data rate shift on ADS1217

julien

Other Parts Discussed in Thread: ADS1217, ADS1216, ADS1218

Dear all,

Component : ADS1217
Datasheet : SBAS260C

It's explain on the ADS1217's datasheet (p17) : that when the decimation register DEC0 & DEC1are set by default (1920) with a 2.4576MHz crystal, the data rate frequency is 10Hz so the /DRDY output should be at the same frequency.

I check that with the following equation(p24) :
Decimation Ratio = Fosc/(mfactor*Fdata)
= 2.4576*10^6/(128*10Hz)
= 1920

There is my registers setups:
    SETUP(00h) = 00001110
    MUX(01h)       = 00001000
    ACR(02h)       = 01111000
    IDAC1(03h)    = 11010000
    IDAC2(04h)    = 00010100
    ODAC(05h)    = 00000000
    DIO(06h)        = 00000000
    DIR(07h)        = 01000000
    DEC0(08h) = 10000000
    M/DEC1(09h)= 01110111
The others register are set by a SELFCAL command.

Problem:

The /DRDY output frequency is 9.4Hz instead of 10Hz...

Questions:

Does this shift is explain on any datasheet? If yes which one?

Does this error is the same with all the components?

Thanks you by advance for all yours support.

Julien

over 16 years ago

0 Rick Downs - WEBENCH Design Center over 16 years ago

TI__Expert 5515 points

Have you checked that the fOSC frequency is really 2.4576MHz? Could it be that your crystal is being "pulled" to a lower frequency by inappropriate load caps? With 2.4576MHz exactly going in, you should get 10Hz out on /DRDY with the register settings you have.

0 julien over 16 years ago in reply to Rick Downs - WEBENCH Design Center

Prodigy 30 points

In fact the /DRDY clock was ok but when I select an other channel I miss two falling edge.

That why i thought that there was a bad frequency on /DRDY.

But i still don't understand why selecting channel change the DRDY clock?...

0 Rick Downs - WEBENCH Design Center over 16 years ago in reply to julien

TI__Expert 5515 points

Well, it took some digging, but there is an explanation: on the ADS1217, unlike its cousins the ADS1216 and ADS1218, the DRDY is suppressed until the filter fully settles. So since you're using the sinc3 filter setting, whenever you change channels the part holds off sending DRDY until the third conversion after the channel change completes. That's why you're missing two DRDYs when you change channels.

0 julien over 16 years ago in reply to Rick Downs - WEBENCH Design Center

Prodigy 30 points

Allright! Thank you for your help.

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data rate shift on ADS1217