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How Delta-Sigma Adcs Work, Part 1

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How Delta-Sigma Adcs Work, Part 1 Texas Instruments Incorporated Data Acquisition How delta-sigma ADCs work, Part 1 By Bonnie Baker Signal Integrity Engineer Analog techniques have dominated signal processing for The DS converter’s primary internal cells are the DS years, but digital techniques are slowly encroaching into modu lator and the digital/decimation filter. The internal this domain. The design of delta-sigma (DS) analog-to- DS modulator shown in Figure 1 coarsely samples the digital converters (ADCs) is approximately three-quarters input signal at a very high rate into a 1-bit stream. The digital and one-quarter analog. DS ADCs are now ideal for digital/decimation filter then takes this sampled data and converting analog signals over a wide range of frequencies, converts it into a high-resolution, slower digital code. from DC to several megahertz. Basically, these converters While most converters have one sample rate, the DS con- consist of an oversampling modulator followed by a digital/ verter has two—the input sampling rate (fS) and the out- decimation filter that together produce a high-resolution put data rate (fD). data-stream output. This two-part article will look closely at the DS ADC’s core. Part 1 will explore the basic topology The DS modulator and func tion of the DS modulator, and Part 2 will explore The DS modulator is the heart of the DS ADC. It is respon- the basic topology and function of the digital/decimation sible for digitizing the analog input signal and reducing filter module. noise at lower frequencies. In this stage, the architecture implements a function called noise shaping that pushes low- DS converters: An overview frequency noise up to higher frequencies where it is outside The rudimentary DS converter is a 1-bit sampling system. the band of interest. Noise shaping is one of the reasons An analog signal applied to the input of the converter needs that DS converters are well-suited for low-frequency, high- to be relatively slow so the converter can sample it multiple accuracy measurements. times, a technique known as oversampling. The sampling The input signal to the DS modulator is a time-varying rate is hundreds of times faster than the digital results at analog voltage. With the earlier DS ADCs, this input-voltage the output ports. Each individual sample is accumulated signal was primarily for audio applications where AC signals over time and “averaged” with the other input-signal sam- were important. Now that attention has turned to precision ples through the digital/decimation filter. applications, conversion rates include DC signals. This dis- cussion will use a single cycle of a sine wave for illustration. Figure 1. Block diagram of DS ADC Sample Rate (fS) Analog ∆Σ Input Modulator fSD/ f = Decimation Ratio Data Rate (fD) Digital Digital Decimator Filter Output Digital/Decimation Filter 13 Analog Applications Journal 3Q 2011 www.ti.com/aaj High-Performance Analog Products Data Acquisition Texas Instruments Incorporated Figure 2. Input signal to the DS modulator Input Input Amplitude Magnitude Time Frequency (a) Time domain (b) Frequency domain Figure 2a shows a single cycle of a sine wave for the In this manner, the quantizing action of the DS modulator input of a DS modulator. This single cycle has voltage is produced at a high sample rate that is equal to that of ampli tude that changes with time. Figure 2b shows a the system clock. Like all quantizers, the DS modulator frequency-domain representation of the time-domain produces a stream of digital values that represent the signal in Figure 2a. The curve in Figure 2b represents the voltage of the input, in this case a 1-bit stream. As a result, continuous sine wave in Figure 2a and appears as a the ratio of the number of ones to zeros represents the straight line or a spur. input analog voltage. Unlike most quantizers, the DS There are two ways to look at the DS modulator—in modulator includes an integrator, which has the effect of the time domain (Figure 3) or in the frequency domain shaping the quantization noise to higher frequencies. (Figure 4). The time-domain block diagram in Figure 3 Consequently, the noise spectrum at the output of the shows the mechanics of a first-order DS modulator. The modulator is not flat. modulator converts the analog input signal to a high-speed, In the time domain, the analog input voltage and the out- single-bit, modulated pulse wave. More importantly, the put of the 1-bit digital-to-analog converter (DAC) are differ- frequency analysis in Figure 4 shows how the modulator entiated, providing an analog voltage at x2. This voltage is affects the noise in the system