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Understanding SAR and delta-sigma ADC signal path

Posted: 18 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:successive-approximation-register? delta-sigma? ADCs?

Typically, you will find the successive-approximation-register (SAR) 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.

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 0�C) and changes linearly at ~ 0.00385 ohms /Ohms/�C (platinum RTD) and able to sense temperatures from C200 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, nonlinear resistance values over a temperature range from C100 to 175�C. A typical thermistor resistance specification at 25�C 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 VOUTC 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.

SAR-ADC signal path
The SAR converter signal path from the sensor to the microcontroller (?C) or microprocessor (?P) comprises a signal conditioning stage, analog gain stage, anti-aliasing filter, a SAR driver amplifier, and the SAR converter (figure 2).

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.

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