DESIGN INFORMATION switched output. The monolithic diode bridge is also temperature controlled, providing a bridge offset error below 10µ V, stabilizing the measurement baseline. The temperature control is implemented using uncommitted diodes in the monolithic array as heater and sensor. Figure 5 details considerations for the diode bridge switch. The bridge diodes tend to cancel each other’s temperature coefficient—unstabilized bridge drift is about 100µV/°C and the temperature control reduces residual drift to a few microvolts/°C. Bridge temperature control is achieved by using one diode as a sensor. Another diode, running in reverse breakdown (V Z ≈ 7V), serves as the heater. The control amplifier, comparing the sensor diode to a voltage at its negative terminal, drives the heater diode to temperature stabilize the array. DC balance is achieved by trimming the bridge on-current for zero input– output offset voltage. Two AC trims are required. The “AC balance” corrects for diode and layout capacitive imbalances and the “skew compensation” corrects for any timing asymmetry in the nomi- nally complementary bridge drive. These AC trims compensate small dynamic imbalances that could result in parasitic bridge outputs. Conclusion This concludes part one of this article. Part two, which will appear in the November issue of Linear Technology magazine, details the settling time cir cuitry and presents results. Both parts represent a distillation of a full-length LTC application note, AN74, Component and Measurement Advances Ensure 16-Bit DAC Settling Time. New 16-Bit Bipolar Output DAC in Narrow SO-16 Package by Hassan Malik Linear Technology introduces its first bipolar, voltage output 16-bit digital to analog converter, the LTC1650. The LTC1650 is available in a narrow 16-pin SO package, making it the smallest bipolar, 16-bit voltage output DAC on the market today. The LTC1650 operates from ± 5V supplies and draws 5mA. It is equipped with a rail-to-rail, low noise, deglitched output amplifier that can be configured to operate in a unipolar or bipolar mode. The mid-scale glitch is under 2nV-s and the full-scale settling time in unipolar mode is 4µ s. The LTC1650 is 16-bit monotonic over the industrial temperature range, 5V 3 8 CLK 7 DOUT 5V MICROWIRE is a trademark of National Semiconductor Corp. 4.096V DVDD 11 RSTOUT REFHI 15 10 AVDD 1.0 0.8 POWER-ON RESET SUPPLY SENSE 16-BIT DAC REGISTER 5 16-BIT SHIFT REGISTER DIN cleared. There are supply brown-out detectors on all three supplies, AVDD, DVDD and AV SS. When any of these supplies drops below 2.5V, the part is cleared, connecting the output to VRST, and the RSTOUT pin changes to a logic low. The 3-wire serial interface of the LTC1650 is SPI/QSPI and MICROWIRE™ compatible. All the logic inputs are TTL/CMOS compatible and the CLK input is equipped with a Schmitt trigger that allows direct optocoupler interfacing. There is also a DOUT pin for daisy-chaining several DACs. The digital feedthrough is 0.05nV-s. 9 CLR 2 VRST 16-BIT DAC + 1 VOUT – 0.6 DNL ERROR (LSB) CS/LD with a typical differential nonlinearity of less than ±0.3LSB. Figures 1 and 2 show a typical application for the part and its DNL curve. The LTC1650 is equipped with an outputspan-setting resistor tied to the UNI/ BIP pin. When this pin is tied to the VOUT pin, the output will swing from REFLO to REFHI; when the pin is tied to REFHI, the output swings from –REFHI to REFHI. The LTC1650 has a user-defined voltage to which its output resets on power-up or when the part is cleared. The voltage on the VRST pin is applied to the output through a transmission gate when the part powers up or is 0.4 0.2 0 – 0.2 – 0.4 – 0.6 – 0.8 – 1.0 6 0 16 4 DGND 12,13 REFLO 14 – 5V Figure 1. LTC1650 block diagram Linear Technology Magazine • August 1998 AVSS UNI/BIP 16384 32768 CODE 49152 65535 1650 TA02 1650 TA01 Figure 2. The LTC1650 bipolar output DAC has ±0.3LSB typical DNL. 33