and facilitates the produc- presented to the integrator, whose output progresses in a tion of a higher-resolution result. negative or positive direction. The slope and direction of The DS modulator shown in Figure 3 acquires many the signal at x3 is dependent on the sign and magnitude of samples of the input signal to produce a stream of 1-bit the voltage at x2. At the time the voltage at x3 equals the codes. The system clock implements the sampling speed, comparator reference voltage, the output of the comparator fS, in conjunction with the modulator’s 1-bit comparator. switches from negative to positive, or positive to negative, Figure 3. First-order DS modulator in the time domain Difference f Analog Amplifier Integrator S Input e xi i + Output to x2 x3 + Digital Filter x4 yi – – VREF Comparator (1-Bit ADC) x4 1-Bit DAC yi = xi – 1 + (ei – ei – 1) 14 High-Performance Analog Products www.ti.com/aaj 3Q 2011 Analog Applications Journal Texas Instruments Incorporated Data Acquisition Figure 4. First-order DS modulator in the frequency domain Sigma ei Delta (Integrator) + x 1-Sample 1-Bit y i Delay ADC i Analog – Output to Input Digital Filter Magnitude Signal 1-Bit DAC Frequency fS Quantization Noise depending on its original state. The output value of the Figure 4 also shows that the combination of the integra- comparator, x4, is clocked back into the 1-bit DAC, as well tor and sampling strategy implements a noise-shaping filter as clocked out to the digital filter stage, yi. At the time that on the digital output code. In the frequency domain, the the output of the comparator switches from high to low or time-domain output pulses appear as the input signal vice versa, the 1-bit DAC responds by changing the analog (or spur) and shaped noise. The noise characteristics in output voltage of the difference amplifier. This creates a Figure 4 are the key to understanding the modulator’s different output voltage at x2, causing the integrator to pro- frequency operation and the ability of the DS ADC to gress in the opposite direction. This time-domain output achieve such high resolution. signal is a pulse-wave representation of the input signal at The noise in the modulator is moved out to higher fre- the sampling rate (fS). If the output pulse train is averaged, quencies. Figure 4 shows that the quantization noise for a it equals the value of the input signal. first-order modulator starts low at zero hertz, rises rapidly, The discrete-time block diagram in Figure 3 also shows and then levels off at a maximum value at the modulator’s the time-domain transfer function. In the time domain, the sampling frequency (fS). 1-bit ADC digitizes the signal to a coarse, 1-bit output code Using a circuit that integrates twice instead of just once that produces the quantization noise of the converter. The is a great way to lower the modulator’s in-band quantization output of the modulator is equal to the input plus the noise. Figure 5 shows a 1-bit, second-order modulator that quan tization noise, ei – ei – 1. As this formula shows, the has two integrators instead of one. With this second-order quantization noise is the difference between the current modulator example, the noise term depends on not just quantization error (ei) and the pre vious quantization error the previous error but the previous two errors. (ei – 1). Figure 4 illustrates the frequency location of this Some of the disadvantages of the second- or multi-order quantization noise. modulators include increased complexity, multiple loops, Figure 5. Block diagram of a second-order DS modulator Integrator Integrator 1-Bit ADC OUT IN + + Σ Σ xi yi – – ei 1-Bit DAC yi = xi – 1 + (ei – 2ei – 1 + ei – 2) 15 Analog Applications Journal 3Q 2011 www.ti.com/aaj High-Performance Analog Products Data Acquisition Texas Instruments Incorporated and increased design difficulty. However, most DS modula- in the final output of the converter. Part 2 of this article tors are higher-order, like the one in Figure 5. For instance, series will discuss how to get rid of this noise with a low- Texas Instruments DS converters include second- through pass digital/decimation filter. sixth-order modula tors. Multi-order modulators shape the quantization noise to References even higher frequencies than do the lower-order modula- 1. R. Jacob Baker, CMOS: Mixed-Signal Circuit Design, Vol. II. John Wiley & Sons, 2002. tors. In Figure 6, the highest line at the frequency fS shows the third-order modulator’s noise response. Note 2. Texas Instruments, Nuts and Bolts of the Delta-Sigma that this modulator’s output is very noisy all the way out at Video Tutorial [Online]. Available: http://focus.ti.com/ docs/training/catalog/events/event.jhtml?sku= its sampling frequency of fS.
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