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ADS1256: Is the ADS1256 right for my application?

Part Number: ADS1256
Other Parts Discussed in Thread: ADS1258, PGA281, PGA280, ADS1261EVM, LM27762

I'm trying to find an appropriate ADC for a data logger system I'm working on...

For my application I need a 24-bit ADC that can take a total of 8 single-ended/16 differential inputs. These may be from a variety of sources of varying magnitude, so I also need a configurable PGA. I will be taking readings from sensors like this (among others), whose output can go negative, so I'm also trying to figure out if I need a bi-polar input range or if the differential inputs remove the need for that. The maximum sample rate I'd need on any one input is 20SPS, but I may wish to oversample. I'm okay with losing a few effective bits due to the PGA/sample rate.

I've come across a few potential ADCs (one being the ADS1256), but none which offer 24-bit resolution and a PGA also offer the number of inputs I require. I'm now thinking the possible routes are:

  1. Use two 4/8 channel ADCs (possibly SPI daisy chained)
  2. Use a single ADC with 8 channel 2:1 analogue multiplexer
  3. Use a larger ADC without PGA, and find external PGAs

If I'm honest I'm very rusty on the AFE side of things and it's been a while since I've needed a dedicated ADC, so I need some advice. 

  • In general, what would be the best approach/architecture here?

  • Can I daisy chain the ADS1256 or is it easiest to just use in parallel?

  • Do I need a Bi-polar ADC or do the differential inputs remove this need?
  • Is there anything else I need to consider for the AFE?


  • Still hoping for some response to this if possible!

  • Hi Craig,

    You might also consider the ADS1258, which offers 8 differential inputs or 16 single-ended inputs (I assume this is what you meant in your original post).

    If you will need to measure voltages that are truly negative with respect to ground, the ADS1258 can also accept a bipolar supply range (AVDD = 2.5V, AVSS = -2.5V). If your sensor is referenced to 0V, then the ADS1258 will be able to measure input signals below ground.

    Finally, the ADS1258 offers a unique MUXOUT / ADCIN feature, so you can connect 1x signal conditioning circuit e.g. an amplifier, between the 16-ch MUX and ADC. You can see examples of this in the ADS1258 datasheet, specifically in the Applications section (Figs 67 and 68).

    I think this ADC might be worth considering for your application, as it offers the channel count and performance you need while also offering the flexibility to choose a signal conditioning circuit that can be tailored to your needs.


  • Hi Bryan,

    Thanks for your response. 

    I think I had initially ruled out the ADS1258 since it had no internal PGA, and instead assumed doubling up on an ADC with half the channels was the best way to achieve my goals.

    Thank you for pointing this out this feature of the ADS1258, I hadn't realised that it was possible to use in this way and it looks like it offers a few benefits (e.g. wider input range). Presumably any 1-channel differential or 2-channel PGA would be appropriate for this?



    I've done a little research and was wondering if the PGA280/PGA281 would be a good fit here?

    My only concern, is that they might require a very wide voltage reference. If I don't intend the output to exceed +-2.5V at any gain level, could I get away with feeding the PGA's VSN/VSP with the same VREFN/VREFP as I'm providing the ADS1258?

    Additionally, I'm not entirely sure how best to provide this bipolar supply to the analog components. Are there any parts you'd recommend if I have 12v, 5v and 3.3v available to the system?

  • Hi Craig,

    I have seen other engineers using the PGA280 with the ADS1258, so this certainly could be a good choice if you want programmable gain. Just keep in mind that the PGA280 requires a high voltage supply; please refer to the datasheet for more information. You would also need to make sure that the input signals to the ADC do not exceed the operating specifications. In other words, even though the PGA280 might be able to accept inputs >5V, the ADS1258 cannot.

    I did not understand the comment about needing a wide VREF. Why would this be necessary?

    You can reference the ADS1261EVM user's guide for an idea for generating +/- supplies:

    This device uses the LM27762 power management IC to generate a +/-2.5V supply off of a 5V or 3.3V unipolar supply.


  • Hi Bryan,

    My wide VREF question was more in relation to the high voltage supply required by the PGA, which is a little bit of a nuissance considering I don't ever need that much range on the input. I was hoping I could just supply it with the same supply as the ADC, but that sounds like it's probably not an option.

    Thanks for the user guide, I'll certainly have a look!


  • Hi Craig,

    What is the signal range your system needs to support?

    Will it be the same on all channels, or different?

    Will you therefore need different gains per channel, or could one single gain suffice?

    If you just could use a fixed gain, you could use a different amplifier that does not necessarily require the HV supply.