I'd like to confirm if there is an 50/60Hz line freq filter inside the device. Customer is concerned when input is coupled with low freq. noise. as i know, some of ADC coming with 50/60Hz filer inside.
Regards
Brian
Hello Brian,
You can get very good 50Hz or 60Hz line frequency rejection by using the SINC3 filter, but you are limited to either 50sps (for 50Hz rejection) or 60sps (for 60Hz rejection). There are other filter settings that can provide some amount of line frequency rejection, but due to tolerance of the power line frequency, the attenuation will not be nearly as good as that provided by the SINC3.
In most cases, the ADS127L11 is used at much higher data rates, and in these cases, there will be little or no attenuation of power line frequencies and a post-digital filter would need to be used to eliminate.
Table 8-7 in the datasheet provides the details for power line rejection when using the SINC3 filter.
Regards,
Keith Nicholas
Precision ADC Applications
Hello Brian,
It depends on which filter and data rate are used with the ADS126x. If using the ADS126x SINC3 filter, then both ADCs will have similar 50/60Hz rejection performance when operating at data rates of 50sps (50Hz) or 60sps (60Hz). The SINC1 and SINC2 filters inside the ADS126x will not be as good, and the SINC4 filter will be better than the ADS127L11.
Since power line frequency can vary by +/-3% or more in different regions of the world, the width of the filter will determine the minimum rejection of power line frequency noise.
The best way to compare performance is to look at the filter response. The below filter response is for the ADS126x (Figures 9-20 and 9-21):
The ADS127L11 SINC3 filter has approximately the same response as the ADS126x SINC3 filter, as shown below in Figure 8-18.
In addition to the SINC filter options inside the ADS126x, the ADS126x also has a simultaneous 50Hz and 60Hz filter option, which the ADS127L11 does not include. Using this filter at 20sps, you can get good rejection for both 50Hz and 60Hz.
Regards,
Keith
