Sense Resistor

Load Current Path

PCB

Source

Trace

Kelvin Sense

Traces to

Amplifer

PCB

Load

Trace

C1

0.1 PF

To Load

+4.5V

to

+76V

ADC

RSN

RSP

GND

LMP8480

R1

R2

LMP8480, LMP8481

www.ti.com

SNVS829A – JUNE 2012 – REVISED AUGUST 2012

Figure 16. Typical Application with Resistive Divider

ERROR SOURCES AND LAYOUT CONSIDERATIONS

The traces leading to and from the sense resistor can be significant error sources. With small value sense

resistors (<100m), any trace resistance shared with the load current can cause significant errors.

Figure 17. “Kelvin” or “4–wire” Connection to the Sense Resistor

The amplifier inputs should be directly connected to the sense resistor pads using “Kelvin” or “4-wire” connection

techniques. The traces should be one continuous piece of copper from the sense resistor pad to the amplifier

input pin pad, and ideally on the same copper layer with minimal vias or connectors. This can be important

around the sense resistor if it is generating any significant heat gradients.

To minimize noise pickup and thermal errors, the input traces should be treated as a differential signal pair and

routed tightly together with a direct path to the input pins. The input traces should be run away from noise

sources, such as digital lines, switching supplies or motor drive lines. Remember that these traces can contain

high voltage, and should have the appropriate trace routing clearances.

Since the sense traces only carry the amplifier bias current (about 7µA at room temp), the connecting input

traces can be thinner, signal level traces. Excessive Resistance in the trace should also be avoided.

The paths of the traces should be identical, including connectors and vias, so that these errors will be equal and

cancel.

The sense resistor will heat up as the load increases. As the resistor heats up, the resistance generally goes up,

which will cause a change in the readings The sense resistor should have as much heatsinking as possible to

remove this heat through the use of heatsinks or large copper areas coupled to the resistor pads. A reading

drifting over time after turn-on can usually be traced back to sense resistor heating.

Copyright © 2012, Texas Instruments Incorporated

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