LMP8481MM-SSLASHNOPB datasheet(14/24 Pages) TI

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VREF

100 k:

2.5V

VREFA

VREFB

100 k:

VREF

100 k:

2.5V

VREFA

VREFB

100 k:

VREF

2.5V

LMP8480, LMP8481

SNVS829A – JUNE 2012 – REVISED AUGUST 2012

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VREF’ = ( VREFA + VREFB ) / 2 (Separate inputs)

(5)

Figure 13. Applying 1:1 Direct Reference Voltage

For mid-range operation VREFB should be tied to ground and VREFA 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.

VREF’ = (VREFA – VREFB) / 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 VREF 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 VREF maximum.

Both VREFA and VREFB 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 VREF 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 RSENSE. 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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