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Analog Angle Article

18-bit, 1.6 Msps serial ADC delivers 101 dB SNR with unipolar supply

Given all the justifiable attention that sigma-delta analog/digital converters get, you might think they are the only analog/digital converter (ADC) topology game in town. Not so : the LTC2379 ADC from Linear Technology Corp. uses the long-established successive approximation register (SAR) converter architecture to deliver 18 bits resolution at up to 1.6 Msps throughput, along with other performance numbers that will be hard for many users to even measure at their own bench and board. Target applications include medical, industrial, and automotive designs—in other words, test and measurement or data acquisition—where performance, power, and size must be balanced and optimized.

Among the specifications of interest to users are:

  • 18-bit, no missing code performance
  • 101 dB SNR with 2-kHz input (which LTC claims to be 4 dB better than their closest competitor) for greater dynamic range and improved noise margin (see Figure 1 below)
  • Operation from -40° C to + 125° C
  • No conversion latency
  • ± 2 LSB maximum integral nonlinearity (INL)
  • Fully differential input, with range spanning ± VREF (which can be between 2.5 and 5.1V)
  • Single 2.5V supply operation, with dissipation of 18 mW at 1.6 Msps and 18 μ W at 1.6 ksps
  • I/O via SPI-compatible port, with daisy chain mode supported
  • Internal conversion clock
  • 1.8 to 5.0 V I/O voltages
  • 16-lead MSOP and 4× 3 mm DFN packages

Figure 1: LTC2379-18 FFT plo t

In addition to the specifications, the ADC uses a technique—invoked at the user's option—called digital gain compression (DGC) to scale the input, mapping the zero-scale code to 0.1× VREF instead of 0 V, and mapping the full-scale code to 0.9× VREF rather than VREF . (For a 0 to 5 V input, this translates to an input range of 0.5 to 4.5 V).

Why do this? It allows excellent performance from a single-ended (unipolar) supply, since the ADC driver amplifiers are no longer approaching the supply rails—and normally, these drivers need several tens of mV of headroom to maintain good linearity at the boundaries. By using DGC, the ADC can generate every code from zero to full scale, while relaxing the output-swing requirements of the ADC driver.

Figure 2 shows a ± 10V analog signal chain using the LTC2379-18, using a unipolar 5 to 12 V supply, which eliminates the negative-rail supply and thus greatly recues power dissipation while proving swing headroom for most ADC drivers.

Figure 2:  LTC2379-18 eliminates negative supply using Digital Gain Compression feature

Price, availability, and support : The LTC2379-18 and compatible 16-bit LTC2380-16 (2 Msps, 96 dB SNR, ± 0.5 LSB INL) are available now, priced at $29.95 and $24.50 each, respectively, in 1000-piece lots. Linear Technology also offers a demo reference design which includes their LTC6350 fast-settling ADC driver and LTC6655 precision reference.

For more information : contact Linear Technology Corp., or go to http://www.linear.com/2379.

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