12-Bit Ultrahigh Speed Multiplying D/A Converter
The AD668 is an ultrahigh speed, 12-bit, multiplying digital-to-analog converter, providing outstanding accuracy and speed performance in responding to both analog and digital inputs. The AD668 provides a level of performance and functionality in a monolithic device that exceeds that of many contemporary hybrid devices. The part is fabricated using Analog Devices' Complementary Bipolar (CB) Process, which features vertical NPN and PNP devices on the same chip without the use of dielectric isolation. The AD668's design capitalizes on this proprietary process in combination with standard low impedance circuit techniques to provide its unique combination of speed and accuracy in a monolithic part.
The wideband reference input is buffered by a high gain, closed loop reference amplifier. The reference input is essentially a 1V, high impedance input, but trimmed resistive dividers are provided to readily accommodate 5 V and 1.25 V references. The reference amplifier features an effective small signal bandwidth of 15 MHz and an effective slew rate of 3% of full scale/ns.
Multiple matched current sources and thin film ladder techniques are combined to produce bit weighting. The output range can nominally be taken as a 10.24 mA current output or a 1.024 V voltage output. Varying the analog input can provide modulation of the DAC full scale from 10% to 120% of its nominal value. Bipolar outputs can be realized through pin-strapping to provide two-quadrant operation without additional external circuitry.
Laser wafer trimming insures full 12-bit linearity and excellent gain accuracy. All grades of the AD668 are guaranteed monotonic over their full operating temperature range. Furthermore, the output resistance of the DAC is trimmed to 100 ohm ± 1.0%. The AD668 is available in four performance grades. The AD668JQ and KQ are specified for operation from 0°C to +70°C, the AD668AQ is specified for operation from -40°C to +85°C, and the AD668SQ specified for operation from -55°C to +125°C. All grades are available in a 24-pin cerdip (0.3" package.