ADS1230: Schematic layout tips welcomed

Hi Charles,

Welcome to the E2E forum!  See my responses below.

Best regards,

Bob B

Charles Cordiez said:

Part Number: ADS1230

Hi team,

I have read a few threads about ADS1230 designs (thanks Bob for your detailed answers on the subject) and I still have a few questions and I am looking for advices/design review.

- In "ADS1230REF User's Guide (Rev. A)" the schematic page 27 uses two 1uF feedthrough caps (C47, C46) and two 100pF (C29, C32) for common mode filtering and one 0.1uF (C7) for differential. Is it normal that it doesn't follow the 10/1 ratio between common and differential caps ? [Bob] A colleague designed the ADS1230REF and I don't know all of the rationale behind the use of the feedthrough caps.  Normally these would be used as transient suppressors.  If I were to redesign this board, I would remove C46 and C47 and instead use C7, C29 and C32.

- Is it good to use a single feedthrough for common mode filtering (that might be more balanced) than two regular caps ? [Bob] Actually the common-mode caps are not all that important and could be left out entirely, but many customers like to use them.  That said the caps that are of the construction similar to the feed-through caps used, can accomplish both the differential and common-mode requirements if installed correctly.  The current ADS1230REF design does not use the caps in this way.  Instead of using as a feedthrough, you instead place as a differential cap.  In this way the single cap acts as both differential and common-mode for EMI type applications.

https://www.johansondielectrics.com/downloads/x2y-filter-and-decoupling-capacitors.pdf

- Should REFN be filtered as REFP or can it be tied to AGND ? [Bob] Although ground noise can be filtered some in this way, I have found no benefit of doing this if the PCB layout is optimal.  So normally I just tie REFN to AGND.

- Should I add caps to complete my RC filter (R1, R2, C1) ? [Bob] Yes you should have analog input filtering for at least antialiasing reasons.  It is also beneficial for EMI/RFI filtering.

- Stupid question : Are there limits to resistor values (R1, R2) is under 10R too low, is over 10k too high ? [Bob] This is not a stupid question at all.  Any resistance and capacitance will require time to settle.  If the resistor is large and the capacitor is also large, then the settling time for changes in the signal output of the sensor will take time before the reading is valid.  Also, if the resistance is large there can be added thermal noise to the measurement, and any input bias current of the ADC can create a voltage drop across the resistance adding to error.  If the resistance is too small, then you may not have the filtering characteristics that you require.  I generally target in the range of 500 to 5k as a starting point.

Regards,

Charles

Hi Charles,

My only comments are with respect to the SPEED pin pulled to +5V instead of +5F.  Also, you may want to add some options for Gain and Speed by creating a voltage divider from +5F to GND and only populating the resistor for the setting you wish to use.  You may find for example, that you need some additional gain and you may want to pull the gain pin high.  So instead of creating a new layout you could just remove the pull-down and add the pull-up.

Best regards,

Bob B