Texas Instruments MSC1210 Multiplexing Channels, 3.Filter Settling, Samples to Discard Filter

Digital Filter

12.11.1 Multiplexing Channels

When the input changes suddenly, it will take a certain amount of time for the output to correctly represent that new input. The amount of time required to correctly represent the new input depends on the type of filter being used. The filters are designed to settle in 1, 2 or 3 data output intervals. Up to an additional full period is required for an accurate sample because a change usually does not take place synchronous with the data output interval. Due to this uncertainty, as a matter of practice, one more cycle is used before the full resolution is obtained. Refer to Table 12−3 for the number of cycles that must be discarded when the input makes a significant shift.

Table 12−3.Filter Settling

Samples to Discard	Filter
1	Fast settling
2	Sinc2
3	Sinc3

Changing the input multiplexer usually creates the same type of step change on the input. The one significant difference is that the timing for the change is more precisely known.

Auto mode can reduce the amount of data that must be discarded, but it also reduces the resolution. Auto mode selects each of the different filter outputs after the input channel has changed. This means that the output uses the fast settling filter for 2 cycles, then sinc2 for the next cycle and finally sinc3 for all remaining cycles, until the channel is changed again.

When switching channels, the settling time must be factored in to determine the total throughput. For example, if the data rate is 20Hz and the filter is sinc3, then with five channels it will give a resulting data rate on each channel of 20Hz / (4 samples per channel) / 5 channels = 1Hz data rate on each channel.

There are many trade-offs, however, that can be evaluated to determine the optimum setup. One of the first criteria is to determine the desired effective number of bits (ENOB). If 18 bits are needed, the same result could be achieved with all three types of filters. Using sinc3, the decimation would be about 200, using sinc2 it would be about 500, and with the fast-settling filter it would be about 1800. With a modulation clock (or sample rate) of 15 625, Table 12−4 shows the output data and channel rates.

Table 12−4.Output Data Rate and Channel Rate

	Data Rate	Channel Rate	Synchronized
Filter	(Hz)	(Hz)	(Hz)
Sinc3 (dec = 200)	78.125	/4 = 19.53	/3 = 26.04
Sinc2 (dec = 500)	31.25	/3 = 10.41	/2 = 15.625
Fast Settling (dec = 1800)	8.68	/2 = 4.34	/1 = 8.68

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