Large currents in distributed systems can cause local ground references to differ significantly, but simulation can help designers spot and resolve problems.

Distributed systems dealing with large currents can have
their “common” ground reference see substantial local variations. In
automotive electronics, for instance, electric motors during cold crank produce
large ground offset noise. The technique of simulating ground offset to test
automotive electronic systems will help developers ensure system reliability.

In a distributed system, each electronic subsystem must able
to communicate with the others reliably, even the in the presence of substantial
noise. High-current-handling systems, such as automotive electronics, face a
special challenge: the presence of large ground offset noise. In such systems
the ground current can be up to 100A or more and often are transient in nature
i.e., they last for short duration. During this transient current, because
ground wires and planes always have some parasitic resistance, the current will
cause a sizable IR voltage drop across the wire. This IR voltage means that
there will be a ground reference difference between communication systems.

Ground Offset

As shown in Figure 1, the high current source (i.e. 12V lead
acid battery) outputs very high current in the orders of hundreds of amperes to
the load (i.e. cranking motor). This current is returned through the ground
wire or chassis, which has low but not zero resistance. Even with a 10 milli-Ohm
ground resistance, the 100A will result in a 1V difference between two
different locations in the ground plane.

Because the car’s communication subsystems connect to the
same ground node, but at different points, there can be as much as a 1V
difference between the ground references of the transmitter and receiver units.
That is, GND-R is 1V higher than GND-T. These communication systems must therefore
be designed to handle such a difference and they must be thoroughly tested
during development.



Figure 1. Ground
offset voltage is caused by high current in ground plane.

To help further understand the ground offset voltage effect,
Figure 2 shows the equivalent circuit of Figure 1 with the 1V IR drop when
there is a 100A ground current.  This
voltage is in between the two communication subsystems. This offset voltage
causes the two communication subsystems ground inputs at a different voltage
level. If the ground voltage difference is large enough, communication could
fail. To verify that a design will work reliably, then, it must be tested with
subsystems operating with ground reference offsets.


2 - Ground offset equivalent circuit

Figure 2. Simplified
equivalent circuit of Figure 1. There is a ground offset voltage in between the
two communication subsystems.

Ground Offset
Simulation and Generation

During product development phase, it is much more
convenience to simulate the ground offset when testing the communication system
than pumping 100A. One way to do so, as shown in Figure 3, is to simulate the
ground offset voltage with a four-quadrant power supply, such as the Accel TS200
or TS250
, inserted between the transmitter ground (GND-T) and the receiver
ground nodes.


3 - ground offset test setup


Figure 3. A
four-quadrant power supply can help simulate offset ground voltage.

Four-Quadrant Power Supply

A four-quadrant power supply is a special voltage supply
that can both sink or source current, regardless of whether the output voltage
is positive or negative. In contrast, a regular power supply can only source
current; it cannot sink current. To further understand 4-qudrant power supply,
Figure 4 shows the 4-qudrant diagram.


4 - The power supply operating realms


Figure 4. Diagram
showing a voltage supply operating in each of the four quadrants.

In the first quadrant, the voltage is positive and the
current is also positive (sourcing current). In the second quadrant, the output
voltage is negative, but the current is still positive (sourcing). In third
quadrant, the voltage is negative and the current is sinking. And finally, the
fourth quadrant is negative voltage but sourcing current. The typical lab power
supply can only operate in the first quadrant. 


5 - Accel 4-quadrant supply


Figure 5. Modulated
Power Supply is used for ground offset testing.

DC Ground Offset

For ground offset voltage simulation as shown in Figure 3, testers
usually use the second and the fourth quadrant. Two test cases are needed for
testing the transceivers. The first test case has the receiver use the higher
ground voltage. To achieve this, testers should set the four-quadrant voltage
supply to positive voltage. The second case is when the transmitter has the
higher ground voltage. In this second case, the power supply should be set to
negative voltage (and sourcing current). For maximum confidence in the system’s
reliability, the transceiver must be thoroughly tested with different ground
offset levels, both positive and negative voltages.

Here is the recommended test procedure:

  • Connect the supply’s positive output terminal to
    the transmitter’s GND-T node.
  • Connect the negative output terminal to a common
  • Connect the receiver ground (GND-R) to the
    common ground.
  • Initially set the supply output to zero volts.
  • Test the communication systems.
  • Increase the output voltage to a higher voltage
    (i.e. +100mV).
  • Repeat step 5 and 6 until communications fail.
    This is the system’s failure point.
  • Repeat steps 1 through 7 for using negative offset
    voltage steps.

AC Ground Offset Testing

Such ground offsets may not only be DC voltages, but are often
AC or transient in nature. Transient ground offsets occur, for instance, when
large surges of current get injected into the ground plane. This surge will
cause the ground to bounce, acting like a noise source that increases the
overall system noise level. This can affect more than the communications
subsystems. For example, large ground noise will reduce the signal-to-noise
ratio in analog and ADC/DAC circuits, or increase the jitter level in digital
systems, as well as corrupting communications integrity.

It is therefore important to thoroughly test all subsystem
and circuits that are exposed to high-level ground noise. The easiest way to
simulate transient noise is to use a AC four-quadrant power supply, such as the
Accel TS250
waveform amplifier. Figure 6 shows the test setup. a signal generator produces
the noise or transient. The waveform amplifier (which can also work as a
4-quadrant power supply) acts like a buffer that, due to its very low output
impedance, can sink or source large currents.


6 - Test setup for transient offset current


Figure 6. Test setup
generating ground noise to simulate large ground transient current.


A four-quadrant voltage supply is crucial to simulate ground
offset voltages that mimic ground plane voltage difference in a high current
system. Further, the simulated offset voltage supply must be able to both sink
and source current. Such simulation is needed to thoroughly test all the sub-systems
(transceiver) in electronic products that handle large currents.