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Linear Technology and Maxim Integrated both recently announced high-precision successive approximation register (SAR) architecture analog-to-digital converters (ADCs) for the industrial and instrumentation markets. They’re alike in some ways, though Linear’s is faster with higher precision while Maxim’s has a smaller footprint and integrates its own voltage reference, saving more board space.
Linear’s LTC2378
Linear’s 1-Msample/s LTC2378-20-1 boasts an internal clock and accepts external references that can range from 2.5 to 5.1 V. Normally, the no-latency, 20-bit differential-input SAR ADC operates from a single 5.5-V supply. For power-critical applications, though, it can be operated down to 2.5 V, where it consumes a maximum of 21 mW. To minimize power consumption, the converter automatically powers down between conversions. The reduction scales with sampling rate.
Digital gain compression (DGC) requires the full-scale input swing to be limited between 10% and 90% of the ±VRef analog input range. The SAR ADC driver then can be powered off a single positive supply as each input swings between 0.5 and 4.5 V. Data output is serial via a daisy-chainable serial peripheral interface (SPI) that supports 1.8-, 2.5-, 3.3-, and 5-V logic, leading to power dissipation that scales with sampling rate.
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The LTC2378-20-1 also offers ±2-ppm integral non-linearity (INL) and –0.5 differential non-linearity (DNL) (max), no missing codes at 20 bits, and a 104-dB signal-to-noise ratio (SNR). Its data-sheet guaranteed minimum signal-to-noise plus distortion (SINAD) with a 5-V reference and a 2-kHz frequency is 101 dB, which calculates out to a 16.48 effective number of bits (ENOB).
The ADC’s guaranteed SNR runs from 101 dB with a 5-V reference to 95.4 dB with a 2.5-V reference voltage. Total harmonic distortion (THD) is –114 dB at 5 V and 113 dB at 2.5 V, and its spurious free dynamic range (SFDR) is guaranteed to be 115 dB at 5 V, all measured with a 2-kHz input signal.
The LTC2378-20-1 comes in a 16-lead mini small-outline package (MSOP) and a 4- by 3-mm dual flat no-lead (DFN) package. In lots of 1000 units, it costs $29.50 each. Linear offers less expensive speed grades as well.
Maxim Integrated’s MAX11156
Maxim Integrated’s MAX11156 is an 18-bit, 500-kample/s, differential input SAR ADC with a precision internal reference that allows for measuring a bipolar input voltage range of ±5 V. With an external reference, it can handle input ranges between ±3.05 V and ±5.19 V. A patented charge-pump architecture permits direct sampling of high-impedance sources. The ADC also guarantees 18-bit no-missing codes. The inputs are sampled with a pseudo-differential on-chip track-and-hold that exhibits no pipeline delay or latency.
Like Linear’s SAR, the MAX11156 delivers its output using a daisy-chainable SPI using 2.5-, 3-, 3.3-V, or 5-V logic. Its INL and DNL are defined in terms of the least significant bit (LSB). Its guaranteed maximum and minumum INLs are +8 and –8 (± 2.5 typical), and its guaranteed maximum and minimum DNLs are +0.9 and –0.9 (±0.5 typical).
The MAX11156’s SINAD is 91.5 dB minimum for a 14.9 ENOB. Its SNR is 93 dB, based on the internal reference. Standard packaging is a 12-pin, 3- by 3-mm thin DFN (TDFN). Unit pricing for the MAX11156 with built-in voltage reference is $16.90.
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