Datasheet

www.ti.com Weigh Scale Mode
Obtaining a calibration weight:
Before the ADS1x31REF can display the mass of an object, it must measure the output of the load cell for
a previously known mass. The known mass can be adjusted.
At power-up, the ADS1x31REF expects a 5kg mass. If this mass is not available, or if the load cell range
is not compatible with this mass, the calibration mass can be changed in the following manner:
1. Determine the mass of the calibration weight. The most accurate way to determine the mass is to
weigh the calibration object on an accurately calibrated scale. If the weight is precalibrated, its given
mass can be used, although this approach is not generally as accurate. (Note that the accuracy of the
ADS1x31REF as a scale directly depends on the accuracy to which the calibration weight can be
measured.)
2. Switch the mode switch to SCALE mode.
3. Press the PARM buttons simultaneously. The ADS1x31REF enters configuration mode.
4. Use the PARM buttons to select the screen which shows UNIT =. This screen allows you to select the
units used for the calibration mass. If the units shown are not correct, adjust them using the VALUE
buttons.
5. Use the PARM buttons to select the screen that shows CW= followed by the calibration mass. On this
screen, you can adjust each digit of the calibration mass separately. Select the digit using the PARM
buttons. The currently-selected digit flashes, and can be adjusted using the VALUE buttons.
6. Adjust the calibration mass to match the mass of the calibration weight.
7. Press the PARM buttons simultaneously to exit the configuration mode.
Preparing Scale mode:
Do the following steps to set up the scale mode:
1. Connect the load cell.
2. If the input shorting jumpers J8 and J7 are connected, disconnect them (see Figure 1).
3. Obtain a weight of known mass.
4. If the mass of the weight is not equal to the configured calibration mass, adjust the calibration mass as
described above.
5. Move the mode switch to SCALE position. If calibration has not been performed, the display reads
NO CAL.
6. Press the CAL button, and follow the calibration procedure given in Section 2.3.
If the calibration is performed properly, and the load cell is connected correctly, the ADS1x31REF will
measure the mass of an object placed on the load cell (provided that the mass of the object is within the
device range).
Weigh scale mode is described in detail in Section 2.
2 Weigh Scale Mode
In weigh scale mode, the ADS1x31REF displays mass.
(1)
Mass is displayed in either SI or avoirdupois
units based on the voltage received from a load cell.
The ADS1x31REF operates in Scale mode when the mode switch is set to the Scale position.
To accurately calculate mass, the ADS1x31REF must have calibration information for the load cell. When
scale mode is first entered, the ADS1x31REF displays NO CAL, because the ADS1x31REF has no
calibration data when it is powered on.
By default, mass is calculated from the average of four successive readings from the ADC. The number of
points for averaging can be adjusted, and averaging can be turned off.
(1)
Load cells do not measure mass directly; they output a voltage proportional to the weight of an object. The mass of an object can be
accurately inferred from this voltage as long as a calibration is accurately performed, the downward force of gravity remains constant (as
it does if the load cell is not moved to a different altitude), and the tilt of the load cell does not change; if either of the latter conditions
change, a new calibration must be performed.
7
SBAU175A–July 2010–Revised August 2011 ADS1131REF and ADS1231REF
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