AD767 USING THE AD767 WITH THE AD588 HIGH PRECISION VOLTAGE REFERENCE
The AD767 is specified for gain drift from 15 ppm/°C to 30 ppm/°C (depending on grade) using its internal 10 volt reference. Since the internal reference contributes the majority of this drift, an external high-precision voltage reference will greatly improve performance over temperature. As shown in Figure 4, the 10 volt output from the AD588 is used as the reference. With a 1.5 ppm/°C output voltage drift the AD588 contributes less than 1/2 LSB gain drift when used with the AD767 over the industrial temperature range. Using this combination may result in apparent increases in full-scale error due to the differences between the internal reference by which the device is laser trimmed and the external reference with which the device is actually applied. The AD767 internal reference is specified to be 10 volts ± 100 mV whereas the AD588 is specified as 10 volts ± 1 mV. This may result in up to 101 mV of apparent full-scale error beyond the ± 25 mV specified AD767 gain error. The 500 Ω potentiometer in series with the reference input allows adequate trim range to null this error.
Figure 5a. Large Scale Settling
Figures 5b and 5c show the settling time for the transition from all bits on to all bits off. Note that the settling time to ± 1/2 LSB for the 10 V step is improved from 2.4 microseconds to 1.6 microseconds by the addition of the 20 pF capacitor.
The AD767 brings out separate analog and power grounds to allow optimum connections for low noise and high-speed performance. These grounds should be tied together at one point, usually the device power ground. The separate ground returns are provided to minimize current flow in low-level signal paths. The analog ground at Pin 5 is the ground point for the output amplifier and is thus the “high quality” ground for the AD767; it should be connected directly to the analog reference point of the system. The power ground at Pin 12 can be connected to the most convenient ground point; analog power return is preferred. If power ground contains high frequency noise beyond 200 mV, this noise may feed through the converter, thus some caution will be required in applying these grounds.
Figure 5b. Fine-Scale Settling, CF = 0 pF
It is also important to apply decoupling capacitors properly on the power supplies for the AD767. The correct method for decoupling is to connect a capacitor from each power supply pin of the AD767 to the analog ground pin of the AD767. Any load driven by the output amplifier should also be referred to the analog ground pin. OPTIMIZING SETTLING TIME
The dynamic performance of the AD767’s output amplifier can be optimized by adding a small (20 pF) capacitor across the feedback resistor. Figure 5 shows the improvement in both large-signal and small-signal settling for the 10 V range. In Figure 5a, the top trace shows the data inputs (DB11–DB0 tied together), the second trace shows the CS pulse, and the lower two traces show the analog outputs for CF = 0 and 20 pF respectively.
Figure 5c. Fine-Scale Settling, CF = 20 pF
Figures 5d and 5e show the settling time for the transition from all bits off to all bits on. The improvement in settling time gained by adding CC = 20 pF is similar.
Figure 5d. Fine-Scale Settling, CF = 0 pF