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Analog Angle Article

Vendor claims DAC is first true 18-bit monolithic device

Milpitas, CA—You can have as many apparent bits in your converter as you want, but are they real? The LTC2757 digital/analog converter (DAC) from Linear Technology Corp. does one thing, and claims to do it very well: the vendor maintains that this is the first true 18-bit DAC available. Key to the “true” claim are the tight DC specifications, as well as AC specifications: maximum ±1 LSB (least significant bit) INL (integral nonlinearity) and ±1 LSB DNL (differential nonlinearity) of the device, thus guaranteeing monotonicity over the full temperature range (0°C to 70°C, or –40°C to 85°C, depending on grade), along with full-scale settling time of 2.1 μsec.

The converter is focused on high-performance applications such as instrumentation, data acquisition, automated test equipment (ATE), and medical devices. Note that designers who require true 18-bit performance, but are using 16-bit DACs, must employ multiple DACs in conjunction with complex calibration schemes.



Figure 1: Block diagram of LTC2757 DAC

(Click on image to enlarge)

The parallel-load DAC (see block diagram above) allows the user to select one of six output ranges (unipolar and bipolar, with a widest-span of ±10 V) via software or pin-strapping; the DAC also features a current-mode output to which the user can add his or her own amplifier matched to the application's special requirements. The DAC also allows the user to read back the loaded internal register, again via the parallel interface.

In addition to the true 18-bit performance (see performance graph below), the analog-output glitch impulse is 1.4 nV-sec (3 V) and 3 nV-sec (5 V), commensurate with the demands of the resolution step size. Maximum supply current is 1 μA, and the DAC operates from either a 2.7 V or 5.5 V unipolar supply. The user can also establish offset and gain trim, via controlling voltage inputs. On power-up, the DAC output automatically settles at 0 V, regardless of selected output range.&#8212by Bill Schweber



Figure 1: Integral nonlinearity (INL) of LTC2757 DAC

(Click on image to enlarge)

Packaging, price, and availability : The LTC2757 DAC is available in a 7 mm × 7 mm, 48-lead plastic LQFP package, and is priced beginning at $25.50 in 1000-piece lots; it is available now.

For more information : contact Linear Technology Corp., http://www.linear.com.

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