DATA ACQUISITION FEATURE
PICKING UP SPEED and accuracy in data acquisition
Alison Steer, product marketing manager for Mixed Signal Products, Linear Technology now part of Analog Devices, discusses the benefits of modern SAR ADC technology
T
oday the precision analogue to digital converter (ADC) market is served
primarily by delta sigma ADCs due to their high dynamic range, precision DC performance and reasonable pricing. A delta-sigma ADC, by design, oversamples the input signal using a delta-sigma modulator followed by a digital decimation filter, resulting in low noise, but slow output data rates. An additional benefit of the oversampling is that the external analogue anti-aliasing filter can be greatly simplified, relying on the digital filter to determine frequency response in the passband. Linear Technology’s modern SAR ADC
technology brings higher performance to precision applications, rivalling the best delta-sigma ADCs in terms of DC specifications (INL, DNL, offset, gain error and stability) while maintaining high sample rates and no-latency operation. Fast sampling SAR ADCs are often used to oversample low bandwidth signals. Traditional oversampling allows the use of a decimation filter (low-pass filter + down-sampling), which increases the dynamic range of the system. Another benefit of oversampling is the relaxed requirements on the analogue anti- aliasing filter. In the absence of oversampling, the analogue anti- aliasing filter is required to have a steep roll-off (sharp transition band), thereby increasing its complexity. Alternatively, oversampling allows the use of a simple low-order analogue filter in combination with a digital filter to create a mixed-mode equivalent anti- aliasing filter with a very steep roll-off. However the penalty is that the burden of this filtering task is placed on the host processor, and requires a faster processor to acquire the data at a much faster rate from the ADC output. For these reasons, Linear Technology is
approaching the precision market differently; blending the high accuracy and speed of their proprietary SAR ADC architecture with integrated digital filters. The latest products are the LTC2508-32 and LTC2512-24. The LTC2508-32 is a 32-bit 1Msps SAR ADC
INSTRUMENTATION | SEPTEMBER 2017
with an integrated, pin-configurable digital filter optimised for low- bandwidth, precision applications. The LTC2512-24 is a 24-bit, 1.6Msps SAR ADC with integrated filter optimised for higher bandwidth applications. The LTC2508-32 achieves a 145dB dynamic range at the slowest output rate of 61sps, while the LTC2512-24 targets 117dB at 50ksps output rate. A key aspect of delta-sigma ADCs is
that the output of the modulator is not directly usable. That is, it is a low resolution signal with shaped quantisation noise and very low SNR. Various techniques are used to shape a delta-sigma modulator’s quantisation noise, pushing it to a higher frequency where it is more easily filtered, with signals of interest occupying lower frequencies in the filter’s passband. The output of the modulator is then low-pass filtered to produce usable conversion results. However, by the sheer nature of its architecture, a delta-sigma modulator suffers from spurious tones in its output spectrum. Try as they may, spurious tones from the modulator can (and do) make an appearance in the passband. Trying to search for a small signal can be nearly impossible in the presence of
Table 1: Properties of filters in LTC2508-32
these spurs. Successive approximation register (SAR) ADCs do not suffer from this shortcoming, and have a near ideal white noise power spectrum. This would make SAR ADCs a better choice for detecting tones or vibrations at incredibly low energy levels. However, many SAR ADCs still suffer from discontinuities in the DC transfer function at the 16- to 18-bit level, thereby compromising DC performance.
The LTC2508-32 and LTC2512-24 have
Figure 1: Step response of the LTC2508-32
well-behaved linearity characteristics, with zero missing codes. This allows applications to take full advantage of the tremendous dynamic range of the filtered output data. With a noise spectral density of only 22nVRMS/√Hz, the LTC2508-32 offers the lowest noise performance of any competitive ADC solution at 24-bit or 32-bit resolution (Figure 1). Unlike a delta-sigma modulator output, the output of this Linear Technology SAR converter has a flat noise power spectral density, with no tones to contend with. This means that the digital filter can be designed arbitrarily for the end application’s requirements, rather than to filter modulator noise and tones. The LTC2508-32 filter is a “spread-sinc”
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