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ADS1258 Crystal Startup Problems

Greg Dunn
Intellectual 475 points
Other Parts Discussed in Thread: ADS1258, ADS1258-EP, ADS1241, ADS1258EVM-PDK

We have been using the ADS1258 in a design for about 2 years. We have built around 150 boards.  We are seeing about 1 in 20 that have crystal startup problems at room temperature.  The crystal never starts oscillating and of course the A/D doesn’t work without a clock.  We are doing temperature testing (-40C to 85C) and initial tests indicate that we will have a much higher failure percentage over temperature.  So far we have tried the following crystals:

 

ECS-.327-7-34B-TR           12.5 pF, 50K max ESR, -40C - 85C operating temperature

MC-306 32.7680K-A0      12.5 pF, spec sheet discrepancy - either 35K or 50K max ESR, -40 - 85C operating temperature

 

We have tried a couple different capacitor values: 12, 22 pF.  These seem to give slightly higher than 32.768 KHz crystal frequency.  We are measuring the frequency at the CLKIO line and then dividing by 480 to calculate the crystal frequency.

 

We are assuming that the PLL multiplier is exactly 480, is that correct?

 

At room temperature, we have seen frequencies in the range below with the crystals and capacitors values above.

 

CLKIO MHz          Crystal KHz

15.728727            32.76818

15.729658            32.77012

  

We have ordered some smaller capacitors (like 4.7 pF as suggested in the spec sheet) but are concerned that they will cause even higher frequencies. We have also seen where smaller crystals have caused the oscillator to not start as well.  The crystal should operate very close to 32.768 KHz when the capacitors are sized properly shouldn't it?

 

Do you have any other suggestions to allow the crystal oscillator to start up reliably over temperature?

 

I also noticed that TI makes an ADS1258-EP version of the chip. Does this part have any differences in the crystal drive circuit or is it simply the same as the ADS1258?  Would there be any benefit to give this part a try?

 

The 35K ohm maximum ESR requirement seems to not be easily achievable with standardly available crystals, especially with the -40C to 85C operating temperature. As shown above, we have tried one of your suggested crystals with no luck.

 

I did notice that the thermal pad on the bottom center of the package is not actually connected to AVss on our board design. There is a pad on the board, but it is electrically floating.  Could this contribute to the crystal not starting up?

 

Below is our schematic for your reference. The traces are very short between the IC, crystal and capacitors.

 

Thanks,

Greg


 

  • 0
    TI__Guru* 97835 points
    Greg,


    I don't usually review ADS1258 posts, but I do have some relevant information. I've written a response to a post for the same type of question for the ADS1241 recently and you should review the post to try the same thing. Below is a link to the post.

    e2e.ti.com/.../1760571

    In particular, my last post in the thread has a document attached. Review section 8.3 of the application note. It describes startup issues with crystals and tests that can be run to determine how robust the crystal setup will be. There is a test method shown with data from Fox Electronics showing some of their results with the MSC12xx. The ADS1258 oscillator circuitry should be similar and you may be able to devise a test to check the start up of the crystal.

    If this doesn't answer your question, please post back.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 475 points
    Joseph,

    Thank you very much, I will print it out and review tonight. I'll let you know what we find out.

    Thanks,
    Greg Dunn
  • 0 in reply to Greg Dunn
    TI__Guru* 97835 points
    Greg,


    Out of curiosity, did you ever run the tests outlined in the applications note? I know other people who have used this but I was checking to see if you had been satisfied with the results.

    Regardless, I hope the application note was of good use to you.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 475 points
    Joseph,

    We have been doing more temperature testing with the MC-306 32.7680K-A0 and things seem to be working pretty well. We are planning to do the recommended tests from the application note this week. I will update with our results when complete. The only unfortunate part is that our board is laid out for a different crystal footprint which didn't work out initially. We are concerned that the mechanical reliability will be compromised using the MC-306 32.7680K-A0 on the current board layout. We were using the ECS-.327-7-34B-TR thru-hole crystal which could easily be tacked on the board. We are seeing the occasional failure with this part however. You don't know of any thru-hole crystals which have the <= 35K ESR and are rated for -40 to +85C do you?

    Thanks,
    Greg
  • 0 in reply to Greg Dunn
    TI__Guru* 97835 points
    Greg,


    I'm sorry, but I don't have a replacement thru-hole crystal for you. I'll check to see if I can find any information on a replacement, and I'll post back if I find anything.

    Let me know how the remainder of you testing goes. Feel free to post back with any further questions.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 475 points
    Just to update you, we have tried both the MC-306 32.7680K-A0 and ECS-.327-7-34B-TR crystals with 4.7pF caps to ground and a variable resistor in series with the crystal as recommended. With both crystals, we could increase the resistance all the way up to 2.25 MOhm and the crystal would still start up reliably. This doesn't really make any sense to us. We expected to see the crystal not start up at some point. Any ideas on what may be going on? Surely we don't have a safety margin of 2.2M / 35K = 62.9.
  • 0 in reply to Greg Dunn
    TI__Guru* 97835 points
    Greg,


    To be honest, I'm not sure. I don't do a lot with crystals to be able to offer much advice besides the application note.

    However, I would note that you've dropped the capacitance surrounding the device so that it's down to 4.7pF which is rather small. There may be additional parasitics that are altering the conditions of the test (either across RG or from the output of crystal directly to ground.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 475 points
    Joseph,

    I agree with the 4.7pF being low. The reason we put them down to 4.7pf is because that is what is recommended in the ADS1258 spec sheet and is also installed on the ADS1258EVM-PDK Evaluation Board. The spec sheet unfortunately does not explain the reasoning of using the smaller capacitor. The application note you sent indicates that removing or reducing the load capacitors can improve crystal startup reliability. Maybe that is why they had to reduce to 4.7pf. I guess removing the allows the crystal to start up easier, but the frequency is probably higher than with the specified load capacitance. The ADS1258 seems to have some inherent start up issues so maybe this improves the situation. We have very short traces between the IC, crystal and caps so I would estimate the parasitic capacitance to be very small. If there is anyway that you could point this message thread out to someone that has expertise with the ADS1258 and crystal issues I would be greatly appreciative.

    Originally we had 18 pF capacitors on the board which gave us 18 / 2 = 9pf plus the stray capacitance which appeared to be close to the 12.5pf spec on the crystal. Earlier testing showed that we seemed to get more reliable startup over temperature if we raised the caps to 22pf so that is how we are currently building the boards - with the ECS-.327-7-34B-TR crystal. We measured the CLKIO frequency to be 15.7287234 which is very close to the 32768 * 480 - assuming the PLL multiplies by 480 as questioned earlier in this thread. We recently have had some boards fail out during temperature testing so we started our current round of testing again to see if we could improve the situation.

    Thanks,
    Greg Dunn
  • 0 in reply to Greg Dunn
    TI__Guru* 97835 points
    Greg,


    I'm not sure why they recommend a lower capacitance. However it is possible that there is extra capacitance on the inverter pins inside the device. Adding the bond pad and the input and output devices of the inverter attached to the crystal, could add a few extra pF to each pin.

    As you are doing, I think that the temperature testing is important for a consistent startup. I'll ask around, but there aren't many people here would have any more opinions in the crystal startup issues you are seeing.


    Joseph Wu