INA282, INA283
INA284, INA285
INA286
SBOS485A –NOVEMBER 2009–REVISED JULY 2010
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COMMON-MODE DYNAMICS AND CURRENT DIFFERENCING
Current sensing is frequently used on totem-pole output stages, such as those of bridge-type motor drives. We
can sense current in one of three locations on a totem-pole output: on the ground side (low-side sensing); on the
power-supply side (high-side sensing); or on the output (phase sensing). Only the output line reports the exact
load current. Obviously, the ground and supply-side sensing report only the current in the individual respective
phases. Figure 42 depicts these various methods on a three-phase motor driver.
Motor
High Side
Current
Sense
High Side
Current
Sense
High Side
Current
Sense
Phase
Current
Sense
Phase
Current
Sense
Phase
Current
Sense
Low Side
Current Sense
Low Side
Current Sense
Low Side
Current Sense
NOTE: Motor drive current sensing can be done on the low side, phase side, or high side. Only the phase output gives
complete information regarding current in the motor, but is subject to common-mode transients that even the best
amplifiers do not reject completely.
Figure 42. Motor Drive Current Sensing
However, sensing on the output is subject to large common-mode voltage steps that result in feedthrough in
even the best amplifiers. The ground and supply-side sensing configurations are free of this problem, thanks to
the static common-mode environments. Sensing either ground or supply alone only provides partial information
regarding motor current, but sense them individually and sum them and we have the same information provided
by phase sensing, with an added advantage of not being subject to transient common-mode artifacts. See
Figure 43 for an illustration of two INA282 family devices connected in this manner. Technically, this
configuration is current differencing, though, because we want the upper sense to report a positive-going
excursion in the overall output, and the negative sense to report a negative-going excursion.
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