System or technology dictates ADC choice
How do you decide which ADC technology to use in your applications before you do a thorough system evaluation?
By Bonnie Baker -- EDN, March 18, 2010
How do you decide which ADC technology to use in your applications before you do a thorough system evaluation? Maybe you prefer SAR (successive-approximation-register) ADCs because you assume they are easy to use and a bit faster than delta-sigma converters. Then again, you may select delta-sigma converters because you assume they are slower but have good resolution. What the heck? Maybe you choose the ADC that you have always used.
When selecting a converter, you usually base your decision on the ENOB (effective number of bits); accuracy; repeatability, or noise; and output data rate. You may assume that SAR ADCs produce accurate outputs with medium output speeds and that delta-sigma converters produce lower-noise output signals with slower output data rates. These assumptions may no longer guide you when deciding between a SAR ADC and a delta-sigma ADC.
Think about changing your design paradigm from focusing on individual devices to considering the complete system. You will find that both ADC architectures might be appropriate for a given application. For instance, if you know the system ENOB, you may find that combining an analog gain stage with a SAR ADC matches the performance of a higher-speed delta-sigma converter.
A system evaluation includes inspecting the system’s sampling speeds, analyzing its accuracy, and comparing its repeatability, or noise-level, capability. To inspect sampling speeds, select a single clock frequency and allow time for the analog components to fully settle before conversion. With system accuracy, you combine the dc performance characteristics into a total-unadjustable-error figure of merit for comparison.
The repeatability evaluation differs from the accuracy evaluation in that it defines how consistently a value from one conversion to the next repeats itself. With a repeatability evaluation, you can combine the noise performance of the signal-chain devices in terms of effective resolution.
|
As we examine the accuracy and repeatability in our system evaluations, we will use Table 1 as our starting point. The circuits in the table encompass handheld-meter, data-logger, automotive-system, monitoring-system, and many other applications. Each system’s gain ranges from one to 128. Column 2 in the table lists the ideal system’s full-scale range referred to the input of the system. The system’s LSB (least-significant bit, Column 3) is equal to the system’s full-scale range divided by the number of system codes: 4096.
In my next column, I’ll delve into the conversion speeds of these designs. In future columns, I’ll examine the differences between a 12-bit SAR ADC, a multiplexed PGA (programmable-gain-amplifier)-SAR ADC, and a 24-bit multiplexed delta-sigma converter. The analog or digital gain range for each system will be 1 to 128V/V, and the power-supply voltage will be 5V. I’ll also investigate the accuracy and repeatability of these systems.
Here is the trillion-dollar question: Which system is best for the listed applications—a PGA-SAR ADC or a delta-sigma ADC? You can reach me at ti_bonniebaker@list.ti.com with your best guess. Be sure to include a description of your application with its basic requirements.
| References |
|
