The AD7770 has 8 simultaneously sampling analog-to-digital converters with each channel having its own mux, PGA and SINC filter. The AD7770 is specified down to DC. However what is most eye catching from a functional safety point of view is a 9th ADC, a SAR ADC at the bottom of the block diagram.

     

Figure 1 - block diagram of the AD7770

The SAR ADC is not as accurate as the sigma-delta ADCs, but it is fast enough to convert the input to the other 8 channels in the time they take to do one conversion on a sigma-delta channel This means you do not have to pause converting on the other channels to get a second opinion on the conversion results. If using the SAR ADC as a diagnostic, then since it is a different architecture to the sigma-delta ADCs there is good protection against CCF (common cause failures.) It is for instance very unlikely that the SAR and sigma-deltas would have an EMI weakness at the same frequency. This is an example of diversity.

Another relevant functional safety features on this part includes that while performance is specified to 105’c, functionally is specified to 125’c and this should facilitate de-rating. While an application might need 0.001% accuracy for normal operation an accuracy of 1% to 10% is typically okay for safety. If your life depends on a system having < 1% accuracy then perhaps you might want to reconsider what you are doing!

For more information on the diagnostic features of the AD7770 see AN-1405 on analog.com. Because this app note does all the talking I can leave this blog a bit short.

The next blog in the series will also be a short one where I introduce the requirements for on-chip redundancy according to IEC 61508-2:2010 Annex E. That then will lead to two further blogs on ICs with on-chip redundancy including the ADSP-CM417 and the AD7902/3. After that I really should do some blogs on functional safety for software but that is still many weeks away.

This blog’s video is not really relevant to the AD7770 but is relevant to ADI (wait for it) – see https://www.youtube.com/watch?v=il1Byvn_vMA

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