校准偏置注册的DS2761-Calibrating the

Number of Readings	128	64	32	16	8	4	2	1
Device 1
Average Current (LSbs)	-1.341	-1.370	-1.414	-1.403	-1.510	-1.775	-2.000	-2.800
Max Offset (LSbs)	-1	-1	-1	-1	-1	0	0	0
Min Offset (LSbs)	-1	-1	-1	-2	-1	-1	-1	-2
Device 2
Average Current (LSbs)	-0.864	-0.873	-0.858	-0.913	-1.010	-1.080	-1.240	-1.640
Max Offset (LSbs)	-1	-1	-1	-1	-1	0	0	0
Min Offset (LSbs)	-1	-1	-1	-1	-1	-1	-1	-2
Device 3
Average Current (LSbs)	-0.386	-0.406	-0.403	-0.426	-0.460	-0.455	-0.730	-0.800
Max Offset (LSbs)	0	0	0	0	0	0	0	0
Min Offset (LSbs)	0	0	0	-1	-1	-1	-1	-2
Device 4
Average Current (LSbs)	-0.347	-0.332	-0.374	-0.331	-0.373	-0.405	-0.540	-0.740
Max Offest (LSbs)	0	0	0	0	0	0	0	0
Min Offset (LSbs)	0	0	0	-1	-1	-1	-1	-2

4. Determine the new offset value

Calculate the average of the 32 readings taken in Step 3. If the decimal portion of the average current is greater than 0.5 LSbs, then round the magnitude up to the next integer, otherwise, round down. For example, if the average is '1.45' round down to '1' and if the average is '1.55' round up to '2'. (Or if the average is '-1.45' round up to '-1' and if the average is '-1.55' round down to '-2'). This example uses a starting point of 0x00h, so the new offset value is simply the rounded average. If the starting point was something different, the rounded average would be added to the starting point to get the new offset value.

As an example, examine the data shown in Table 1 for Device 1. The starting point was 0 and the average of 32 readings of the current register was -1.414 LSbs. The average would be rounded to -1 and the two's compliment value of -1 which is 0xFFh would be written to the Offset Register. If the starting point was -2 then the average of 32 readings would have given .586 LSbs. The rounded average would be +1 LSb, which would be added to the starting point of -2, to obtain the new offset value of -1 and 0xFFh would again be written to the Offset Register.

When properly calibrated, the Offset Register will typically have a value in the range of +2 LSbs to -4 LSbs (in two's complement format that range is 0x02h to 0xFCh). It is possible to offset up to ±127 LSbs, however, if the value of the Offset Register is larger than ±10 LSbs, it may be necessary to verify the circuit for any leakage current and verify that the current is being read properly.

5. Write and copy the new offset value

The value that was calculated in Step 4 should then be written to the Offset Register in two's complement format and copied to EEPROM.

6. Verify accuracy

With the new value in the Offset Register, Steps 2 and 3 may be repeated to verify the accuracy of the calibration. Using ±.5 LSbs as the rounding point in Step 4, the user assures the average current of being within ±1 LSb when reading variations are taken into account.

Summary

The Current A/D of the DS2761 is capable of measuring a 15.625μV drop across the sense resistor. To ensure this accuracy, the current offset should be calibrated after assembly into the pack. For greatest accuracy, multiple measurements should be made over the longest acceptable period of time with the control FETs disabled and all loads removed. This will guarantee no extraneous current is flowing in the system during calibration and that a precise calibration may be obtained.