Datasheet

G =
V – V
OUT1 OUT2
100 mV – 20 mV
V RTI (refe red to inp
OS
r ut) =
V
OUT1
G
– 100 mV
( (
INA270-Q1, INA271-Q1
SBOS401B –JULY 2007–REVISED FEBRUARY 2010 www.ti.com
Accuracy Variations as a Result of V
SENSE
and Common-Mode Voltage
The accuracy of the INA270 and INA271 current-shunt monitors is a function of two main variables: V
SENSE
(V
IN+
– V
IN–
) and common-mode voltage, V
CM
, relative to the supply voltage, V
S
. V
CM
is expressed as
(V
IN+
+ V
IN–
)/2; however, in practice, V
CM
is seen as the voltage at V
IN+
because the voltage drop across V
SENSE
is usually small.
This section addresses the accuracy of these specific operating regions:
Normal Case 1: V
SENSE
≥ 20 mV, V
CM
≥ V
S
Normal Case 2: V
SENSE
≥ 20 mV, V
CM
< V
S
Low V
SENSE
Case 1: V
SENSE
< 20 mV, –16 V ≤ V
CM
< 0
Low V
SENSE
Case 2: V
SENSE
< 20 mV, 0 V ≤ V
CM
≤ V
S
Low V
SENSE
Case 3: V
SENSE
< 20 mV, V
S
< V
CM
≤ 80 V
Normal Case 1: V
SENSE
≥ 20 mV, 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.
(1)
Where:
V
OUT1
= Output voltage with V
SENSE
= 100 mV
V
OUT2
= Output voltage with V
SENSE
= 20 mV
Then the offset voltage is measured at V
SENSE
= 100 mV and referred to the input (RTI) of the current-shunt
monitor, as shown in Equation 2.
(2)
In 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
= 12 V; for V
CM
≥ 12 V, the output error is at its minimum. This case is
also used to create the V
SENSE
≥ 20 mV output specifications in Electrical Characteristics.
Low V
SENSE
Case 1: V
SENSE
< 20 mV, –16 V ≤ V
CM
< 0; and
Low V
SENSE
Case 3: V
SENSE
< 20 mV, V
S
< V
CM
≤ 80 V
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
is in these regions.
As V
SENSE
approaches 0 mV, 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
= 60 mV for
V
SENSE
= 0 mV. As V
SENSE
approaches 20 mV, V
OUT
returns to the expected output value with accuracy as
specified in Electrical Characteristics. Figure 3 illustrates this effect using the INA271 (Gain = 20).
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