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ADS8401 1.25Mbps Specifics

Other Parts Discussed in Thread: ADS8405, ADS8406, THS4031

Hi:

I have many legacy boards that use an 16-bit ADS8401 sampling at 1 MHz  that is performing poorly as far as its SNR. The PDS says SNRAD of the ADS8401 is about 85dB. In our legacy system, it unfortunately measures about 60dB, presumably due to layout and pre-drive circuitry issues.

1.   Why is the ADS8401 discontinued or not recommended for new designs? Was this a business decision or a technical one? If technical, what were the issues with the chip that led to its replacement by the ADS8405 or others like it?

2. What drive amplifier is recommended for the ADS8401? The datasheet  doesn't seem to show one.

3. What does it mean to have 16 bits of resolution but 15 bits of no miising codes(NMC)? What's the NMC term anyways?

 

Regards,

David

  • Hi David,

    How are you going about testing the SNR? There are many things that could be occurring to give you a poor SNR. If you have a picture, or better yet, the time-domain raw data (code outputs) of the ADS8401 then we can look at the FFT of the output signal and have a good idea of where your noise is coming from. Some other questions I have regarding your application would be:

    • Are you using coherent sampling or non-coherent sampling, which requires the use of windowing to remove some the noise added when non-coherent sampling is performed?
    • Are you using an external reference voltage, or the internal reference voltage and is it stable? Unstable references could be the result of layout issues.

     

    Regarding your questions:

    1. The ADS8401 does have a rare timing glitch that causes only a few output codes to be wrong on occasion. At room temperature you see this glitch occur after about 12 hours of normal operation, then after one or two wrong codes the part continues with the correct output codes. This glitch appears more often at higher ambient temperatures (60 deg C). This glitch was fixed in the ADS8405 and ADS8406. The linearity was also improved in the ADS8405/6.
    2. The drive amplifier requirements for the ADS8401 are partially dependent on what the input signal is that you're trying to measure and whether ac or dc performance is more important to you. So far it seems that ac performance (SNR, THD, etc...) are the parameters you are most concerned about. The ADS8405 shows an application example with the THS4031 100MHz High speed amplifier, but you should be able to suffice with a slower but higher dc precision amplifier. By slowing your sample rate down to 1MSPS and increasing the acquisition time to 350ns or 500ns, a 6 MHz op amp driver will be enough. If you don't increase the sampling time and use the minimum 150ns, you may need an amplifier with a bandwidth closer to 25 MHz.
    3. A missing code is when a legitimate A/D converter output code that should exist is not available. An increase in the analog voltage can produce an unexpected smaller or the same digital output code. See the figure below, the code '100' will not occur as it should for an analog input near mid-scale (and in fact won't occur for any analog input), this results from internal IC mismatches that produce a large enough non-linearity to skip a code. This is a common occurrence with higher resolution parts (>= 16 bits). Parts with no missing codes must undergo stricter process and quality control requirements. At these high resolutions the missing code adds a small "noise" to your signal with an order of magnitude similar to the quantization noise. This noise is accounted for in the SNR specified in the datasheet.

    Best regards,
    Chris