HCPL-7860 Datasheet, PDF (16/18 Pages) Agilent(Hewlett-Packard) – Isolated 15-bit A/D Converter

For a two-terminal shunt, as the value of shunt resistance
decreases, the resistance of the leads becomes a signifi-
cant percentage of the total shunt resistance. This has two
primary effects on shunt accuracy. First, the effective resis-
tance of the shunt can become dependent on factors such
as how long the leads are, how they are bent, how far they
are inserted into the board, and how far solder wicks up
the lead during assembly (these issues will be discussed in
more detail shortly). Second, the leads are typically made
from a material such as copper, which has a much higher
tempco than the material from which the resistive element
itself is made, resulting in a higher tempco for the shunt
overall. Both of these effects are eliminated when a four-
terminal shunt is used. A four-terminal shunt has two ad-
ditional terminals that are Kelvin-connected directly across
the resistive element itself; these two terminals are used
to monitor the voltage across the resistive element while
the other two terminals are used to carry the load current.
Because of the Kelvin connection, any voltage drops across
the leads carrying the load current should have no impact
on the measured voltage.
Several four-terminal shunts from Isotek (Isabellenhütte)
suitable for sensing currents in motor drives up to 71
Arms (71 hp or 53 kW) are shown in Table 3; the maximum
current and motor power range for each of the PBV series
shunts are indicated. For shunt resistances from 50 m:
down to 10 m:, the maximum current is limited by the
input voltage range of the isolated modulator. For the 5
m: and 2 m: shunts, a heat sink may be required due to
the increased power dissipation at higher currents.
When laying out a PC board for the shunts, a couple of
points should be kept in mind. The Kelvin connections
to the shunt should be brought together under the body
of the shunt and then run very close to each other to the
input of the isolated modulator; this minimizes the loop
area of the connection and reduces the possibility of stray
magnetic fields from interfering with the measured signal.
If the shunt is not located on the same PC board as the
isolated modulator circuit, a tightly twisted pair of wires
can accomplish the same thing.
Also, multiple layers of the PC board can be used to in-
crease current carrying capacity. Numerous plated-through
vias should surround each non-Kelvin terminal of the shunt
to help distribute the current between the layers of the PC
board. The PC board should use 2 or 4 oz. copper for the
layers, resulting in a current carrying capacity in excess of
20 A. Making the current carrying traces on the PC board
fairly large can also improve the shunt’s power dissipa-
tion capability by acting as a heat sink. Liberal use of vias
where the load current enters and exits the PC board is
also recommended.
Table 3. Isotek (Isabellenhütte) Four-Terminal Shunt Summary.
Shunt Resistor
Shunt Resistance
Tol.
Part Number
m:
%
PBV-R050-0.5
50
0.5
PBV-R020-0.5
20
0.5
PBV-R010-0.5
10
0.5
PBV-R005-0.5
5
0.5
PBV-R002-0.5
2
0.5
Note: Values in brackets are with a heatsink for the shunt.
Maximum RMS Cur-
rent
A
3
7
14
25 [28]
39 [71]
Motor Power Range
120 V_AC - 440 V_AC
hp
kW
0.8 - 3
0.6 - 2
2-7
0.6 - 2
4 - 14
3 - 10
7 - 25 [8 - 28] 5 - 19 [6 - 21]
11 - 39 [19 - 71] 8 - 29 [14 - 53]
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