ADS1241 Crystal driver specification

Helge,


I'm not aware of any specifications on the driver for the crystal oscillator. We have made recommendations for specific crystals in the datasheet as shown in Table 2 on page 12 of the datasheet (three ECS and one CTS crystal). I know that the device was tested with these crystals.

In the past, there was an EVM for this device which used the HC49 crystal made by CTS, which I believe is the same as listed in the datasheet (with the vendor number listed as MP042S).


Joseph Wu

Helge,


I found the datasheet for the CTS crystal here:

www.ctscorp.com/.../008-0308-0.pdf

I wasn't able to find the datasheets for the ECS crystals, and the crystals may have changed series names or become obsolete. However, I believe it might be similar to these:

www.ecsxtal.com/.../hc49ux.pdf
www.ecsxtal.com/.../hc49u.pdf

The ESR depends on the frequency, but the max ESR that these crystals will be about 300 Ohms for the 2.4576 MHz crystal, and about 50-55 Ohms for the 4.9152 MHz crystal. Based on the part number you provided, I believe they are using the latter frequency.

One thing that I have seen is that often is that the boards work better with a capacitive load that is less than what is mentioned in the datasheet. With these crystals, it's often operated with about 20 pF of load capacitance and there is already some parasitic capacitance on the board which may take up some of the load.

I would recommend having them drop the load capacitance to half of the value they are using now and seeing if this helps. Note that if they use a oscilloscope to monitor the crystal in/out pins, the probe adds an extra 10-20 pF to the load. It might be better just to monitor the /DRDY pin to check that the device is operating.

As for the specific crystal that the customer is using, it seems that an ESR of 150 Ohms is higher than than the other recommended crystals operating at 4.9152 MHz. I was finally able to find the datasheet as one of the ACT surface mount crystals.

After decoding the part number, it seems that the crystal is supposed to have a 16pF load capacitance (shown by the RO designation in the PO4915HOGROFC-PF part name). I would have them lower the capacitance to 10 pF or less to see if that helps, but I think the ESR may still be too high for that frequency.


Joseph Wu

Helge,


Thanks for the email. There are a few things that I want to point out. In the last post, you mention that reflowing the solder help solve the problem. That's likely to be a cold solder joint and I'd think of that as a problem in their manufacturing.

Second if they have a significant parasitic capacitance at either the XIN or XOUT pins, then this is an additive capacitance and you should really reduce the chip load capacitance to compensate for it. I'd consider dropping the load capacitance to 10pF or less. It should be easy to at least test it.

If you look at the EVM schematic, you'll see that at XIN C21 is only 12pF for the crystal which is rated to a 20pF load. I believe that C19 is used for the experimental kickstart and isn't a part of the circuit as a default. It does look like the ATS042 has a higher ESR, but I will say that the crystals recommended in the datasheet all list something lower. If you lower the capacitive load, it's likely that the crystal could take a higher ESR.


Joseph Wu