On-Chip Calibration Advantages of New Synchronous SAR ADCs

►As shown in Figure 17a, UUT #1 performs very well in calibration, which is comparable to the on-chip calibration. This means that its actual input impedance (RIN) is very close to the typical value.

► UUT #2 to #4 show some deviation, which means the actual input impedance (RIN) is slightly higher than the typical value.

►On -chip calibration (shown in dark blue in all 4 plots) keeps the total error for all devices and RFILTER values ​​below 0.03%.

When using calibration coefficients in the back-end controller, the actual input impedance of the PGA is not taken into account, which means that differences from device to device will cause post-calibration errors. However, on-chip calibration measures the input impedance internally, so the calibration result is more accurate and is independent of the RFILTER and actual RIN impedance placed in front. This lower post-calibration error helps us achieve a more efficient, easy-to-use and accurate system design, which is another advantage in addition to the advantage of "not having to perform post-processing on each individual ADC data point in the controller to avoid wasting resources."

in conclusion

The resistive input synchronous sampling ADC is a complete solution with all signal chain blocks on chip, providing excellent AC and DC performance, easy to use, and can be directly connected to the sensor. As some applications dictate, external resistors need to be added in front of the analog input. These external resistors will increase the accuracy error of the system, resulting in longer time to market and additional calibration costs. Analog Devices has introduced the new AD7606B series of resistive input ADCs to help solve this problem. The solution includes a larger input impedance and on-chip calibration function to help reduce the errors caused by external resistors.

References

1 Eamonn J. Byrne. U.S. Patent No. 10,312,930: ADC Digital Gain Error Compensation. Analog Devices, Inc., June 2019.

About the Author

Lluis Beltran Gil graduated from the Polytechnic University of Valencia with a bachelor’s degree in electronic engineering in 2009 and a bachelor’s degree in industrial engineering in 2012. After graduation, Lluis joined Analog Devices in 2013 as an applications engineer in the Precision Converters Group in Limerick, supporting the development of temperature sensors. Currently, Lluis works in the SAR ADC applications team in the Precision Converters Group at ADI in Valencia, Spain. He can be contacted at lluis.beltrangil@analog.com.