ADS7828 Read Problem

Kevin Shaw

Other Parts Discussed in Thread: ADS7828

Hello,

I'm reading unusual values out of the ADS7828. I've monitored the I2C bus and the values are actually being read out of the device. There seems to be ranges of input voltages where the ADC will output the same value continuously and then start sending a more correct value once the input voltage being sample rises or falls below this range. Also, I very rarely see the LSB of the data being set to a "1". For example, data is like 0xA20, 0xA24, 0xA2C, etc...This issue has been seen on more than one board so it's not a bad part. Is there any reason (timing of conversion, etc...) that could cause this? The I2C bus access appears to be correct. A graph of the data illustrates the problem:

The first graph is a voltage measurement. The last two graphs are of a temperature sensor.

Thank you.

over 13 years ago

Tom Hendrick over 13 years ago

TI__Guru**** 182671 points

Hi Kevin,

Can you describe your ADS7828 hardware for us? Are you using one of our EVM's or is this your own PCB? This sort of plateau effect is usually caused either by the reference, the ADC input drive or something with grounding. Is it possible for you to send along a schematic and/or layout if this is your own hardware?

Kevin Shaw over 13 years ago in reply to Tom Hendrick

Prodigy 40 points

Hi Tom,

I have attached a graphic file of the portion of the schematic with the ADS7828. This probably doesn't provide enough information but unfortunately I'm not able to provide the entire schematic.

Thanks,

Kevin

Tom Hendrick over 13 years ago in reply to Kevin Shaw

TI__Guru**** 182671 points

Hi Kevin,

My apologies for the late response here. Have you made any headway with the plateau issues you were seeing?

Kevin Shaw over 13 years ago in reply to Tom Hendrick

Prodigy 40 points

Hi Tom,

I bought the ADS7828 EVM and I plan to do some testing using a Xilinx ML405 board to drive the I2C bus. However, my client is pushing to do a demo late next month and we have been working towards that event. The ADS7828 is not involved with the demo so I've put that on hold for now. I'll post any findings once I get back into it.

Thanks,

Kevin

Tom Hendrick over 13 years ago in reply to Kevin Shaw

TI__Guru**** 182671 points

Hi Kevin -

How is your project going?

Kevin Shaw over 13 years ago in reply to Tom Hendrick

Prodigy 40 points

Hi Tom,

We're still working towards the demo. I haven't gotten back to the ADC issue yet.

Thanks,

Kevin

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ADS7828 Read Problem