circuit impedance will lead me to a short?

[danny davis]

Will measuring each IC chips pins impedance lead me to a short?

When measuring with a DVM meter set to ohms, measuring each IC pin referenced to ground, is this measuring the impendance?

IC chips either logic TTL , CMOS or op amps

I'm trying to measure the impedance of each IC pin that is " in circuit" , but what is my reference? when measuring the impedance of an IC pin? the reference should be where?

Will this lead me to finding a short circuit?

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if there is a short, the impedance or in circuit resistance of the IC pin will change?

 

[betwixt]

You would be measuring resistance not impedance. Firstly, short circuits between IC pins is extremely rare. If you did try to measure resitance it wouldn't really tell you anything useful unless the reading was very close to zero Ohms because the resitance is measured by injecting a current from the meter across the pins and measuring the resulting voltage drop. There may be thousands of internal connections to the pin and even the slightest tolerance difference in manufacture could result in widely different readings while the device is still within specification. Also, as most pins will connect to semiconductor junctions, the voltage will depend on the measurement current. You could find that two different test meters gave completely different readings simply because they tested at different currents.

If yu try to measure in-circuit the problem is further compounded by parallel current paths through other compompnents. In commercial automated test systems a system called 'guarding' is used to help isolate paths thrugh components so they can be individually measured but even that is only partially sucessful in some cases and involves considerably more extra measurment circuitry and connections to inject currents at other parts of the board to divert the measurement current.

Brian.

 

[danny davis]

RC network on the ground buss going to the Earth , this RC network is a common mode rejection RC network for the whole circuits ground , the RC network is on the ground buss connect with Earth

Have you seen this or heard of this?



Is this ground Impedance, my manager called it common mode reject the RC network of the whole circuits ground to earth

He says that it helps out with the potential difference and that the whole circuit is not floating

What does he mean by this?

 

[betwixt]

It assumes the power source is isolated does not have any connection to Earth and therefore stray capacitance and resistance could make the circuits ground node 'float' at any voltage present on the power input wires. Usually it is used where a direct connection to Earth would not be suitable but a weak leakage path is acceptable. So it isn't strictly speaking a common mode rejector, just a path for leakage currents to discharge through to keep the circuits ground at safe potential.

Brian.