These CMOS devices incorporate wideband low-distortion, track-and-hold amplifiers to ensure excellent dynamic performance throughout and beyond the Nyquist band.
Averaging is used especially in the first and second MDAC to ensure that the calibration is noise-free. Data Latency Because each sample must propagate through the entire pipeline before all its associated bits are available for combining in the digital-error-correction logic, data latency is associated with pipelined ADCs.
Any error made at that conversion is suppressed by the large 44 cumulative gain preceding the 4-bit flash.
This fact is often exploited to save additional power by making the pipelined stages progressively smaller. Therefore, the input to Stage 2 occupies only half the range of the 3-bit ADC in Stage 2 that is, when there is no error in the first 3-bit conversion in Stage 1.
Thus the final stage only needs to be more than 4-bits accurate. This also results in less data latency.
Nonetheless, pipelined ADCs of various forms have improved greatly in speed, resolution, dynamic performance, and low power in recent years.
The 3-bit output is then fed to a 3-bit DAC accurate to about 12 bitsand the analog output is subtracted from the input. In general, higher speed CMOS pipelined ADCs tend to favor a lower number of bits per stage as low as just one bit per stage so that the interstage gain is only 2because it is difficult to realize wideband amplifiers of very high gain in CMOS.
During normal conversions, those error terms are recalled from the RAM and used to adjust the outputs from the digital-error-correction logic. Undersampling, popular in digital receiver design, is possible with these ADC families.
Component Accuracy Digital error correction does not correct gain or linearity errors in the individual DAC and gain amplifiers. The first stage of the pipelined ADC is responsible for the most significant bit, and the seventh stage gives the least significant bit of the digital output.
As long as this gained-up residue does not overrange the subsequent 3-bit ADC, it can be proven that the LSB code generated by the remaining pipeline when added to the incorrect 3-bit MSB code will give the correct ADC output code.
This pipelining action is the reason for the high throughput. In Figure 1, notice that the 3-bit residue at the summation-node output has a dynamic range one-eighth that of the original Stage 1 input VINyet the subsequent gain is only 4.
The major factor limiting MDAC accuracy is the inherent capacitor mismatch. Depending on the region in which the input to the flash ADC falls, the residue voltage is calculated as follows and is fed to the next stage as the input voltage.
This gained-up residue continues through the pipeline, providing three bits per stage until it reaches the 4-bit flash ADC, which resolves the last 4LSB bits. Different Variations The example in Figure 1 shows that there can be many variations of pipelined ADCs, depending, that is, on several variables: The ADC then gives a digital output corresponding to the region in which the analog input falls.
Resolutions range from eight bits at the faster sample rates up to 16 bits at the lower rates. This "residue" is then gained up by a factor of four and fed to the next stage Stage 2.
This process is called "1-bit overlap" between adjacent stages. These resolutions and sampling rates cover a wide range of applications, including CCD imaging, ultrasonic medical imaging, digital receivers, base stations, digital video for example, HDTVxDSL, cable modems, and fast Ethernet.
Although each stage generates three raw bits in the Figure 1 example, because the interstage gain is only 4, each stage Stages 1 to 4 effectively resolves only two bits. If one of the comparators in the first 3-bit flash ADC has a significant offset when an analog input close to the trip point of this comparator is applied, Pipelined adc design thesis an incorrect 3-bit code and thus an incorrect 3-bit DAC output would result, thus producing a different residue.
The article also briefly compares pipelines ADCs to other data converter architectures. A purely bipolar implementation would be more complicated and would suffer mainly from resistor mismatch in the current-source DAC and the interstage gain amplifier.
The digital error correction will not correct for errors made in the final 4-bit flash conversion. The highest sampling rates a few hundred Msps or higher are still obtained using flash ADCs.This thesis analyzes standard and low voltage design issues for pipelined ADCs and proposes a fully-differential implementation of the OpAmp Reset Switching.
Master’s Thesis Modeling and Implementation of A 6-Bit, 50MHz Pipelined ADC in CMOS By QAZI OMAR FAROOQ Department of Electrical and Information Technology Faculty of Engineering, LTH, Lund University acquisition is driving the pipelined ADC design towards higher speed. Pipelined ADC Stage Power Dissipation & Noise •Typically pipeline ADC noise dominated by inter- stage gain blocks •Sub-ADC comparator noise translates into comparator threshold.
Design of an 8-bit Successive Approximation Pipelined Analog to Digital Converter (SAP-ADC) in 90 nm CMOS A thesis submitted in partial fulfillment. This thesis explores a pipelined ADC design that employs a variety of low- power techniques such as dynamic residue amplification and incomplete settling in a unique way to maximize the speed while maintaining low energy (98 fJ/conv-step).
A V 25MSPS Pipelined ADC Using Split CLS with Op-amp Sharing by Visu Vaithiyanathan Swaminathan A Thesis Presented in Partial Fulfillment of the Requirements for the Degree.Download