Analog-To-Digital Converters: Part 3 of 6 by Bonnie C Baker, Senior Applications Engineer, Texas Instruments Incorporated
Prior to the introduction of controllers and processors, we used analog circuits to implement all types of filters. You would implement analog filters in hardware, usually with a few amplifiers and some capacitors and resistors. If your filter design was somewhat ineffective, you simply yanked out some of your components and soldered new ones in until the performance was satisfactory.
Then came the digital filter. A digital filter is capable of duplicating the frequency response of any analog filter in the digital domain, which would suggest that the analog filter is dead. Is this too good to be true? In the sprit of goodwill, this TechNote will save you when you still find noise in your system after installing your digital filter. Often you may find that the performance of a digital filter is less than ideal. But, more importantly, you need an analog low-pass filter (LPF) in every circuit where an analog-to-digital conversion occurs. This is true even if you are only measuring dc signals. This is also true if the ADC has a successive approximation register (SAR), dual slope. And, yes, even a delta-sigma (∆-∑, DS) topology. The placement of a LPF in the circuit must always be on the analog side, or in front of the converter.
