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ADC Basics, Part 2: SAR & Delta-Sigma ADC Signal Path

Generally, you will find the SAR (successive-approximation-register) and delta-sigma (??) analog-to-digital converters (ADCs) in lower frequency applications. The signal chain for these applications starts with the sensor that usually produces a low output voltage, or current signal. These signals require amplification and filtering before digitization.

We will find that the SAR and delta-sigma converter signal paths handle the conditioning of this low sensor signal in dramatically different ways. In this article, I elaborate on the signal characteristics of several representative sensors and signal chain components for each converter type. As we look at these signal characteristics, we will come to terms with the application demands placed on each converter type and how those converters rise to the occasion.

Sensor electrical characteristics

The signal chain for sensor applications starts with the sensor. Figure 1 shows several sensors that take advantage of their environment by changing what they see or feel into an electrical signal.


Click on image to enlarge.

Figure 1. Typical sensors used in SAR and delta-sigma ADC signal conditioning circuits.

The modeling symbol for the RTD (resistor-temperature-device) and thermistor is a resistor. The RTD resistance is relatively small (typically 100 ohms at 0ºC) and changes linearly at ~ 0.00385 Ohms /Ohms/ºC (platinum RTD) and able to sense temperatures from –200 to 800?C. Note the small change in the RTD resistance per degree Celsius. The appropriate maximum excitation current source for the RTD element is ~1 mA. Following a conversion from resistance to voltage, the RTD signal will require further amplification.

The negative temperature coefficient (NTC) thermistor (thermally sensitive resistor) generates higher, non-linear resistance values over a temperature range from –100 to 175ºC. A typical thermistor resistance specification at 25ºC is 10 kOhm. One generates a measurable thermistor voltage by building a simple voltage divider across the power supply.  

The construction of a thermocouple uses two dissimilar metals such as chromel and constantan (type E) or nicrosil and nisil (type N). The two dissimilar metals are bonded together at one end of both wires with a weld bead. The exposure of the bead to a thermal environment creates a temperature difference between the bead and the other end of the thermocouple wires. In this environment, an electromotive force (EMF) voltage appears between the two wires. The EMF voltage can range in the tens of millivolts region across their temperature ranges. However, the delta EMF voltage compared to a one degree Celsius change is in the tens of microvolts. The thermocouple also requires a signal gain in the signal path prior to digitization.

Engineers use resistive bridge circuits to model pressure and load sensors. When a positive pressure or load is applied to the four-element bridge, two of the opposing elements respond by compressing and the other two change to a tension state. The designer can apply a voltage or current excitation source to the high side of this resistive bridge. Although the magnitude of excitation affects the dynamic range of the sensor output, the maximum difference between VOUT+ and VOUT– generally ranges from tens to several hundred millivolts.

Photodiodes and their associated preamps are the bridge between a basic optical event and electronics. Photosensing circuits are used in systems such as CT scanners, blood analyzers, smoke detectors, position sensors, IR pyrometers, and chromatographs. In these circuits, photodiodes generate a small nanoamp to microamp current, which is proportional to the level of illumination. A preamplifier converts the current output signal of the photodiode sensor to a usable voltage level.

All sensors described require excitation sources and signal conditioning circuitry to transform their small signals into useable voltage levels for the ADC at the end of the signal path. The remainder of this article describes the general signal chain for the SAR-ADC and delta-sigma-ADC.

The SAR-ADC signal path

The SAR converter signal path from the sensor to the microcontroller (µC) or microprocessor (µP) comprises a signal conditioning stage, analog gain stage, anti-aliasing filter, a SAR driver amplifier, and the SAR converter (Figure 2 ).


Click on image to enlarge.

Figure 2. SAR-ADC signal path: Biased, sensor small-signals are amplified and filtered to provide an appropriate input signal to the SAR converter.

The signal conditioning stage provides sensor-biasing and level-shifting, as needed. The purpose of the analog gain stage in this signal path is to match the sensor’s voltage output range to the SAR converter’s input range. In doing this a designer can take full advantage of the possible number of bits in the converter. For instance, if your sensor produces a total output range of 50 mV and the input range of the ADC is five volts, the desired analog gain for this stage is ~100 V/V. The type of analog devices that fit into this circuit position are operational or instrumentation amplifiers.

In Figure 2 , an anti-aliasing filter (AAF) follows the analog gain stage. This third-to-fifth order anti-aliasing (analog) filter removes superimposed analog signal noise before it reaches the SAR converter. Before the signal changes to a digital word, the AAF reduces the high frequency noise that could be aliased back into the digital signal. Analog filtering is a critical portion of the data acquisition system. If an analog filter is not used, signals outside half of the ADC’s sampling bandwidth are aliased back into the signal path. Once a signal is aliased during the digitalization process, it is impossible to differentiate between noise with frequencies in-band and out-of-band.

The SAR driver amplifier presents the signal to the SAR-ADC. This driver amplifier provides a stable signal to the converter with ample driving capability. Once the SAR-ADC derives a final digital word, the microcontroller or microprocessor further filters the signal or translates that signal to a usable digital value. SAR converters are frequently the architecture of choice for medium-resolution applications. SAR A/D converters range in resolution from eight to 18 bits with sampling speeds less than 10 Msps.

The delta-sigma-ADC signal path

Delta-sigma converters are ideal for converting signals over a wide range of frequencies from DC to several megahertz with very high resolution. Figure 3 shows the basic sensor signal path for a delta-sigma ADC. These converters range in resolution from 12 to 24 bits with sampling speeds as high as 50 Msps. Notice that this block diagram seems to be simpler than the SAR-ADC signal path in Figure 2 , but that is far from true. All functions described for the SAR-ADC signal path remains. However, they have been absorbed inside the delta-sigma converter.


Click on image to enlarge.

Figure 3. Delta-sigma ADC signal path: Small sensor signals are filtered to provide an appropriate input signal range to the delta-sigma converter.

The digital or process gain replaces the analog gain cell in Figure 2 . One can achieve process gain by moving the position of the LSB in the digital output word. If you think about it, there are 4096 individual 12-bit converters across the output range of a 24-bit converter. You can also use the 24 bits of the delta-sigma converter to substitute the analog functions of gain and level shift into this digital engine.

The “easy-to-design” anti-aliasing filter for the delta-sigma converter attenuates the major higher frequency noise contributors inside the analog input signal. The corner frequency for this resistive/capacitive anti-aliasing filter is the delta-sigma’s output data frequency.

Further capability of the delta-sigma converter provides sensor excitation sources and internal analog gain stages.

Conclusion

In this article, the general characteristics of various sensors and the basic SAR-ADC and delta-sigma ADC signal paths are shown. In this general overview it appears that the delta-sigma ADC provides the signal path of choice. This type of converter has the least amount of signal conditioning circuitry. However, I encourage you to not jump to a conclusion so fast. As they say, the devil is in the accuracy, repeatability, and cost details when selecting either converter. The next article of this series will dive into the inner-workings of the SAR-ADC.

References
1. “System or technology dictates ADC choice,” Baker, Bonnie, EDN, March 18, 2010.
2. “Delta-sigma ADCs in a nutshell,” Baker, Bonnie, EDN, December 14, 2007.
3. Baker, B., “A Baker's Dozen: Real analog solutions for digital designers,” Burlington, MA: Elsevier/Newnes, 2005.
4. For more information about these and other ADCs, visit www.ti.com/adc-ca.

Related article:
ADC Basics (Part 1): Does your ADC work in the real world?

1 comment on “ADC Basics, Part 2: SAR & Delta-Sigma ADC Signal Path

  1. ljsiodfjwe
    September 28, 2015

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