1. Field of the Invention
The present invention relates to electronics. More specifically, the present invention relates to analog to digital converters.
2. Description of the Related Art
The function of an analog to digital converter (ADC) is to accurately convert an analog input signal into a digital output represented by a coded array of binary bits. The output bits are generated by processing the analog input signal through a number of comparator steps. An N-bit digital output can be produced in one step with 2N-1 parallel comparators (flash ADCs) or, at the limit, by N sequential steps with a single comparator (successive approximation ADCs). Flash ADCs provide higher speed of conversion, but are limited by higher input capacitance, power consumption, and device yield constraints associated with the high number of comparators in the circuitry. At the other extreme, successive approximation ADCs are simple in structure, and may be very accurate, but they have very slow conversion times due to the serial nature of the conversion process.
Subranging ADCs provide an intermediate compromise between flash ADCs and successive approximation ADCs. Subranging ADCs typically use a low resolution flash quantizer during a first or coarse pass to convert the analog input signal into the most significant bits (MSB) of its digital value. A digital to analog converter (DAC) then generates an analog version of the MSB word, which is subtracted from the input signal at a summing node to produce a residue or residual signal. The residue signal is sent through one or more fine passes (through the same quantizer or additional low resolution quantizers) to produce the lower significant bits of the input signal. The lower significant bits and the MSB word are combined by digital error correcting circuitry to produce the desired digital output word.
There is a requirement to produce high dynamic range, low power ADC integrated circuits (IC) for the military communications market, as well as for commercial applications such as the cellular basestation market. Currently available ADCs do not meet the needs of the marketplace.
In particular, typical flash or subranging ADCs may utilize a flash quantizer for quantizing the analog signal. It is often preferable to use a differential signal path to improve system performance. Prior art differential quantizers typically include two equal resistor ladders that spread the positive and negative inputs of the differential analog input signal. Each resistor ladder includes a plurality of serially connected resistors and a single reference current source for maintaining a uniform current. A bank of comparators then compares signals tapped from both resistor ladders. In the prior art quantizer, the current sources are employed as current sinks, which loads down the hold amplifier driving the quantizer and requires too much power. Hence, there is a need in the art for an improved quantizer that requires less power than prior art quantizers.
In addition, subranging ADCs typically include a summing node circuit to generate the residue signal. Conventional summing node circuits include offset current sources and a DAC to generate an analog signal representing the coarse pass of the ADC, which is subtracted from the input signal by a summing amplifier to produce the residue signal. This design dissipates too much power, plus the offset current sources are noisy and bandlimited, which creates settling problems with the offset currents. Hence, there is a need in the art for an improved summing node design for subranging ADCs that requires less power than prior art approaches.
Furthermore, subranging ADCs may exhibit significant nonlinearity errors that tend to repeat in response to an analog input signal. The repetition produces spurs in the ADC's frequency response that distort the signal and reduce its spur free dynamic range. Because the spurs tend to lie very close to the signal frequency, it is difficult and expensive to remove them using conventional filtering techniques. New trim methodologies for reducing the static differential nonlinearity (DNL) and integral nonlinearity (INL) are required in order to achieve the desired performance. Hence, there is a need in the art for a system or method for trimming a subranging ADC.