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ADS1282: Need help for Data variation after specific time

Miteshkumar Dalabhai Patel
Intellectual 1900 points
Part Number: ADS1282
Other Parts Discussed in Thread: OPA4277, ADS1118, ADS131M02, ADS1262, ADS1256

Hi..

One of my customer is working on high resolution temperature controller & they have considered ADS1282 for their project.

They have used OPA4277 as front end & made differential amplifier using 2 opamps in buffer mode. No gain from OPAMP, only low pass filter is implemented with OPAMPs. Design is working fine accept one issue.

After power on when they are capturing the data (clock - 1 MHz), after 30 samples they see the drop in count & hence they have to discard the 30 samples initially. After that it remains on the same level. This variation is about 80 to 100 counts. We would like to know what could be the issue or is it the known behavior.

Pls. find below snapshot.

  • 0
    TI__Mastermind 18900 points

    Hello Miteshkumar,

    From the description this sounds like they aren't compensating for the digital filter delay for the FIR to fill with data. There's some more info in the FIR section but I've grabbed a couple sentences from the modulator overrange section which is similar to a power up.

    Note, that truly settled data or latency from actual settled data will take 62 samples as shown in the electrical characteristics table. 

    Best,

    -Cole

  • 0 in reply to Cole Macias
    TI__Intellectual 1900 points

    Cole,

    If user wants use 2 channels & switch from one channel to another one then how many samples needs to discard? If they have to wait for 62 samples then part is no more usable in multi channel measurement.

    Is my understanding correct? At present my customer is using for single channel but in future they might looking to use both the channel.

    Mitesh 

  • 0 in reply to Miteshkumar Dalabhai Patel
    TI__Mastermind 18900 points

    Hello Mitesh, 

    Yes, your understanding is correct. 

    This device's typical application will use an array of the ADS1282s to measure multiple inputs. Switching to channel 2 is primarily used for calibration and checking if the device and sensor are functional. Not saying this you need to use the ADS1282 this way, but this is pretty typical for the targeted use case.

    I'm not sure what your customer's primary objective is with this device but the ADS128x family does a really good job and accurately measuring really slow AC signals. It doesn't have simultaneously sampling (SimSam) channels like a device such as the ADS131M02 nor does it have faster settling through digital filters like an ADS1118.

    Of course, accuracy is the tradeoff between these considerations so I'd encourage you to take a look at other devices. Most people use this for accuracy but some will bounce off of it when they understand the tradeoffs for the devices TI can offer.

    Best,

    -Cole

  • 0 in reply to Miteshkumar Dalabhai Patel
    TI__Mastermind 18900 points

    Hello Mitesh,

    I did forget the fact that you can bypass the FIR filter and only use the SINC filter using FILT[1:0]. This will bump up the sampling frequency quite a bit and kill the noise performance. But it doesn't have as bad as delays if you want to switch between the inputs.

    Most customers bypass the FIR because they build their own FIR (or another SINC) in software so they can use the best of both worlds. Unfortunately, I don't have any great digital filtering material to send your way if that is an option for the customer.

    As for the other TI devices that might be a good trade off, have them check out ADS1256 and ADS1262.

    Best,

    -Cole

    Edit: what End Equipment is this for? Might have a better recommendation