Analog Angle Article

Monolithic DAC offers 18/20-bit performance, targets high-end MRI and related applications

Norwood, Mass.—Systems which require improved performance and superior image resolution in applications such as scans for magnetic resonance imaging (MRI), as well as other high-end situations, will benefit from the 20-bit performance of the AD5791 digital/analog converter (DAC) from Analog Devices, Inc. Other targeted applications for this DAC include test and measurement systems, high-end scientific and aerospace instrumentation, communications equipment, and industrial control systems where precision and high-resolution performance are priorities.

This increased converter resolution improves image resolution and quality by enabling more precise control of the high-field-strength magnetic gradient, which is a key parameter in overall MRI system performance. A higher-performance converter with guaranteed specifications also reduces the need for complex circuits to compensate for errors and nonlinearities, and drift, as well as the need for special calibration, employing sophisticated algorithms, while the system is taken off line (and is thus non-productive).

Block diagram of AD5791

(Click on image to enlarge)

The monolithic device features 20-bit apparent resolution (18-bit true) with ±2 bits integral nonlinearity (INL), and guaranteed monotonic performance with ±1 LSB maximum differential nonlinearity (DNL). Input to the DAC is via a 3-wire, 50-MHz serial interface which is compatible with SPI, QSPI, Microwire, and various DSP standards. On the output side, the unbuffered voltage output can drive a 60 kΩ load, and settles in under 1 μsec. Internal reset circuitry ensures that the DAC output goes to zero volts at power-on, and is in a known impedance state.

The AD5791 DAC operates from a VDD supply range of 7.5V to VSS + 33V, while the VSS supply range is VDD – 33V to -2.5V, and a VCC supply range of 2.7V to 5.5V. It requires positive and negative external references, with a positive reference of 4V to VDD ” 2.5V, and a negative reference input from VSS + 2.5 V to 0V. Low-frequency noise is 0.025 ppm and output drift is 0.05 ppm/°C; designers can set the output for standard unipolar (+5V, +10V) or bipolar (±5V, ±10V) ranges. Operating temperature range is -40°C to +105°C.—Bill Schweber

Price, packaging, and availability : The AD5791 DAC is available in a 20-lead TSSOP, priced at $38 (1000-piece lots); samples are available now.

For more information : contact Analog Devices, Inc.,♦

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