ADS1246 reading problem

Cynthia Chen

Other Parts Discussed in Thread: ADS1246

I am using an ads1246 for my application to read voltage. It seems everything runing ok, I can set the registers and read them back with the correct values. The problem is the reading is not right. Here is some of the data:

intput voltage (AINP) adc reading

0 0x107

5mV 0x119

10mV 0x115

15mV 0x114

20mV 0x25d7

25mV 0x6ab3

30mV 0xa9bc

It seems like there is a threashold around 20mV, below that level, the ADC has no reading. Even after that level, the slope doesn't look correct also. I tried continuous conversion mode and single conversion mode , tried different data rates, it all behave the same. Does anybody have any ideas about this?

Thanks in advance for any help.

Cynthia

over 13 years ago

0 Bob Benjamin over 13 years ago

TI__Guru** 113765 points

Cynthia,

It appears that you are trying to read a single ended measurement. There is a common mode input restriction which prevents the device from reading near AVSS and AVDD. Please see the Design Notes page for more information:

http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/input-voltage-range-requirements-for-the-ads1248-and-ads1148-families.aspx

Best regards,

Bo bB

0 Luis Chioye over 13 years ago

TI__Guru 54556 points

Hello Cynthia,

Welcome to the Forum! One possibility will be that the measurement is outside the common-mode range of the internal PGA of the ADS1246. The nternal PGA has a has a common-mode voltage requirement. and if you are measuring a single-ended signal with respect to ground, the ADS1246 needs to be configured using bipolar supplies +/-2.5V. For more information on the common-mode requirements please look through the link below.

Is this a single ended measurement referred to GND or is it a differential measurement? What voltage reference are you using for conversion? What is the PGA gain set to?

http://e2e.ti.com/support/data_converters/precision_data_converters/w/design_notes/input-voltage-range-requirements-for-the-ads1248-and-ads1148-families.aspx

If possible, please send a schematic and we could review it.

Thank you,

Luis

0 Cynthia Chen over 13 years ago in reply to Luis Chioye

Prodigy 30 points

Hi Luis and Bob,

Thanks for your reply. Actually we use differential input, sorry for the misleading. Here is the ads1246 part of our schematic.

Thanks again for your help.

Cynthia

0 Bob Benjamin over 13 years ago in reply to Cynthia Chen

TI__Guru** 113765 points

Cynthia,

All of the B side inputs to the mux are connected to ground, which in essence makes this a single ended input to the ADS1246.

Best regards,

Bob B

0 Cynthia Chen over 13 years ago in reply to Bob Benjamin

Prodigy 30 points

Looks like our VCM is out of range. I talked to our hardware engineer, he is going to change the AVDD to +2.5v, AVSS to -2.5v and the reference voltage to 2.048v. Thanks for all your help. If you see other problem in our design, please let me know.

Reagrds,

Cynthia

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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ADS1246 reading problem