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AM26LS32AC: noise problem

Davoud Karimi
Prodigy 20 points
Part Number: AM26LS32AC

Hi,

I am using AM26LS32ACN IC to interface an encoder signal and pass it to a counter. 

The motion section of the board works fine, which means that the IC is working perfectly. 

However, in another section of the board I am communicating with an ADC IC using SPI protocol. I realized that AM26LS32 is making noise and due to that SPI communication is interrupting.

When I remove the AM26LS32 everything is working fine but with that installed on the board the noise on power line is awful. I tested the power line of the board (3.3V in my case) with an oscilloscope and it is about 1V!!!!!!

One strange behavior if the IC is that sometimes the noise dies out, which makes the SPI work fine.

I attached a 0.1uF capacitor between pins 8 and 16, but it did not solve the problem.

One observation that I cannot find an explanation is that I turn on the power supply and connect a capacitor with the same polarity (positive to pin 16 and negative to pin 8) the noise exists. Then (with the power still on) I change the polarity (interchange the capacitor's and IC pins) and suddenly the noise goes away. The capacitor is a tantalium 100UF 16V.

Below I have attache a schematic portion of the board related to AM26LS32.

  • 0
    TI__Guru 62910 points

    Hi Davoud,

    That's very strange! I'm curious to know more about the noise. Does it seem random in nature, or does it correlate with any other signals (such as a receiver channel output or differential input)? Does it occur with or without a signal cable connected to the system?

    I'm also not sure how to interpret the results of the test with a capacitor. Does it make the noise go away only if the polarity is inverted while the system is powered, or does inverting the polarity seem to resolve the issue on future power ups as well? Is there a "good" and "bad" polarity or does just the act of inverting seem to be beneficial?

    Regards,

    Max

  • 0 in reply to Max Robertson
    Prodigy 20 points

    Hi Max,

    The noise occurred without a signal cable connected to the board. The source of the noise seemed to be the malfunctioning of the IC.

    I found the source of the problem and solved it. I had put a capacitor (100nF) between the OUTD and the ground and it seemed to interrupt the function of the IC. I removed the capacitor and the problem was solved. Now the noise is gone and everything works perfect.

    Thanks for your reply and I appreciate your time

  • 0 in reply to Davoud Karimi
    TI__Guru 62910 points

    Davoud,

    Thanks for following up with the solution - I'm glad you were able to find it quickly!  100 nF is a high load for a logic output, and so I could understand how if the output were toggling between states this may result in high loading on VCC (to charge/discharge the capacitance) which could skew the voltage.  And, since there was no input signal applied the differential input voltage would have been close to 0 V for each channel, which is the high/low switching threshold.  Perhaps it was a combination of this and the VCC loading that created a kind of unstable circuit where the OUTD frequently toggles and the corresponding ICC interacts with the external regulator/PCB power distribution to create voltage ripple.

    Max