Design Methodology for MFB Filters in ADC Interface Applications MFBFILTER-CALC (ACTIVE)
Design Methodology for MFB Filters in ADC Interface Applications
(ZIP 700 KB)
23 Feb 2005
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While the Multiple FeedBack (MFB) filter topology is well-known, its application to very high dynamic range analog-to-digital converter (ADC) interfaces requires a careful consideration of component value selection. This application report develops the ideal transfer function, then introduces a component selection methodology and discusses its impact on noise and distortion. The impact of the amplifier Gain Bandwidth Product on final pole locations is also included, showing several examples. A complete high dynamic range, differential I/O filter for wide dynamic range ADC driving is then designed. A simple design spreadsheet embodying this design approach is available for download with this application report.
The spreadsheet comprises five related worksheets:
- MFBdesign: This worksheet allows the user to select a part number, set target filter specifications, and design the component values. This design sequence has been shown in the examples used in this application note.
- PartSelection: This worksheet contains the list of amplifiers with their required specifications and allows the designer to select a part and then load the needed data in the MFBdesign sheet. The part number must be entered in the MFBdesign sheet exactly as listed here (NOTE: Selection is case-sensitive). Also, this sheet shows the resulting loop gain plots and final phase margin. This sheet is where a CT value would be selected for noise gain control if a non-unity-gain stable amplifier is selected.
- NoiseGainPlot: The noise gain calculations are performed on this worksheet, with the plot replicated in the PartTable sheet.
- PartAolgain: Each of the device open-loop gain over frequency values are tabulated.
- PartAolphase: Each of the device open-loop phase values are tabulated.