ADS1258: Nucleo G071RB & RS485 & I2C

Hi Yoeri Caarls,

Some comments about your schematic:

• You mentioned having 0 ohm resistors in your schematic that you will populate / not-populate, however usually there is a default configuration. Right now for example I see you have one 0ohm to ground and one 0ohm to 3.3V on the CLKSEL pin, which will likely put this pin into an unknown state (and therefore the ADC). I would expect to see a DNP / DNI / etc. on one of these components (there may be more) so the default configuration is known
• I would not use a shunt reference for the ADS1258. The recommended reference circuit is included in the ADS1258 datasheet. You can also use a REF6xxx, which has an integrated reference buffer incase you don't want to use something like a REF5xxx + buffer
• The input filtering has a very low cutoff frequency (your comment says 159Hz), which might impact the frequency response of the ADC digital filter. The external filters will also take a long time to settle, which you typically want to avoid with a higher speed, multi-channel, multiplexed ADC like ADS1258. Your design is not necessarily wrong, you just need to consider these factors when choosing the final component values.
• I would not recommend using a crystal oscillator with the ADS1258. You can search on e2e and find dozens of threads where engineers have difficulty getting the 32kHz crystal to oscillate. Instead I would recommend using a 16MHz clock oscillator, especially if you want to drive both ADCs with a single clock source.
• The default configuration of the GPIOs is that they are inputs, and therefore should not be left floating. You can tie these pins to ground if you are not planning on using any of them
• I did not review the RS485 or Nucleo sections of your schematic

-Bryan

Hi Yoeri Caarls,

Yoeri Caarls said:

The 0 Ohms to the GND is a backup if i need to make it LOW, it will normally be connected to the 3,3V so it will be pulled HIGH. So the other 0 Ohm resistor is to disconnect is from the 3,3V. So it should not be a problem.

I understand that some of the 0ohm resistors might not be included in the final design. However, usually these are labeled as DNP on a schematic so it is clear which components are populated and which are not. Otherwise, the schematic can be confusing. I would consider making your intentions as clear as possible on your schematic in the future

Yoeri Caarls said:

The crystal oscillator of 32 kHz is used in the datasheet, so i figured it would be a well suited component. In the first case i want to use the clock of the MCU, which is 16 MHz if i am not mistaken. So the crystal is as a backup, but i can change the crystal to a 16MHz variant if that works better.

To be clear, if you use a crystal, it has to be 32kHz as specified in the datasheet. You cannot use a 16MHz crystal, you can only use a 16MHz clock oscillator that feeds directly into the CLKIO pin (and then also configure CLKSEL to use an external clock). I do see the connections where you allow for an external clock, this is good because it allows you to try both options

Yoeri Caarls said:

In the schematic of the ADS1258 the MUXOUTP and MUXOUTN are connected to the ADC directly, but in the other one they are not connected and need to be routed to the ADCINP and ADCINN. Can you verify which one is correct?

I'm not sure what you mean by this, both ADCs in your schematic have MUXOUTx connected to ADCINx through 0ohm resistors. The ADC has a BYPAS bit that either connects MUXOUTx and ADCINx together internally or uses the external connections in case you want to put some circuitry between the MUXOUT and ADCINx pins. See the register map for more information, as well as the applications section for an example of how you might use the external MUXOUT connections. Again, if you just want to feed the output of the MUX directly into the ADC, then you can use the BYPAS bit

-Bryan