LMP8640
LMP8640HV
SNOSB28F –AUGUST 2010–REVISED APRIL 2013
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One of the project constraints requires RS<10mΩ, it means that the 20.5mΩ will be discarded and hence the
50V/V and 100V/V gain options are still in play.
Third step – Shunt resistor selection
At this point an error budget calculation, considering the calibration of the Gain, Offset, CMRR, and PSRR, helps
in the selection of the shunt resistor. In the table below the contribution of each error source is calculated
considering the values of the Electrical Characteristics table at 5V supply.
Table 2. Resolution Calculation
ERROR SOURCE
CMRR calibrated ad mid VCM range
PSRR calibrated at 5V
RS = 4.1mΩ
77.9µV
8.9µV
RS = 8.1mΩ
77.9µV
8.9µV
Total error (squared sum of contribution)
Resolution (Total error / RS)
78µV
78µV
19.2mA
9.6mA
Table 3. Accuracy Calculation
ERROR SOURCE
RS = 4.1mΩ
182µV
RS = 8.1mΩ
182µV
Tc Vos
Nosie
216µV
216µV
Gain drift
75.2µV
293µV
151µV
Total error (squared sum of contribution)
Accuracy 100*(Max_VSENSE / Total Error)
320µV
0.7%
0.4%
From the tables above is clear that the 8.2mΩ shunt resistor allows the respect of the project's constraints. The
power burned on the Shunt is 820mW at 10A.
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