ADS1148-Q1: supply current requirement especially during startup

Hi,

if the supply current is limited, then the ADS1148-Q1 supply voltage creates a valley together with the current limit. The valley looks similar to a MOSFET gate voltage.

If the current limit/budget is e.g. 10mA, then the valley might take something like 5..10ms. If the supply current is more limited, then the time duration of the valley increases. If the time duration is longer than a certain value, then the ADS1148-Q1 fails working.

Br,

Tamas

Hi Tamas Fenyvesi,

I am not sure what you mean by "valley" in your previous post. Are you referring to something like a brownout condition where the supply fails to ramp to the final voltage and instead gets stuck at an intermediate voltage for some time? Again, it would be helpful to see this on a scope so we can more clearly understand the issue you are seeing.

Anecdotally I have seen that other, similar ADCs can require 10mA or more of instantaneous current. It therefore makes some sense that you are having issues when you restrict the supply current limit to <10mA.

Are you able to increase the supply current limit in your system to accommodate for these short bursts of current?

-Bryan

Hi Bryan,

yes, exactly as you wrote: the supply voltage gets stuck at a certain voltage for a while rather than reaching the recommended range by a continuous ramp.

We modified the supply current source for allowing more current. Anyway, given we are talking about more than 20 times higher current than what the datasheet communicates, we need valid specs that covers the scattering and the full temperature range for a correct calculation and sizing.

Br,

Tamas

Hi Tamas Fenyvesi,

Typically we see our ADCs being paired with power supplies + low noise LDOs that can regulate 100s of mA of current, even if the ADC does not require this amount of current. Therefore, this isn't a common challenge. I am glad you were able to modify the power supply to increase the current limit. Did this resolve the issues you were having?

As I stated in my previous post, the 10 mA value is just anecdotal evidence from other devices, it is not meant to be a hard requirement for the ADS1148-Q1. In fact, the ADS1148-Q1 could be quite different, as I am unaware of any test data that identifies the instantaneous current draw for the ADS1148-Q1. I presume you could measure instantaneous current consumption using a high-speed scope if this more detailed information is required for your application.

-Bryan

Hi Bryan,

(Regarding technical part, an automotive LDO is easy for positive, but much less options are available for the negative. On the other hand, ADS1148 can be used in micropower/wearable environment, e.g. medical (ECG), where the bottleneck is not necessarily the LDO.)

Whether LDO or something else, also whichever application area, if the current consumption is so much higher than the datasheet value even temporarily, then the datasheet ought to include hints as a very minimum. (Not to mention that the manufacturer ought to exactly know any potential causes of such an effect for avoiding any internal latch-up in my logic.) Any relevant estimation needs deep knowledge of internal structure and technology - which only the manufacturer has.

It isn't satisfactory to conclude "folks usually don't see such effect - but some do", since it is products with responsibilities.

So, what's the specification, please?

Thanks,

Tamas

Hi Tamas Fenyvesi,

Let me see if I can look into this in more detail. Please note that this may take some time e.g. a week or more, before I am able to get the setup and equipment ready, take data, and summarize the findings

Also please note that should we provide data on this issue, it is not guaranteed since it is not included in the ADC datasheet. Therefore you use it at your own risk. I just want to level-set expectations so there is no confusion moving forward.

-Bryan

Hi,

Thanks and looking forward to hearing from you.

Please also note that we've seen large variance in the inrush current, therefore some kind of analysis seems more usable than experimental verification of one/few pieces.

Kind regards,

Tamas

Hi Tamas Fenyvesi,

In the meantime then, can you share your system schematic including the ADC, any signal conditioning circuitry, and the power supply? It would be worth also checking to make sure this is not a different applications issue.

If you do not want to share this information on a public forum, you can send it to me via private message. Please hover over my user name and then select "Request friendship". This will allow us to share messages privately

-Bryan

Hi Tamas Fenyvesi,

I just wanted to let you know that we are still looking into this issue, but I have nothing concrete to share with you at the moment.

In the meantime, would you be willing to share your schematic including the power supply so we can double-check for any applications issues? You can use the process I outlined in my last post if you do not want to share your schematic in a public forum.

-Bryan

Hi,

Thanks for updating. I'll send the related schematic part soon - I've been just flooded a bit - we all know this situation

Kind regards,

Tamas.

Thanks, we will continue working on our end, no rush on the schematic.

-Bryan