Datasheet

V
REF
¶
100 k:
2.5V
5V
V
REFA
V
REFB
100 k:
V
REF
¶
100 k:
2.5V
V
REFA
V
REFB
100 k:
V
REF
2.5V
LMP8480, LMP8481
SNVS829A –JUNE 2012–REVISED AUGUST 2012 www.ti.com
V
REF
’ = ( V
REFA
+ V
REFB
) / 2 (Separate inputs) (5)
Figure 13. Applying 1:1 Direct Reference Voltage
For mid-range operation V
REFB
should be tied to ground and V
REFA
can be tied to VS or an external A/D
reference voltage. The output will be set to one-half the reference voltage. For example, a 5V reference would
result in a 2.5V output “zero” reference.
Figure 14. Applying A Divided Reference Voltage.
V
REF
’ = (V
REFA
– V
REFB
) / 2 (6)
When the reference pins are biased at different voltages, the output will be referenced to the average of the two
applied voltages.
The reference pins should always be driven from clean, stable sources, such as A/D reference lines or clean
supply lines. Any noise or drifts on the reference inputs are directly reflected in the output. Care should be taken
if the power supply is used as the reference source so as to not introduce supply noise, drift or sags into the
measurement.
It is possible to set different resistor divider ratios by adding external resistors in series with the internal 100K
resistors, though the temperature coefficient (tempco) of the external resistors may not tightly track the internal
resistors and there will be slight errors over temperature.
REFERENCE INPUT VOLTAGE LIMITS
The maximum voltage on either reference input pin is limited to VCC or 12V, whichever is less.
The average voltage on the two V
REF
pins, and thus the actual output reference voltage level, is limited to a
maximum of 1.5V below VCC, or 6V, whichever is less. Beware that supply voltages of less than 7.5V will have a
diminishing V
REF
maximum.
Both V
REFA
and V
REFB
may both be grounded to provide a ground referenced output (thus functionally duplicating
the LMP8480).
It should be noted that there can be a dynamic error in the V
REF
to output level matching of up to 100µV/V.
Normally this is not an issue for fixed references, but if the reference voltage is dynamically adjusted during
operation, this error needs to be taken into account during calibration routines. This error will vary in both
amplitude and polarity part-to-part, but the slope will generally be linear.
SELECTION OF THE SENSE RESISTOR
The accuracy of the current measurement depends heavily on the accuracy of the shunt resistor R
SENSE
. Its
value depends on the application and is a compromise between small-signal accuracy, maximum permissible
voltage drop and allowable power dissipation in the current measurement circuit.
The use of a “4-terminal” or “Kelvin” sense resistor is highly recommended. See the ERROR SOURCES AND
LAYOUT CONSIDERATIONS below.
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