How to choose the right converter based on system and technology

How do you decide which ADC technology to use in your application before you do a thorough system evaluation? You might first choose a SAR converter (SAR-ADC) because you think they are easy to use and a bit faster than a delta-sigma converter. Next, you might choose to use a delta-sigma converter because you think they have better accuracy, even though they are slower. Or without giving it a second thought, you might choose the ADC you use regularly.

When choosing a converter , you usually make some decisions based on the effective number of bits (ENOB), accuracy, repeatability (noise), and output data rate. Your assumption might be that the SAR-ADC produces an accurate output with a moderate output speed, while the DS converter produces a lower noise output signal with a lower output data rate.

These assumptions may no longer guide your choice between a SAR-ADC and a DS-ADC. Think about how you can change your design paradigm—shifting your focus from individual devices to the entire system. You will find that both ADC architectures may be suitable for a particular application. For example, if you know the system ENOB, an analog gain stage combined with a SAR-ADC can match the performance of a high-speed DS converter.

System evaluation includes checking the system sampling speed (a detailed system accuracy analysis) and comparing the repetitive positioning accuracy (noise level) performance of your system. Some issues that affect the system sampling speed are the selection of the single clock frequency and allowing time for the analog components to fully settle before conversion. In terms of system accuracy, you can compare DC performance characteristics in conjunction with the total unadjusted error (TUE) figure of merit. Repetitive positioning accuracy is different from the accuracy assessment, which defines how consistent the value obtained from one conversion is with its next repetition. With repetitive positioning accuracy, you can combine the noise performance of the signal chain components with the effective resolution (ER).

Next time we will look at some of the specific differences between a 12-bit SAR and multiplexed PGA (PGA-SAR) and a 24-bit multiplexed DS converter. All systems have a gain range (analog or digital) of 1 to 128 V/V and a 5V supply voltage.

As we study the accuracy and repeatability of these two systems, we can use Table 1 as a starting point. In Table 1, the system gain ranges from 1 to 128. The second column of the table shows the full-scale range (FSR) of the ideal system, which is the equivalent input (RTI) of the system. Finally, the system least significant bit (LSB, column 3 of the table) is equal to the system's FSR divided by the number of system codes (4096).

Table 1 This table contains ideal FSR and LSB values ​​that can be used when evaluating accuracy (TUE) and repeatability (noise) in system evaluations using gains from 1 to 128.

Some of the applications that the circuits described in this article enable include handheld meters, data loggers, automotive systems, and monitoring systems. Next time, we will take a closer look at the conversion speed of these two designs. Later, we will study the accuracy (TUE) and repeatability (noise) of these systems.