 34 at that node.

The positive input terminal 40 being adapted to receive the positive portion of the differential analog input signal is connected to a second buffer amplifier 42. The output of the second buffer amplifier 42 is connected to a second resistive spreading ladder 44. Second resistive spreading ladder 44 likewise, employs two resistors R3 and R4 of equal value being connected in series and adapted for spreading the differential analog input signal and providing a second series of comparison signals. Similarly, a reference current source 46 is coupled to the output of the second resistive ladder 44. A level shifted replica of the positive portion of the analog signal is generated at each node of the second resistive spreading ladder 44. Again, the amount of level shift is equal to the sum of the voltages developed across the resistors R3 and R4 by the reference current source 46 at that node.

Comparators C1, C2, and C3 are coupled to the first and second resistive spreading ladders 34 and 44 for comparing the differential comparison signals of the positive and negative portions of the differential analog signal and providing an output response therefrom. The comparators C1, C2, and C3 are adapted to receive comparison signals provided from the positive portion of the input signal at the positive terminal of each comparator. Likewise, comparison signals provided from the negative portion of the input signal are received by the negative terminal of each comparator and thereby form the comparator threshold. In response, each comparator produces a high output when the positive portion of the differential signal exceeds the comparator threshold. Alternately, a low output response is provided when the positive signal is less than the comparator threshold.

The negative terminal of comparator C1 is adapted to receive a comparison signal from the negative portion of the differential signal before it is applied to resistors R1 and R2 of the first resistor spreading ladder 34. The positive terminal of comparator C1 is adapted to receive a comparison signal from the positive portion of the differential signal after the signal has been applied to both resistors R3 and R4 of the second resistive spreading ladder 44.

The negative terminal of comparator C2 is adapted to receive a comparison signal from the negative portion of the differential signal between resistors R1 and R2, after having been partially level shifted by the first resistor R1 of the first resistive spreading ladder 34. Similarly, the positive terminal of comparator C2 is adapted to receive a comparison signal from the positive portion of the differential signal between resistors R3 and R4, after having been partially level shifted by the first resistor R3 of the second resistive spreading ladder 44.

Comparator C3, likewise, receives comparison signals in a similar manner. The negative terminal of comparator C3 receives a comparison signal from the negative portion of the differential signal after having been level shifted by both first and second resistors R1 and R2 of the first resistive spreading ladder 34. The positive terminal of comparator C3 receives a comparison signal from the negative portion of the differential signal before being applied to either resistors R3 or R4 of the second resistive spreading ladder 44. Reference current sources 36 and 46 are connected to the ends of both the first and second resistive spreading ladders 34 and 44, respectively. Both current sources 36 and 46 are of equal value and provide a uniform and equal flow of current through each of resistive networks 34 and 44. (The quantization step size of this analog to digital conversion is simply two times the voltage developed across the resistor by the reference current source.)

The outputs of comparators C1, C2, and C3 are adapted to be coupled to a binary encoder 26 for providing an N-bit binary output code 28. As shown in FIG. 2, logic gates may be easily employed to provide for a two-bit binary output code 28. However, alternate binary encoding means such as a ROM decoder are available and are more preferable for higher bit quantizers, since the large number of logic gates required for a higher order bit quantizer results in a more complex circuit design.

The output of comparator C1 is coupled to the first input terminal of a first AND-gate 50. First AND-gate 50 having its second input terminal coupled to the output of comparator C2. The output of comparator C2 is inverted and coupled to the first input terminal of a second AND-gate 52. The second input terminal of AND-gate 52 being coupled to the output of comparator C3. The outputs of first and second AND-gates 50 and 52 are coupled to the first and second input terminals of an OR-gate 56. The output response of OR-gate 56 provides the least significant bit D0 of the binary output code 28. The output of comparator C2 is further connected to a buffer 54. The output response of buffer 54 provides the most significant bit D1 of the binary output code 28.

In operation, a differential analog input signal is applied across the positive and negative input terminals 30 and 40. Both the positive and negative portions of the differential analog input signal pass through separate buffer amplifiers 32 and 42 and resistive spreading ladders 34 and 44. Resistive spreading ladders 34 and 44 divide the incoming signal into differential comparison signals. Comparators C1, C2, and C3 compare the differential comparison signals on the first resistive spreading ladder 34 with the differential comparison signals on the second resistive spreading ladder 44. If the voltage tapped from the positive input portion of the differential analog signal is greater than the voltage tapped from the negative input portion (i.e., above the comparator threshold), the comparator provides a high output response therefrom. Likewise, if the positive input voltage portion of the differential signal is less than the negative input voltage portion (i.e., below the comparator threshold), the comparator provides a low output response. Note that when the differential input voltage is zero, that is, the positive input voltage is equal to the negative input voltage, the mid-scale comparator C2 threshold is teased. Essentially, the differential voltage levels of the comparison signals trigger the comparators in an incremental manner as the differential input signal is changed.

The outputs of comparators C1, C2, and C3 are then applied to a network of logic gates which perform the binary encoding. The binary encoding provides a two-bit binary output code. The implementation of a two-bit binary output code provides for four possible outputs. Given the 2-bit example in FIG. 2, wherein the resistors (R) and current (I) in each resistive ladder are of equal value, the four possible digital output codes for various analog input voltages (V.sub.IN) are provided in the following logic diagram:

A higher bit order may be implemented in order to attain higher output resolution. In doing so, a greater number of resistors and comparators are required and an alternate binary encoding means may be preferred. In all, an N-bit binary output resolution will require at least 2.sup.n -1 comparators and 2.sup.n -2 resistors for each resistive ladder, the resistors being divided equally between the buffer amplifier and the current source in each resistive spreading ladder.

In view of the foregoing, it can be appreciated that the present invention enables the user to achieve analog-to-digital conversion of a differential signal with a more simplified comparator design. Thus, while this invention has been described in connection with a particular example thereof, no limitation is intended thereby except as defined by the following claims. This is because the skilled practitioner will realize that other modifications can be made without departing from the spirit of this invention after studying the specification and drawings.