Datasheet

y
OFFSET
[CHx_SCAL_
OFFSET]
Uncalibrated
VIN
Calibrated
ADC_DOUT
GAIN
[CHx_SCAL_
GAIN]
±
LMP90100
SNAS510P –JANUARY 2011–REVISED MARCH 2013 www.ti.com
A data-flow diagram of these coefficients can be seen in Figure 46.
Figure 46. System Calibration Data-Flow Diagram
There are four distinct sets of System Calibration Offset and System Calibration Gain Registers for use with
CH0-CH3. CH4-CH6 reuse the registers of CH0-CH2, respectively.
The LMP90xxx provides two system calibration modes that automatically fill the Offset and Gain coefficients for
each channel. These modes are the System Calibration Offset Coefficient Determination mode and the System
Calibration Gain Coefficient Determination mode. The System Calibration Offset Coefficient Determination mode
must be entered prior to the System Calibration Gain Coefficient Determination mode, for each channel.
The system zero-scale condition is a system input condition (sensor loading) for which zero (0x00_0000) system-
calibrated output code is desired. It may not, however, cause a zero input voltage at the input of the ADC.
The system reference-scale condition is usually the system full-scale condition in which the system's input (or
sensor's loading) would be full-scale and the desired system-calibrated output code would be 0x80_0000
(unsigned 24-bit binary). However, system full-scale condition need not cause full-scale input voltage at the input
of the ADC.
The system reference-scale condition is not restricted to just the system full-scale condition. In fact, it can be any
arbitrary fraction of full-scale (up to 1.25 times) and the desired system-calibrated output code can be any
appropriate value (up to 0xA00000). The CHx_SCAL_GAIN register must be written with the desired system-
calibrated output code (default:0x800000) before entering the System Calibration Gain Coefficient Determination
mode. This helps in in-place system calibration.
Below are the detailed procedures for using the System Calibration Offset Coefficient Determination and System
Calibration Gain Coefficient Determination modes.
System Calibration Offset Coefficient Determination mode
1. Apply system zero-scale condition to the channel (CH0/CH1/CH2/CH3).
2. Enter the System Calibration Offset Coefficient Determination mode by programming 0x1 in the SCALCN
register.
3. LMP90xxx starts a fresh conversion at the selected output data rate for the selected channel. At the end of
the conversion, the CHx_SCAL_OFFSET register is filled-in with the System Calibration Offset coefficient.
4. The System Calibration Offset Coefficient Determination mode is automatically exited.
5. The computed calibration coefficient is accurate only to the effective resolution of the device and will
probably contain some noise. The noise factor can be minimized by computing over many times, averaging
(externally) and putting the resultant value back into the register. Alternatively, select the output data rate to
be 26.83 sps or 1.67 sps.
System Calibration Gain Coefficient Determination mode
1. Repeat the System Calibration Offset Coefficient Determination mode to calibrate for the channel's system
offset.
2. Apply the system reference-scale condition to the channel CH0/CH1/CH2/CH3.
3. In the CHx_SCAL_GAIN Register, program the expected (desired) system-calibrated output code for this
condition in 24-bit unsigned format.
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