Analog Devices has introduced industry's most accurate monolithic digital-to-analogue converter.
Designed in Edinburgh, the AD5791 20bit DAC has 1ppm resolution and accuracy, sub 1ppm noise and 1µs refresh.
"It provides four times greater accuracy and four times more resolution than competing converters," claimed ADI. "1ppm analogue system design typically entail combining multiple lower-resolution DACs with a significant number of discrete components and support ICs."
The architecture is R-2R, with proprietary precision thin film resistors built between metals layers, product marketing manager Brendon Cronin told EW.
Underneath, the silicon comes from an enhanced CMOS process.
Accuracy is specified as +/-1 LSB max, and operation is guaranteed monotonic with a +/-1 LSB max differential non-linearity.
Low frequency noise is 0.025ppm low frequency noise and output drift is 0.05ppm/C.
The output can be configured for 5 or 10V unipolar or +/-5 V or +/-10V bipolar output ranges although bipolar operation requires a mixture of on and off-chip components - see green box on diagram.
"The green bit is patented and enables it to go bipolar very easily with an external amplifier. The external amplifier does not have to be optimised for input bias current so the designer does not have to select the best on the market," product marketing manager Brendon Cronin told EW. "There is no net system INL error in our architecture because it is insensitive to external op amp ideality."
Why not integrate the amplifier?
"It could be optimised for noise, drift, or other parameters, but it might not be optimised for what the customer wants," said Cronin.
The device is aimed at controlling 'gradient' coils in MRI scanners, as well as instrumentation, industrial control and test.
Gradient coils are modulated rapidly and precisely to aim the MRI scan.
Current through these coils is set by a precision DACs in what is basically an open-loop system, said Cronin. Before this device, more than one DAC has had to be used, with local feedback and control to get enough accuracy.
Supply voltage is +/-7.5 to +/-16.5V, and the chip is specified from -40 to +125C.
