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EXERCISE 22 : BDO & GROUND CLIP (BACKDRIVING
NOISE)
ACTION
Identify the Ground clip and attach to the test cable as shown in Schematic. Place clip on IC
U15 (74161). Ensure that the BDO LOW is attached to TP84 DIS CLK.
Press START on the IC Tester. Observe the failures and then apply the Ground Clip to the
GND point near to the Ground pin of the IC TP93.
DESCRIPTION
This device has a clock signal running to it that gives output failures but also the Analysis box
suggests that a Ground Clip may be required. The Ground Clip is used to reduce backdriving
noise. This noise is caused by current surges when backdriving IC outputs. On the training
board we have used a 74S00 (a relatively high current output IC) to drive a clock signal
into a 74LS161. As part of the 74LS161 test, the SYSTEM 8 pin drivers must output a clock
signal to the CLK pin (pin 2) to test the functionality of the IC. In this situation, the existing
clock on the board under test is "backdriven" by the SYSTEM 8 pin driver. This process
requires larger than normal current of a magnitude that depends on the device being
backdriven. It is particularly large when driving a pin that is in the LOW state since the output
sink current of most logic families is higher than their source current. This large current can
cause "ground bounce" on the board, which may cause double clocking of a sequential device
such as a counter or shift register. If double clocking occurs, the test will FAIL because the
test program gets out of step with the operation of the IC under test.
To remove this noise, the ground clip must be used as it provides a shorter ground return
path back to the SYSTEM 8 pin driver. The ground clip should be placed as near as possible
to the GND pin of the IC under test.
