选择合适的系列电压基准源的绝对精度电压输出

• Using an adjustable reference whose output is always above 4.096V when all circuit tolerances are considered
• Using a force/sense DAC with the gain set slightly higher than necessary
• Adding an output buffer with gain

The MAX6162 reference tempco error is calculated as 625ppm (125°C × 5ppm/°C), and the typical temperature hysteresis value of 80ppm is used directly. The long-term-stability specification is doubled to a more conservative 160ppm, because no burn-in is specified for the application and it is never calibrated once it leaves the factory.

We find Design C's worst-case reference output current variation to be 293μA (2.5V/[14kΩ||14kΩ], remember there are two DACs driven by the reference), which is used directly in the load-regulation calculation:

Load-Regulation Error	= 293μA × 0.9mV / mA = 264μV (max)
	= 106 × 264μV / 2.048V = 129ppm (max)

Because reference-load regulation is proportional to the reference output voltage, it can be calculated at either the voltage reference (264μV/2.048V) or the DAC output ((2 × 264μV)/(2 × 2.048V)).

The power supply is constant in this application, so the line regulation is assumed to be 0ppm. With the bandwidth for Design C specified as 0.1Hz to 10Hz, we use half of the 22μVp-p low-frequency (1/f) noise specification (peak value) to arrive at a noise contribution of 5ppm at the reference output (106 × (22μV/2)/2.048V)). As mentioned previously, we get the same 5ppm answer if the calculation is referred to the DAC output, because the equation is just multiplied by 2.0/2.0.

Moving on to the MAX5154 DAC error terms, the A-grade INL is ±0.5LSB, which is 122ppm on the 12-bit scale. The DAC gain error is ±3LSB(244ppm), but it is ignored because it was already accounted for in the digital reference/DAC gain calibration mentioned earlier in this step and we don't want to count it twice. The MAX5154 gain-error tempco has a typical value of 4ppm/°C, which gives us a total of 500ppm (125°C × 4ppm/°C). The DAC output noise is not specified for the MAX5154, so it is ignored. We recognize that this could present a problem, but our experience with Design B indicates that DAC noise is usually a relatively small contributor to the total error. Measurements can be performed to confirm this assumption.

The worst-case error for Design C is calculated as 1865ppm, and the RSS error is 874ppm. With a target-error specification of 977ppm, the current design is marginally acceptable at best, especially given that some typical values were used and the DAC output noise was not considered. The details of Design C will not be rehashed here, because the important points have already been covered. However, some options for improvement are as follows:

• Use the MAX6191 instead of the MAX6162, because it has better load regulation (0.55μV/μA versus 0.9mV/mA), temperature hysteresis (75ppm versus 80ppm), and long-term stability (50ppm versus 80ppm). The end result would be a 1750ppm worst-case error and an 858ppm RSS error, which is a net change of 115ppm and 16ppm, respectively. This is a slight improvement, but may not be enough.
• Re-examine the overall system-accuracy specifications to determine if any parameters can be relaxed. The existing design could be the best choice in terms of accuracy versus cost.
• Reduce the temperature range if the entire extended range is not needed. For example, if the range can be reduced from -40°C to +85°C down to -10°C to +75°C, the worst-case error drops to 1505ppm and the RSS error becomes 648ppm. This is because much of the error