ADS1210: Vref, Vbias and Xin specifications

Hi Suzuki-san,

See my response below.

Best regards,

Bob B

Takumi Suzuki1 said:

Part Number: ADS1210

Hi,

Let me ask you following questions about Vref/Vbias, Xin specifications.

1) Vref

    The Vref value of the specification is from 2.4V(min) to 2.6V(max), and the temperture specified is from -40 to +85℃.

    My question is that the Vref value is fixed at one value of between 2.4V and 2.6V within -40 to +85℃, correct?

    For example, once the value is 2.55V fixed at 25℃, the value is almost same as 2.55V with drift(25 ppm/℃) during the temperature. (not changed)

    Or the value is possibly to cahnge to between 2.4V and 2.6V with temperature drift??

[Bob] We do not have capability to trim the reference to a specific voltage.  When tested at final test, the reference output voltage can be from 2.4V minimum to 2.6V maximum at 25 deg C.  The drift specification comes from simulation data.  Life test data of 90 devices shows the mean shifts by about -1mV, and standard deviation is about 7mV across devices.  From the initial test data it appears that the initial mean of the reference was about 2.4887V. 

2) Vref drift

     The specification is 25ppm/℃, but this is "typ" value.

     Will you tell me the max value, if possible?

[Bob] Unfortunately I do not have the original simulation data available.  As the internal reference has issues with initial accuracy, noise and drift it is recommended to use an external reference for the best level of precision.

3) VBIAS

    VBIAS is generated from Vref, I think. [Bob] That is correct.

    So I suppose the "min-max" value range has almost same relation with Vref.

    Why is the "drift" value 50ppm/℃ against the one of Vref, 25ppm? (2 times large) [Bob] As there is an op amp involved to buffer the reference as well as add a gain of 1.333, the reference drift is a factor, but you must also include the drift of the op amp and gain resistors.

4) Xin

    Will you tell me the specification of Xin error/margin at 10MHz?

    For example, the margin should be within +/-50ppm.

[Bob] As this is an old design, and as I recall the typical crystal could be 100ppm.  50ppm is common today, but 30ppm is also available.  I believe any of these types of crystals should work.  The error will be relative to the change in frequency of the modulator and output data rate relative to the chosen decimation.  I doubt that the difference is easily discernible, but unfortunately I do not have any data to support that conclusion.  Typically clock jitter is a more dominant factor with respect to the modulator and error. This would be a consideration when using a crystal oscillator as input instead of a standard crystal.  In most cases a few nanoseconds of jitter will not be an issue for low data rates and DC type measurements, and for higher data rates the quantization noise will most likely dominate.

As I know this device is too old to check the specifications, I would be also glad even though your answer is just general infomation.

Thank you for your support in advance.

Best Regards,