ACPL-790A-500E Datasheet, PDF (13/15 Pages) AVAGO TECHNOLOGIES LIMITED – Precision Isolation Amplifiers

become a larger percentage of the signal amplitude. The
selected value of the sense resistor will fall somewhere
between the minimum and maximum values, depending
on the particular requirements of a specific design.
When sensing currents large enough to cause significant
heating of the sense resistor, the temperature coefficient
(tempco) of the resistor can introduce nonlinearity due to
the signal dependent temperature rise of the resistor. The
effect increases as the resistor-to-ambient thermal resis-
tance increases. This effect can be minimized by reducing
the thermal resistance of the current sensing resistor or
by using a resistor with a lower tempco. Lowering the
thermal resistance can be accomplished by reposition-
ing the current sensing resistor on the PC board, by using
larger PC board traces to carry away more heat, or by
using a heat sink.
For a two-terminal current sensing resistor, as the value of
resistance decreases, the resistance of the leads become a
significant percentage of the total resistance. This has two
primary effects on resistor accuracy. First, the effective
resistance of the sense resistor 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 leads 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 overall.
Both of these effects are eliminated when a four-terminal
current sensing resistor is used. A four-terminal resistor
has two additional 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 connec-
tion, any voltage drops across the leads carrying the load
current should have no impact on the measured voltage.
When laying out a PC board for the current sensing
resistors, a couple of points should be kept in mind. The
Kelvin connections to the resistor should be brought
together under the body of the resistor and then run
very close to each other to the input of the ACPL-
790B/790A/7900; this minimizes the loop area of the
connection and reduces the possibility of stray magnetic
fields from interfering with the measured signal. If the
sense resistor is not located on the same PC board as the
isolation amplifier circuit, a tightly twisted pair of wires
can accomplish the same thing.
Also, multiple layers of the PC board can be used to
increase current carrying capacity. Numerous plated-
through vias should surround each non-Kelvin terminal of
the sense resistor 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
sense resistor’s power dissipation capability by acting as a
heat sink. Liberal use of vias where the load current enters
and exits the PC board is also recommended.
Shunt Resistor Connections
The typical method for connecting the ACPL-790-
B/790A/7900 to the current sensing resistor is shown in
Figure 21. V_IN+ (pin 2) is connected to the positive terminal
of the sense resistor, while V_IN– (pin 3) is shorted to GND1
(pin 4), with the power-supply return path functioning
as the sense line to the negative terminal of the current
sense resistor. This allows a single pair of wires or PC board
traces to connect the isolation amplifier circuit to the sense
resistor. By referencing the input circuit to the negative
side of the sense resistor, any load current induced noise
transients on the resistor are seen as a common-mode
signal and will not interfere with the current-sense signal.
This is important because the large load currents flowing
through the motor drive, along with the parasitic induc-
tances inherent in the wiring of the circuit, can generate
both noise spikes and offsets that are relatively large
compared to the small voltages that are being measured
across the current sensing resistor.
If the same power supply is used both for the gate
drive circuit and for the current sensing circuit, it is very
important that the connection from GND1 of the ACPL-
790B/790A/7900 to the sense resistor be the only return
path for supply current to the gate drive power supply
in order to eliminate potential ground loop problems.
The only direct connection between the ACPL-790-
B/790A/7900 circuit and the gate drive circuit should be
the positive power supply line.
Differential Input Connection
The differential analog inputs of the ACPL-790B/790A/7900
are implemented with a fully-differential, switched-capac-
itor circuit. In the typical application circuit (Figure 21),
the isolation amplifier is connected in a single-ended
input mode. Given the fully differential input structure,
a differential input connection method (balanced input
mode as shown in Figure 24) is recommended to achieve
better performance. The input currents created by the
switching actions on both of the pins are balanced on
the filter resistors and cancelled out each other. Any noise
induced on one pin will be coupled to the other pin by the
capacitor C and creates only common mode noise which
is rejected by the device. Typical value for Ra and Rb is 10
: and 22 nF for C.
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