Why signal level is shifted on PCB?

[EDA_hg81]

In the design, two PCB are connected by a 50 pins flat flex connector, which provides Grounds and Control signals from master board to slave board.

We have two shipment PCB with the same design. Right now the problem is the Ground of control signal of the second shipment PCB has a 0.5V voltage level shift from real Ground which is hard for slave board to recognize the signals correctly.

Only difference between the first PCB and second PCB is the flat flex connector.

Do you think flat flex connector is the reason make the Ground of control signal shifted 0.5V higher ?

Thanks.

 

[spikeyang]

v = I * R

It depends on the impedance between the two grounds and the current.

If the measurement is correct, for a 500mA current draw of the daughter board, R = 0.5/0.5 = 1 Ohm, maybe quite appropriate.

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And you didnt mention if the two boards have the same or separate power supply. Different power supply and uncommon ground also can make the ground potential difference.

 

[EDA_hg81]

The master board provide the power supply for the slave board.

Thanks.

 

[Iouri]

might be you should use separate connector for the power

 

[paurelian]

It is a good idea to use as many ground pins when connecting one slave board to a motherboard. If it's possible, in order to avoid ground shifting between boards, use 30 - 40 % from the number of pins for ground connection. Normally when the boards are new the contact resistance is about 10 miliohms, but during product life this contact resistance will depreciate.

 

[EDA_hg81]

I have found out since the chip on slave board is short to high, all signals have been shifted.

Thanks