FEATURE TEST & MEASUREMENT
matching invariably degrades and may limit the ADC’s linearity. The LTC2378-20 achieves high INL
performance by implementing a proprietary architecture that makes the INL independent of CDAC capacitor mismatch. This makes the device robust to the type of temperature variations and package stress effects that are present in harsh industrial environments. Furthermore, the comparator circuit
is designed to balance speed, power and noise, such that the LTC2378-20 achieves a 104dB signal-to-noise ratio (SNR), consuming only 21mW at 1Msps without introducing any cycle latency. The power consumption of the LTC2378- 20 family of SAR ADCs is proportional to the sampling rate, so that they consume only microwatts when operated at 1ksps.
ACCURACY AND SPEED The LTC2378-20 can achieve a level of accuracy previously available only with much slower ADC architectures, such as delta-sigma or multi-slope ADCs. High-channel-count ATE systems often employ slow ADC architectures for precision DC measurements, with multiplexers allowing a single meter to service many inputs. The ADC conversion time can often be adjusted over a wide range to trade speed for resolution. However, measurement resolution is often limited to less than 16 bits at sample rates above 100ksps. The LTC2378-20 can take a million
readings per second, with 2.3ppm noise resolution (standard deviation of noise, 104 dB SNR) of each reading. Results from multiple readings of the same analogue signal may be combined digitally to improve the noise resolution, yielding performance exceeding that of multi-slope ADCs. For example, by averaging blocks of 10 samples, the LTC2378-20 effectively operates at 1Msps/10=100ksps with a 0.7ppm noise resolution (114dB SNR). Delta-sigma and multi-slope ADCs may
36 DECEMBER/JANUARY 2017 | INSTRUMENTATION
The LTC2378-20 configured to simultaneously read and average 10 analogue inputs for a 100ms observation period
be configured to average an input signal during an observation/integration period to suppress noise and interference. An observation period of 100ms is often used to simultaneously suppress 50Hz and 60Hz line-frequency interference, resulting in a throughput of only 10 samples per second. Accordingly, it takes a full second to service 10 multiplexed channels with one multi-slope ADC. The averaging operation can be
performed with a simple adder that is easy to implement in either programmable logic or in a processor. Thus, the LTC2378-20 enables significant increases in measurement speed, while maintaining the key advantages of prior architectures. Because a single LTC2378-20 device can potentially replace several discrete components required for a multi-slope design, a valuable degree of design freedom opens up for balancing cost, board space, and channel count.
CONFIGURABLE ADC SYSTEM SIMPLIFIES SIGNAL CHAIN Some ATE systems may require the ability to evaluate a signal with great precision, and also with some bandwidth. More bandwidth implies more noise, so such systems typically require two digital data streams: one with low noise, low bandwidth and high accuracy; and one with higher noise, higher bandwidth and lower accuracy. The conventional approach is to use separate ADCs for each data stream, where each ADC is optimised for either accuracy or bandwidth/noise. The LTC2378-20 is optimised for both objectives and a single ADC may be used for both data streams. Used in conjunction with a digital back- end, the LTC2378-20 provides high performance at any sampling rate up to 1MSPS. When oversampling a SAR ADC,
anti-aliasing requirements are relaxed just the same as they are for delta-sigma ADCs. Historically, however, SAR ADCs have not been able to match delta-sigma ADCs in terms of linearity. That has now changed. The LTC2378-20 sets a new standard with an improved INL, opening up many new exciting opportunities in ATE applications. Two very different and separately optimised ADCs can be replaced by one high performance SAR ADC, thus simplifying system design.
Linear Technology
www.linear.com
RUGGED DESIGN MAINTAINS MEASUREMENT PERFORMANCE IN TOUGH ENVIRONMENTS
VTI Instruments’ RX1032 Rugged Thermocouple Measurement Instrument has been developed for use in tough environmental conditions. The instrument incorporates advanced cold junction
compensation (CJC) circuitry and thermal management. It has a temperature accuracy of ±0.12°C, data acquisition rates of 1,000 samples/sec/channel and operating ranges from -40°C to +70°C. Military-grade connectors provide reliable power and network connectivity, while all thermocouple and sensor inputs are routed through a protected quick release panel for easy access. Multiple RX1032 instruments can be distributed and synchronised using IEEE-1588 timestamp codes
with typical accuracies of <200μS, ensuring that the acquired data is correlated across the entire test article. The instrument comes with the industry-standard LXI Ethernet interface and IVI compliant software drivers. Its EX10xx application programming interface (API) allows software developed for the laboratory to be easily transported for use in the test cell. The construction of the RX1032 allows the measurement system to be mounted on test pylons, in test
cells or in other harsh environments, making this instrument suitable for a wide range of applications, including large scale engine test, HALT/HASS, rocket motor reliability and wind tunnel tests.
VTI Instruments
www.vtiinstruments.com
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