AD767 The AD767 reference output should be buffered with an external op amp if it is required to supply more than 0.1 mA output current. The reference is typically trimmed to ± 0.2%, then tested and guaranteed to ± 1.0% max error. The temperature coefficient is comparable to that of the full-scale TC for a particular grade.
BIPOLAR CONFIGURATION (Figure 3)
This configuration will provide a bipolar output voltage from –5.000 to +4.9976 volts, with positive full scale occurring with all bits ON (all 1s). STEP I … OFFSET ADJUST Turn OFF all bits. Adjust 100 Ω trimmer R1 to give –5.000 volts output.
If an external reference is used (10.000 V, for example), additional trim range must be provided, since the internal reference has a tolerance of ± 1%, and the AD767 full-scale and bipolar offset are both trimmed with the internal reference. The gain and offset trim resistors give about ± 0.25% adjustment range, which is sufficient for the AD767 when used with the internal reference.
STEP II … GAIN ADJUST Turn ON all bits. Adjust 100 Ω gain trimmer R2 to give a reading of +4.9976 volts. STEP III … BIPOLAR ZERO ADJUST (Optional) In applications where an accurate zero output is required, set the MSB ON, all other bits OFF, and readjust R1 for zero volts output.
It is also possible to use external references other than 10 volts. The recommended range of reference voltage is from +8 to +10.5 volts, which allows both 8.192 V and 10.24 V ranges to be used. The AD767 is optimized for fixed-reference applications. If the reference voltage is expected to vary over a wide range in a particular application, a CMOS multiplying DAC is a better choice. Reduced values of reference voltage will also permit the ± 12 volt ± 5% power supply requirement to be relaxed to ± 12 volts ± 10%. It is not recommended that the AD767 be used with external feedback resistors to modify the scale factor. The internal resistors are trimmed to ratio-match and temperature-track the other resistors on the chip, even though their absolute tolerances are ±20%, and absolute temperature coefficients are approximately –50 ppm/°C. If external resistors are used, a wide trim range (± 20%) will be needed and temperature drift will be increased to reflect the mismatch between the temperature coefficients of the internal and external resistors.
Figure 3. ± 5 V Bipolar Voltage Output INTERNAL/EXTERNAL REFERENCE USE
The AD767 has an internal low-noise buried Zener diode reference which is trimmed for absolute accuracy and temperature coefficient. This reference is buffered and optimized for use in a high-speed DAC and will give long-term stability equal or superior to the best discrete Zener reference diodes. The performance of the AD767 is specified with the internal reference driving the DAC since all trimming and testing (especially for full-scale error and bipolar offset) is done in this configuration.
Small resistors may be added to the feedback resistors in order to accomplish small modifications in the scaling. For example, if a 10.24 V full scale is desired, a 140 Ω 1% low-TC metal-film resistor can be added in series with the internal (nominal) 5k feedback resistor, and the gain trim potentiometer (between Pins 6 and 7) should be increased to 200 Ω. In the bipolar mode, increase the value of the bipolar offset trim potentiometer also to 200 Ω.
The internal reference has sufficient buffering to drive external circuitry in addition to the reference currents required for the DAC (typically 0.5 mA to Ref In and 1.0 mA to Bipolar Offset). A minimum of 0.1 mA is available for driving external loads.
Figure 4. Using the AD767 with the AD588 High Precision Reference