CD4051B-Q1: Input and output wave abnormal

Xiao,

Figure 2 shows that there is a DC offset shift (not expected) of the input voltage up to 8 V compared to the Figure 1 offset 4V. Because the input voltage (11V) is greater than the supply voltage (8V), the output signal is clipped due to the CD4051B-Q1 internal clamp diodes (this is expected).

A normal operating passive FET switch like the CD4051B-Q1 cannot create a DC offset.

1) What happens if you remove the CD4051B-Q1 from the abnormal board and short the signal path?

2) Have you removed the CD4051-Q1 from the abnormal board and replace with new unit? What happens?

3) Does taking the CD4051-Q1 from the abnormal board cause the normal board to become abnormal?

4) Do you have a schematic of your board? In the past I have seen external didoes cause DC offset.

5) Where are probing in the device? Are they using the same scope 1M ohm termination?

Thank you,
Adam

Adam
Thanks for response. Following are some input for your questions
1) What happens if you remove the CD4051B-Q1 from the abnormal board and short the signal path?
>>Remove CD4051, short the signal path. the signal is normal
2) Have you removed the CD4051-Q1 from the abnormal board and replace with new unit? What happens?
>> Following with the part.
3) Does taking the CD4051-Q1 from the abnormal board cause the normal board to become abnormal?
>>Yes
4) Do you have a schematic of your board? In the past I have seen external didoes cause DC offset.
>> I will send you schametic, when I get it.
5) Where are probing in the device? Are they using the same scope 1M ohm termination?
>> yes.

Xiao,

How many parts do you have that are showing this behavior out of how many built?  

I will wait for the schematic to see if there is anything that can cause an increase in DC offset voltage.  Typically when a signal switch is damaged the symptom is lower voltages due to excess leakage in the part.  I've never seen a signal switch increase the voltage.  

Are you able to test the suspect unit for damage?  A quick way to do this will be to power up the device, place a DC signal on the signal paths, toggle the switch with the control logic, and observe any high current through the pins.   If placing a new part on the board solves the issue and placing the suspect part on a good board causes issue we may want to start getting ready to submit it for return.

Thank you,

Adam

Xiao,

Do you have any updates on the additional steps you have taken?

Do you have a schematic we can review?

Thank you,
Adam

Xiao,

We haven't heard back from you so I will assume this has been resolved and close the thread. If you have additional questions you may reply here to re-open the thread or create a new one.

Thank you,
Adam

New Microsoft Word Document.docxhello Adam

Sorry for late reply.  I also test it in customer board. and update some new information . The output signal offset changed high, but the amplitude change small. It seemed there have a weak pull up to power.

Xiao,

This sounds like the part might be damaged as mentioned above. 

If you remove the part from the customer's board and test it by itself, what is the leakage current you see going into the pin you think has a weak pull up to supply?  

You can test the leakage by placing Vdd at 18V and set the logic control pins to turn on the suspect signal path.  Then you can place a DC 18V on the suspect pin and measure the current flowing through that pin to see if it is within the spec.  

Thank you,

Adam

Xiao,

Do you have any update on the testing?

Thank you,
Adam

Xiao,

We haven't heard back from you in several weeks so I'm going to assume you resolved your issue. I will close this thread. If you have additional questions you may reply here to reopen it or create a new one.

Thank you,
Adam