How to connect ADS8331/2 unused inputs

Marc: That is correct, floating input pins not only cause problems for analog but digital products also. They can oscillate and cause additional power consumption. Input if left floating every time the floating pin logic state toggles, additional input/output (I/O) current consumption will be the result. Please see the following TI document regarding unused pins guidelines in general.  http://www.ti.com/litv/pdf/sprab14

Regards,

Naser

Naser, I don't think that the introduction of the TI document http://www.ti.com/litv/pdf/sprab14  in this thread is correct.  People might be confused.  This thread refers to the analog inputs of an ADC, ADS8332.  The document refers to the logic inputs of a DSP.  That is a big difference.  We all know that logic inputs should not be left unconnected, due to the high input impedance of CMOS circuits, but this is not the subject at hand.  The question was posted about the analog inputs of ADS8332.  There are 8 inputs and they can be selected sequentially. The design engineer might not need all of them so he might leave some unconnected. 

Many engineers, including me, prefer to have some ADC unused inputs in their initial prototype to be able to test the ADC with different external signals if things don’t go well. For example, if I see a noisy measurement, I just connect a battery to an unused input and test the ADC behavior. Also, having some unused ADC input signals brings flexibility, especially if requirements change during the development.  Question is, can you just put some test points in the inputs so that you can easily connect a signal to any of them, or should you tie them to ground? 

Personally, I believe that there is no harm to leave the ADC inputs unconnected or, at minimum, just put a small cap on them.  This is in agreement with the ADS8332 evaluation board and other ADC EVMs from TI.  TI’s information about the ADS8332 input stage is scarce, but I believe that the oscillations seen on the inputs are not oscillations per se.  As the input channels are switched, the unconnected ones may show voltage which is due to the charge of the sample-and-hold capacitor after the mux. Normally, I would assume that the sample-and-hold capacitor be discharged during the channel switch.  However, since I do not know how the ADS8332 analog input is designed, this is just an assumption, and I am sure a TI engineer can better explain the ADS8332 analog input stage behavior.  If TI recommends that the ADS8332 analog unused inputs be connected to ground, as in the previous posts, I would like to know why.    

HI Adrian:

Thanks for the follow up. We appreciate your input and feedback. My post meant to give general guidelines and comparison form what we have seen or experienced throughout the years on the digital side. Sorry if that caused confusion here.  Although I  may agree with you and we may also can debate it on the analog side ( See my suggestion below from page 11 of the document regarding AIN0,1,2,3 for the on chip ADC pins), I believe my statement regarding digital floating input pins proper termination is still true and valid.

AIN0 I Analog input channel Analog input.   Leave None   Input 0 unconnected.

AIN1 I Analog input channel Analog input.    Leave None Input 1 unconnected.

AIN2 (BGA only) I Analog input channel Analog input. Leave None Input 2 (BGA package only) unconnected.

AIN3 (BGA only) I Analog input channel Analog input. Leave None Input3. (BGA package only) unconnected

Regards,

Naser

Hi Adrian,

It really depends on the input structure of the ADC, but general 'best practice' guidelines are that any input (either digital or analog) should be terminated.  On an ADC where there is a multi channel mux, you can still pick up/couple noise onto the sample and hold capacitor from an unused channel.  This can cause issues with performance beyond 12/14-bit levels.  Often an uncommitted ADC channel tends to drift towards mid rail, but that's not always the case - if there is a discharge path, you may see something closer to one of the rails. 

Grounding unused inputs via resistor or capacitor or taking them to a mid-rail or reference voltage is often a better choice - it still gives you a chance to use an uncommitted input via the good old 'blue wire' and often improves overall performance/noise of the system.  It really depends on the ADC and its input structure though so I'll try to get you specifics for the ADS8332. 

Thank you, Tom.  This is what I was looking for.  Looking forward for the ADS8332 input specifics.

So...what was the conclusion? You gave us three answers.  Grounded, terminated, and that application note says float (which was for a different device).  That datasheet provides no recommendations.  The device has 8 channels -- taking out those termination resistors would be awesome.  Thanks.

Bump.  

TI employees have provided two contrasting suggestions with no final outcome.