capacitor comparison product brief 10/06/99 page - 1 of 1 product brief audio codec capacitor decoupling performance comparison introduction sigmatel has consistently provided the industry?s highest performance ac?97 audio codecs for applications in pcs, settop boxes, dvd players, pdas, and other audio enabled equipment. recent improvements in the designs from sigmatel now offer the option of significantly reducing the size and number of external decoupling capacitors used to filter internal voltages. audio subsystem costs are now even lower with this option for fewer and smaller external components. the charts below indicate the expected performance levels expected for a typical pci sound card design. these results can also be applied to the increasingly popular motherboard designs with ac?97 audio on-board. it should be noted that motherboards can have additional noise sources not present on sound cards. in the case of a high noise environment, the decoupling capacitor values can be slightly increased typical sound card values to improve performance. cap2 reduction effect the cap2 input on sigmatel codecs is used to decouple the adc reference voltage. the standard value for cap2 is listed as +10uf in most sigmatel documentation. as is apparent from the chart, values down to 0.01uf have no appreciable effect on adc performance. cap2 and vrefcap combined reduction effect the cap2 input on sigmatel codecs is used to decouple the adc reference voltage, while the vrefcap decouples the primary internal reference voltage. the standard value for cap2 and vrefcap is listed as +10uf in most sigmatel documentation. as is apparent from the chart, both capacitors can be reduced to values as small as 0.01uf with only a minor effect on adc performance. sigmatel inc integrating mixed-signal solutions audio performance vs cap2 value 74 76 78 80 82 84 86 88 90 10pf 100pf 1000pf 0.01uf 0.1uf 1.0uf 10uf cap2 value audio performance (db) adc thd+n 0db adc -60db dr adc snr vref decoupled with 1.0uf and 0.1uf audio performance vs cap2/vrefcap value 72 74 76 78 80 82 84 86 88 90 10pf 100pf 1000pf 0.01uf 0.1uf 1.0uf 10uf cap2/vrefcap value audio performance (db) adc thd+n 0db adc snr adc -60db dr vref and cap2 decoupled with same values
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