Improve ADC accuracy by adjusting the reference voltage

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Improve ADC accuracy by adjusting the reference voltage

Abstract: In this design guide, the circuit uses a 10-bit ADC, resistor divider, and an external reference to increase the actual accuracy of the ADC to 13 bits. This article shows an example of scaling a 10-bit ADC to a 13-bit design by voltage scaling. The MAX159 10-bit ADC, the MAX5420 resistor divider, and the MAX6141 voltage reference are described separately.

In order to increase flexibility, the data acquisition board should be suitable for different input voltage ranges. When using the same acquisition circuit to process low-amplitude signals, it is often necessary to increase the resolution by several bits, thus increasing the system cost.

Using the simple circuit presented in this application note, you can increase the actual accuracy to 13 bits with a low-cost 10-bit ADC.

figure 1

The 1 LSB (least significant bit) of the ADC is FSR/2n, where n is the number of bits. The FSR (full scale) depends on the voltage reference amplitude. The MAX159, an external reference, is a low-power, 108ksps serial ADC packaged in μMAX?-8 with an input range of 0 to VDD + 50mV. The wide input range allows the use of reference scaling techniques to accommodate different input ranges.

The low-cost, 3-terminal voltage reference output is scaled by a digitally programmable resistor divider (MAX5420) that provides an accurate voltage divider ratio (1, 2, 4, 8). The partial pressure ratio accuracy is 0.025% to 0.5%, depending on the selected device grade (A, B, C). The voltage division ratio is determined by digital inputs D1 and D0 as follows:

Table 1.

The MAX6141 voltage reference provides a 4.096V output voltage. When the partial pressure ratio is 1, the 1LSB is: 4.096/1024 = 4mV. The voltages corresponding to 1LSB at different partial pressure ratios are shown in the table below.

Table 2.

The effective resolution in this circuit is still 10 bits. However, the actual accuracy is improved compared to the 4.096V FSR system. Even at a partial voltage ratio of 8, one LSB is still larger than the typical noise floor (300 μV) of the converter. Ensure that ADC performance is not limited by the LSB.

μMAX is a registered trademark of Maxim Integrated Products, Inc.