By Len Staller
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Embedded Systems Design
(02/24/05, 05:24:00 PM EST)
Reading ADC Specification Numbers
The ADC specifications posted in data sheets serve to define the performance of an ADC in different types of applications. The engineer uses these specifications to define if, how, and in what way the ADC should be used in an application. Performance specifications can also be a guarantee that an ADC will perform in a certain way. If a specification is labeled as a maximum or minimum, this is implied. For example, in the ADC specification shown in Table 1, the data sheet excerpt gives an INL error maximum of 1 LSB. This should mean the manufacturer has tested the ADC and is stating that INL error should not be greater or less than 1 LSB. Besides minimum and maximum, specifications listed as typical are also given. This is not a guarantee but simply represents typical performance for that ADC. For example, if a data sheet specifies 2 LSB INL in the "Typical" column, there's no implied guarantee that the engineer won't find the ADC with higher INL error.Though a typical number is not a guarantee, it should give the designer an idea of how the ADC will perform, since these numbers are generally derived from the manufacturer's characterization data or are expected by design. Typical numbers are more helpful when the manufacturer gives the standard deviation from the mean of the tested specification. This gives the engineer more information on how the ADC's performance can be expected to deviate from the numbers posted as typical. Keep this in mind when comparing ADC data sheets, especially if the specification is critical to your design. An ADC with a typical 2 LSB INL may yield higher INL error than expected, making a 12-bit ADC effectively a 10-bit ADC—caveat emptor!
Len Staller serves as an applications engineer for Silicon Laboratories' microcontroller products. Previously, he was an applications engineer for Cygnal Integrated Products, which was acquired by Silicon Laboratories in 2003. Staller has a bachelor's degree in electrical engineering from The University of Texas at Austin. He can be reached at Len.Staller@silabs.com.
Reference:
- http://www.embended.com/
- http://en.wikipedia.org
