Datasheet

-5 5
15 25 35
45 55
65
75
85 95 105
Time(ms)
8200
8190
8180
8170
8160
8150
8140
8130
8120
8110
8100
8090
8080
8070
8060
8050
OutputCode(LSB)
OFFSETCORRECTION
TimeResponse
8181
Offsetof
10LSBs
8192
Finalconvergedvalue
Offsetcorrection
convergestooutput
codeof8192
Offsetcorrection
begins
ADS4126 , ADS4129
ADS4146 , ADS4149
SBAS483G – NOVEMBER 2009–REVISED JANUARY 2011 www.ti.com
OFFSET CORRECTION
The ADS412x/4x has an internal offset corretion algorithm that estimates and corrects dc offset up to ±10mV.
The correction can be enabled using the EN OFFSET CORR serial register bit. Once enabled, the algorithm
estimates the channel offset and applies the correction every clock cycle. The time constant of the correction
loop is a function of the sampling clock frequency. The time constant can be controlled using the OFFSET CORR
TIME CONSTANT register bits, as described in Table 12.
Table 12. Time Constant of Offset Correction Loop
TIME CONSTANT, TC
CLK
OFFSET CORR TIME CONSTANT (Number of Clock Cycles) TIME CONSTANT, TC
CLK
× 1/f
S
(sec)
(1)
0000 1M 4ms
0001 2M 8ms
0010 4M 16.7ms
0011 8M 33.5ms
0100 16M 67ms
0101 32M 134ms
0110 64M 268ms
0111 128M 537ms
1000 256M 1.1s
1001 512M 2.15s
1010 1G 4.3s
1011 2G 8.6s
1100 Reserved —
1101 Reserved —
1110 Reserved —
1111 Reserved —
(1) Sampling frequency, f
S
= 250MSPS.
After the offset is estimated, the correction can be frozen by setting FREEZE OFFSET CORR = 1. Once frozen,
the last estimated value is used for the offset correction of every clock cycle. Note that offset correction is
disabled by a default after reset.
After a reset, the device is in low-latency mode and offset correction is disabled. To use offset correction:
• First, disable the low-latency mode (DIS LOW LATENCY = 1).
• Then set EN OFFSET CORR to '1' and program the required time constant.
Figure 123 shows the time response of the offset correction algorithm after it is enabled.
Figure 123. Time Response of Offset Correction
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