ADS1263: Saturation on power-up

Part Number: ADS1263

Does the ADS1263 have the ability to short the inputs temporarily?

Hi Ron,

Can you explain what you mean by saturation? Are you getting a full-scale output code from the ADC? Can you provide more details on what occurs before and after this event? Sharing a schematic and register settings would help as well.

You can always short the inputs together by setting both channels of the INPMUX register to the same value (for example, set both to AIN0). The ADC also shorts the inputs together during a self-offset calibration (SFOCAL1) as described in section 9.4.6.2 of the datasheet.

-Bryan

Brian,

By changing the .22uF cap to a .1uF, the warm up time reduced significantly.  By changing it to .024uF, it reduced it further.  By saturation, I am getting a max ADC value.  This is being used for an electro-magnetic flowmeter.  The fluid is grounded and I am using +-2.5 volt supplies (using switchers followed by LDOs).

I currently have replaced the 10.0M resistors with 20.0M.  C64 is a networks so that the 1000pF is matched.

I only need a 20Hz response rate.  

A TI application engineer indicated that I will need the 20M input resistors.  Should the 1000pF be reduced in size?

When I had the .22uF cap in place, I would get a 5 minute warm up curve that looked exponential in shape.  It followed the high raw values start at ADC15600 to around 3000 where the differential volt measure would stabilize.  

I welcome a call at 608-235-7503. 

I am using a gain or 32 and a sync filter.  I take two readings at a 7.5Hz rate.  I have positive magnetic field generating a positive value followed by a negative magnetic field generating a negative value.  The two values are added and the steady state noise drops out leaving the signal.  

I am not auto doing an auto zero or a self calibration at the moment.  But we will implement self-offset per your suggestion.

Register values are:

All I do is power up the circuit and monitor flow from my magmeter.  It stabilizes after 5 minutes if it does not saturate (max ADC value).  

Ron

Hi Ron,

Do you have a schematic to share? You referenced C64 in your thread but I did not see an accompanying picture.

If you would like, you can contact your TI FAE and we can work this issue offline. I am a bit unclear as to what you want your circuit to do and how it is currently not working.

-Bryan

Hi Ron,

It looks like you might have edited your previous post to include additional information. But any images, etc. did not come through. I see something specifically saying "Register values are:", but there is nothing after this.

I usually need to save images to my hard drive and then attach them instead of trying to insert them directly into the post.

-Bryan

I managed to reduce the start up time significantly by reducing the differential cap from .22uF to 10,000pF NPO.  However, I still get max ADC values for 20 or more seconds on power-up.  If I momentarily short the diff input together, the ADC values are normally below 5000 and I get good differential readings.  I will try to connect suggestion above.  It seems like it would work.  I have a 1000pF X7R cap array (so the caps will be matched) for the common mode capacitors.  Would it be better to change them to NPO's and if so what tolerance would you suggest?  I appreciate your help on this.  It is very important to this project.

Ron

Hi Ron,

NPO caps should be fine. We usually use 5% C0G caps for the input filters on our EVMs, so I would change the CM caps as well.

-Bryan

Hi Ron,

Also, can you just provide a quick drawing of your input circuit so I can see where you are applying the voltages and how the input passives are connected to the ADC?

-Bryan

Shorting the inputs above take care of the input amp but it does not short out the external differential capacitor that I think is the problem.  Do I need to add an analog switch to remove the charge on the differential cap on start-up?

Also, to accommodate a high variability in the input resistance, would you recommend placing a unity gain high input impedance op amp on each of the IN+ and IN- inputs?  These would have be very good op amps to not introduce new noise.  But because this is a mag-meter, any constant noise or offsets would be subtracted out because the input signal is reversed and then every two reading are substracted from each other canceling steady state offsets and noise and leaving 2X the signal input.

Ron

Hi Ron,

Can you just provide a quick drawing of your input circuit so I can see where you are applying the voltages and how the input passives are connected to the ADC? I cannot really comment on the need for a switch without knowing how the input circuitry is configured.

The ADS1263 already has high input impedance since you are using it in a gain of 32 - you can see in the datasheet it is 1Gohm (typ) with the PGA enabled. I do not think an additional buffer would be required. In fact, we have other customers using the ADS1263 for flow meter applications where the sensor input is connected directly to the ADC without the need for active circuitry (passive filtering, etc. is required of course)

-Bryan

Thanks for the note on the PGA.  I think I still need to momentarily short the 10,000pF differential cap to eliminate the saturation on power up.  The only way to get around it would be to reduce the size of it and the common mode caps even further and rely on the digital filtering for noise reduction.  Does this make sense to you and if so, would you recommend 100pF common mode and 1000pF differential cap?

P.S.  If I put an analog switch on the front end to momentarily short it during power-up, I worry about the addition noise that it would generate on the input.  Perhaps I am being paranoid?

Ron

Hi Ron,

Without being able to see what your input structure looks like, it will be hard to determine if your suggestions will work, so I doubt my comments will be very helpful. Can you please help answer my questions below so I can assist you further?

• Can you at least share a drawing of the input conditioning circuit (everything between the system input terminals to the ADC) if you do not feel comfortable sharing an entire schematic?
• What type of sensor are you using? A part number would be useful
• What is the output voltage of the sensor upon power-up?

You may always share this information via private message, though I believe you need to request me as a friend first.

-Bryan

The cap that I think produces max values (saturation) is C8 which was changed from .22uF to .01uF but still produces saturation.  It seems like I need to place an analog switch from ANI0 to ANI1 and active it upon startup for a second.  I have changed the 10M input resistors to 20M.  Is that OK?  What size do they have to be to prevent input offset currents from causing a problem on this flowmeter application?

This should be the last of my questions to come to a resolution on this.

Thank you.

Ron

Hi Ron,

Since I cannot see how you are connecting the passives to the ADC, I cannot comment further without the information I requested in my last post.

-Bryan

C 8 is now a 10,000pF.  I have to short it out upon power up to prevent saturation.  Do I need to put an analog switch on the front end to automate this?

Ron