18-bit ADC offers high resolution

Engineers at Texas Instruments (India) have designed a high-speed, monolithic 18-bit, 500-kHz ADC, functioning at 100 mW, which they claim as an industry-first. The device, called the ADS8381, includes an 18-bit capacitor-based successive approximation register (SAR) ADC with inherent sample and hold.

The chip is targeted at applications in high-end medical instrumentation, IC test equipment, optical power monitoring, and wide dynamic range data acquisition applications that require the handling of a large amount of data at high speed and at a very high resolution. The ADS8381's 18-bit no missing code performance improves image resolution, while the high dynamic range improves the overall efficiency of the data acquisition systems. The ADC has also integrated on-chip a high-performance reference buffer.

While current alternatives to this chip are implemented using an external programmable or fixed gain amplifier to scale the signal before it passes through a low-resolution 16-bit ADC, the ADS8381 does away with this requirement. To speed up the device and increase resolution, TI India has implemented the SAR using three 6-bit DACs.

The ADS8381 offers a full 18-bit interface, and also a 16-bit option where data is read using two read cycles. Also provided is an 8-bit bus option using three read cycles.

The conversion clock for the ADS8381 is generated internally, as the chip uses an internal oscillator generated clock which controls the conversion rate, and in turn the throughput of the converter. No external clock input is required. The architecture of this ADC is based on charge redistribution, which inherently includes a sample/hold function. The conversion time of 1.6 µs is capable of sustaining 500 kHz throughput. The analog input is provided to two input pins, +IN and -IN. When a conversion is initiated, the differential input on these pins is sampled on the internal capacitor array. While a conversion is in progress, both inputs are disconnected from any internal function. The ADS8381's nonlinearity is less than ±9.5 PPM over the first octal input range (±2.5 LSB at the 18-bit level).

The ADS8381 can operate with an external 4.096-V reference for a corresponding full-scale range of 4.096V. It is available in a 48-pin TQFP package and is characterized over the industrial -40° to 85°C temperature range. The ADS8381 is sampling now, with production scheduled for the third quarter of this year. The device will be priced at US$19.99 in 1,000 piece quantities.


Texas Instruments (India)
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