SAR ADCs control the world
Design goals for today’s industrial control systems demand that systems are faster, more accurate and smaller, while consuming less power and providing greater reliability. Designers of these systems must choose components that ensure that these goals are met. One of the central components in an industrial control system is the analog-to–digital converter (ADC).
System designers have several types of ADCs to from which to choose, including pipeline, delta-sigma and SAR (successive approximation register) architectures. Without getting into the details of how each ADC works there are several characteristics that must be considered when selecting an ADC. Pipeline ADCs are capable of very fast conversion times, can digitize very fast input signals with low distortion but draw high supply currents, have poor signal-tonoise ratios (SNR) and pipeline delays (a delay of a fixed number of samples between the instant the input is sampled and the time the data is available). Poor SNR can be overcome with averaging at the expense of lowering the effective sample rate. Pipeline delay obscures the real-time nature of the data, making fine tuning of control loops difficult. Delta-sigma ADCs excel in applications requiring high precision and low noise, but their low sampling rate limits their use to applications near DC. SAR ADCs are capable of conversion rates from a few megahertz down to DC and can handle input signals from DC to tens of megahertz with good SNR and low distortion. SAR ADCs can sample on an as needed basis and then deliver that data without any pipeline delays, providing timely feedback for control systems, resulting in a tight control loop with good transient response.
One example is the LTC2379-18 from Linear Technology, a 1.6 Msps, 18-bit SAR ADC that consumes only 18 mW. The 16-pin 4×3 mm MSOP device operates from a 2.5 V supply and has a fully differential input range of +/-2.5 V to +/-5.1 V that is set by Vref. With a guaranteed total harmonic distortion (THD) less than -115 dB, a signal-to-noise ratio greater than 98 dB, integral nonlinearity (INL) less than 2LSBs, no missing codes at 18-bits and guaranteed operation up to 125˚C, the LTC2379-18 provides the speed, accuracy, low power consumption and reliability required by today’s control systems.
The finer grained control required by modern systems necessitates the ability to measure smaller time increments. This ability to measure smaller time increments, in a real-time loop, is limited by the ADCs’ maximum sampling rate. The maximum sampling rate is inversely related to the sum of the ADC conversion time and acquisition time. Serial ADCs are usually thought of as being slower than parallel ADCs because of the time required to shift out the data. In the case of a serial ADC the data is usually transferred during the acquisition period. If the data transfer time is less than the acquisition time then the serial ADC’s maximum sampling frequency is no slower than a parallel ADC with similar conversion and acquisition times. The LTC2379- 18 has a minimum acquisition time of 200 ns, which is approximately the same as the minimum data transfer time of 180 ns, indicating this part is optimized for maximum sampling frequency.
That same need for finer grained control also necessitates higher accuracy in control systems. When moving up from 16-bit to 18-bit performance, it is important to look at more than just the number of bits. Don’t be fooled by “marketing bits.” Make sure that the ADC is specified with no missing codes over the full temperature range. An ADC with good SNR provides more noise margin when making measurements, which reduces the need for averaging. Less averaging results in control loops with smaller delays, making them more stable. DC applications require good INL and DNL specifications, while AC applications require a good THD specification. The FFT of Figure 1 shows the typical performance of the LTC2379-18, which includes SNR of 101.2 dB and THD of -120 dB.
Figure 1: 32k point FFT shows low distortion and noise of LTC2379-18.
The LTC2379-18 is compatible with the SPI standard and is capable of interfacing with 1.8 V, 2.5 V, 3.3 V and 5 V logic families. A daisy chain mode shown in Figure 2 allows multiple LTC2379-18s to share the SPI and Busy lines, which is useful in instances where large numbers of converters might otherwise make the number of signals required impractical. Chain mode is also useful when synchronizing the data from several channels, which is necessary for maintaining phase information between channels. The Busy line can be eliminated if the digital host is able to wait the maximum conversion time before starting the data transfer, further reducing the line count from four to three.
Figure 2: LTC2379-18 CHAIN mode allows multiple ADCs to communicate with host processor using only four wires.
As control systems become more complex and the number of channels increase as space requirements shrink, reducing system power demands becomes more important. In addition to shrinking operating costs, reducing power requirements also simplifies thermal management. Selecting components with built-in power management features makes reducing power easier. As an example, the LTC2379-18 automatically powers down after a conversion, resulting in even lower power dissipation at low sample rates.
Single supply ADCs usually have an analogue input range that ranges from ground to Vref. Due to driver headroom requirements, this means that the driver driving the ADC requires a supply voltage at least a few hundred millivolts greater than Vref and a negative supply a few hundred millivolts below ground. This is true even with rail-to-raildrivers because distortion increases as the output nears the plus or minus supplies. Until now this meant either running with a single supply and throwing away thousands of codes near ground and V+ to keep distortion low or running on a split supply and consuming more power.
A digital gain compression feature, available on the LTC2379-18, allows a zero to full-scale ADC output swing to be achieved with an input that ranges between 10% and 90% of the +/-Vref analog input voltage. For a 5 V reference, this means that the analog input range is 0.5 V to 4.5 V while still maintaining all 262,144 output codes, as shown in Figure 3. Compressing the analog input range gives the ADC driver more headroom above ground and below the positive supply voltage. This feature allows the LTC2379-18 buffer to be powered from a single supply, resulting in significant power savings.
Reliability is one of the most important goals in designing control systems. Customers want to buy reliable products. Increasing noise margins, making more accurate readings, reducing the number of signal lines and power supplies, lower power consumption and good thermal management all make for a more reliable system. The selection of high quality components, including the ADC is equally important. Care should be taken to ensure that all critical ADC specifications such as INL, DNL, SNR and THD are fully guaranteed, not just typical specifications.
Equally important is that the specifications be guaranteed over the full temperature range in which the system must operate.
The LT6350 can be used to buffer and convert large true bipolar signals which swing below ground to the ±4 V differential input range of the LTC2379-18, with digital gain compression enabled in order to maximize the signal swing that can be digitized. Figure 4 shows the LT6350 used to convert a ±10 V true bipolar signal for use by the LTC2379-18. In this case, the first amplifier in the LT6350 is configured as an inverting amplifier stage, which acts to attenuate and level shift the input signal to the 0.5 V to 4.5 V input range of the LTC2379-18. In the inverting amplifier configuration, the input impedance is set by resistor RIN. RIN must be chosen carefully based on the source impedance of the signal source. Higher values of RIN tend to degrade both the noise and distortion of the LT6350 and LTC2379-18 as a system. Lower values of RIN may be difficult for the signal source to drive. The resistors on the inputs of the first amplifier in the LT6350 must be selected to achieve the desired attenuation, common mode output voltage and to maintain a balanced input impedance. The circuit of Figure 4 has an SNR of 99 dB and a THD of -95 dB.
Figure 4: LTC2379-18 and LT6350 accept a ±10 V input signal while running off a single 6 V supply.
Guy Hoover is applications engineer, mixed signal products at Linear Technology – www.linear.com
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