Datasheet

Sense Resistor
Load Current Path
PCB
Source
Trace
Kelvin Sense
Traces to
Amplifer
PCB
Load
Trace
V
SENSE
V
CC
= +4.5V to +76V
C1
0.1 PF
V
OUT
To Load
+4.5V
to
+76V
R
SENSE
I
SENSE
ADC
RSN
RSP
GND
LMP8480
V
IN
+
V
IN
- V
REF
V
SENSE
V
CC
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.
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