ZHCSGA6A February 2017 – June 2017 ADS114S06 , ADS114S08
PRODUCTION DATA.
The device uses a very low-drift PGA and modulator in order to provide very low input voltage offset drift. However, a small amount of offset voltage drift sometimes remains in normal measurement. The ADC incorporates a global chop option to reduce the offset voltage and offset voltage drift to very low levels. When the global chop is enabled, the ADC performs two internal conversions to cancel the input offset voltage. The first conversion is taken with normal input polarity. The ADC reverses the internal input polarity for a second conversion. The average of the two conversions yields the final corrected result, removing the offset voltage. The global chop mode is enabled using the G_CHOP bit in the data rate register (04h). Figure 73 shows a block diagram of the global chop implementation. The combined PGA and ADC internal offset voltage is modeled as VOFS.
The first conversion result is available after the ADC takes two separate conversions with settled data. When using the low-latency filter, data settles in a single conversion. When the global chop mode is enabled, the first conversion result appears after a time period of approximately two conversions. When using the sinc3 filter, data settles in three conversions. If the global chop mode is enabled, the first conversion result appears after a time period of approximately six conversions.
In continuous conversion mode with the global chop mode enabled, subsequent conversions complete in half the time as the first conversion completed. Data for alternating inputs are pipelined so that averaging appears on each ADC data cycle. Conversion times using the global chop mode are given in Table 18 and Table 19.
NOMINAL
DATA RATE(1) (SPS) |
FIRST DATA CONVERSION PERIOD
FOR GLOBAL CHOP MODE(2) |
SECOND AND SUBSEQUENT CONVERSION
PERIODS FOR GLOBAL CHOP MODE |
||
---|---|---|---|---|
ms(3) | NUMBER OF
tMOD PERIODS(3) |
ms(3) | NUMBER OF
tMOD PERIODS(3) |
|
2.5 | 813.008 | 208130 | 406.504 | 104065 |
5 | 413.008 | 105730 | 206.504 | 52865 |
10 | 213.008 | 54530 | 106.504 | 27265 |
16.66 | 120.508 | 30850 | 60.254 | 15425 |
20 | 113.008 | 28930 | 56.504 | 14465 |
50 | 40.313 | 10320 | 20.156 | 5160 |
60 | 33.820 | 8658 | 16.910 | 4329 |
100 | 20.313 | 5200 | 10.156 | 2600 |
200 | 10.313 | 2640 | 5.156 | 1320 |
400 | 5.313 | 1360 | 2.656 | 680 |
800 | 2.813 | 720 | 1.406 | 360 |
1000 | 2.313 | 592 | 1.156 | 296 |
2000 | 1.313 | 336 | 0.656 | 168 |
4000 | 0.813 | 208 | 0.406 | 104 |
NOMINAL
DATA RATE(1) (SPS) |
FIRST DATA CONVERSION PERIOD
FOR GLOBAL CHOP MODE(2) |
SECOND AND SUBSEQUENT CONVERSION
PERIODS FOR GLOBAL CHOP MODE |
||
---|---|---|---|---|
ms(3) | NUMBER OF tMOD PERIODS(3) | ms(3) | NUMBER OF tMOD PERIODS(3) | |
2.5 | 2400.508 | 614530 | 1200.254 | 307265 |
5 | 1200.508 | 307330 | 600.254 | 153665 |
10 | 600.508 | 153730 | 300.254 | 76865 |
16.66 | 360.508 | 92290 | 180.254 | 46145 |
20 | 300.508 | 76930 | 150.254 | 38465 |
50 | 120.508 | 30850 | 60.254 | 15425 |
60 | 100.445 | 25714 | 50.223 | 12857 |
100 | 60.508 | 15490 | 30.254 | 7745 |
200 | 30.508 | 7810 | 15.254 | 3905 |
400 | 15.508 | 3970 | 7.754 | 1985 |
800 | 8.008 | 2050 | 4.004 | 1025 |
1000 | 6.313 | 1616 | 3.156 | 808 |
2000 | 3.313 | 848 | 1.656 | 424 |
4000 | 1.813 | 464 | 0.906 | 232 |
In global chop mode, sequences are similar to taking consecutive single-shot conversions and swapping the input on each conversion. Output data are averaged using the last two data read operations by the ADC with the inputs swapped. Figure 74 shows the time sequence for the ADC using global chop mode.
Because the digital filter must settle after reversing the inputs, the global chop mode data rate is less than the nominal data rate, depending on the digital filter and programmed settling delay. However, if the data rate in use has 50-Hz and 60-Hz frequency response notches, the null frequencies remain unchanged.
The global chop mode also reduces the ADC noise by a factor of √2 because two conversions are averaged. In some cases, the programmable conversion delay must be increased, DELAY[2:0] in the gain setting register (03h), to allow for settling of external components.