1989 vintage ADC replacement

Discussion in 'General Electronics Chat' started by kubeek, Jun 6, 2013.

  1. kubeek

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    Sep 20, 2005
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    My dad's got a precision lab-grade scale, but recently it stopped working. Tracking the original manufacturer I got hold of the service manual for the scale, and figured out that the fault lies in the ADC module as it doesn't output any data and the pin is floating.

    Now the ADC is a quite simple 15-bit multiple ramp converter module, but it is potted into the case quite well, and since it was made in the early 90s I doubt I can get a spare from anywhere, so the final choice is to replicate one.

    As you can see from the DS5601 datasheet, the ADC simply does the conversion on its own, and then you can read out the data serially, the conversion takes 10ms so this should be easy to achieve.
    The bigger question is what ADC should I choose? The main problem probably being linearity and low drift. I found this LTC2389-18 one at Linear which seems that should be far superior to the original, but I am a little worried that the very fast conversion time could introduce noise into the system.
    Any thoughts?
     
  2. kubeek

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    I came to conclusion that the scale uses 16x sampling and averaging to get it's required 19bits of precision.
    So the question is, if I use a higher precision and lower noise ADC than the original, would I have to do dithering or something in order for the scale to get the correct result with this sampling?
     
  3. t06afre

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    To me it looks like the ADC datasheet is not complete. As it looks as the ADC expect some settings from the controller unit. Unless you get a replacement for the ADC5601. I think it would be a hard fix to do
     
  4. kubeek

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    In the diagram on page two, I would say the ADC is completely autonomous and the only thing you can do with it is read the contents of the shift register, which is valid after the adc sets the end of conversion signal.
    Or what makes you think the datasheet is incomplete? I doubt that they could fit into the 50x50x10mm footprint any more featuers than it already has.
     
    Last edited: Jun 7, 2013
  5. kubeek

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    Well, here is the replacement I made. The reference voltage is 6.95V and the input is from about +1V to -2V so I had to make a bit of level and gain conditioning.
    Any thoughts on the way it is connected and the layout? I finally decided to use LTC2440. (attached is also a schematic and board file from Eagle 6.4)
     
    Last edited: Jun 11, 2013
  6. t06afre

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    I might be wrong here[​IMG], but was not the ADC that your weight just 16 bit. And to get higher resolution several reading had to be averaged. So would it not be a problem using an ADC with higher resolutions than 16 bit since the controller inside the weight would never expect or read more than 16 bit
     
  7. kubeek

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    I thought about this, and someone from eevblog forum advised to basically divide the ADC value into the 15bit chunks in such way that they will add up to the actual 19bit value that the scale presumably makes out of them.
    But this is of course subject to further modification based on how it will actually work, I will probably start with just sending arbitrary values and seeing how the scale reacts.
     
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