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SBOS381 FEBRUARY 2007
www.ti.com
9
Normal Case 1: V
SENSE
≥
20mV, V
CM
≥
V
S
This region of operation provides the highest accuracy.
Here, the input offset voltage is characterized and
measured using a two-step method. First, the gain is
determined by Equation 1.
V
OUT1
100mV
G
V
OUT2
20mV
where:
V
OUT1
= Output Voltage with V
SENSE
= 100mV
V
OUT2
= Output Voltage with V
SENSE
= 20mV
Then the offset voltage is measured at V
SENSE
= 100mV
and referred to the input (RTI) of the current shunt monitor,
as shown in Equation 2.
V
OS
RTI (ReferredToInput)
V
OUT1
G
100mV
In the Typical Characteristics, the
Output Error vs
Common-Mode Voltage
curve shows the highest
accuracy for the this region of operation. In this plot,
V
S
= 12V; for V
CM
≥
12V, the output error is at its minimum.
This case is also used to create the V
SENSE
≥
20mV output
specifications in the Electrical Characteristics table.
Normal Case 2: V
SENSE
≥
20mV, V
CM
< V
S
This region of operation has slightly less accuracy than
Normal Case 1 as a result of the common-mode operating
area in which the part functions, as seen in the
Output Error
vs Common-Mode Voltage
curve. As noted, for this graph
V
S
= 12V; for V
CM
< 12V, the Output Error increases as V
CM
becomes less than 12V, with a typical maximum error of
0.005% at the most negative V
CM
= 16V.
Low V
SENSE
Case 1:
V
< 20mV, 16V
≤
V
CM
< 0; and
Low V
SENSE
Case 3:
V
SENSE
< 20mV, V
S
< V
CM
≤
80V
Although the INA270 family of devices are not designed for
accurate operation in either of these regions, some
applications are exposed to these conditions. For
example, when monitoring power supplies that are
switched on and off while V
S
is still applied to the INA270
or INA271, it is important to know what the behavior of the
devices will be in these regions.
As V
SENSE
approaches 0mV, in these V
CM
regions, the
device output accuracy degrades. A larger-than-normal
offset can appear at the current shunt monitor output with
a typical maximum value of V
OUT
= 60mV for
V
SENSE
= 0mV. As V
SENSE
approaches 20mV, V
OUT
returns to the expected output value with accuracy as
specified in the Electrical Characteristics. Figure 3
illustrates this effect using the INA271 (Gain = 20).
Low V
SENSE
Case 2: V
SENSE
< 20mV, 0V
≤
V
CM
≤
V
S
This region of operation is the least accurate for the
INA270 family. To achieve the wide input common-mode
voltage range, these devices use two op amp front ends in
parallel. One op amp front end operates in the positive
input common-mode voltage range, and the other in the
negative input region. For this case, neither of these two
internal amplifiers dominates and overall loop gain is very
low. Within this region, V
OUT
approaches voltages close to
linear operation levels for Normal Case 2.
V
O
(
V
SENSE
(mV)
0.40
0.36
0.32
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0
0
20
2
6
8
10
12
14
18
4
16
Actual
Ideal
Figure 3. Example for Low V
Cases 1 and 3
(INA271, Gain = 20)
This deviation from linear operation becomes greatest the
closer V
SENSE
approaches 0V. Within this region, as
V
SENSE
approaches 20mV, device operation is closer to
that described by Normal Case 2. Figure 4 illustrates this
behavior for the INA271. The V
OUT
maximum peak for this
case is determined by maintaining a constant V
S
, setting
V
SENSE
= 0mV and sweeping V
CM
from 0V to V
S
. The exact
V
CM
at which V
OUT
peaks during this case varies from part
to part. The maximum peak voltage for the INA270 is
0.28V; for the INA271, the maximum peak voltage is 0.4V.
(1)
(2)