ADS1262: ADS1262 strange internal reference behaviour

Hi Ilhan,

Welcome to the TI E2E Forums, and thank you for sharing your schematic via email!

A constant 70mV error with a varying input signal would seem to be more of an offset error than a gain error. Typically, if the reference voltage is inaccurate you'd notice a gain error that scales proportionally with the input signal amplitude. This makes me think you might be having more than just one issue...

First off, how are you measuring and verifying the 70 mV offset? If the ADC is in disagreement with the signal source setting, or if the signal source is being measured with a DMM, you also need to question the accuracy of the source/DMM to know which device to believe. Since the ADS1262 is a high precision device, you need to compare the voltage to a DMM with comparable accuracy. Might it possible this 70 mV offset is actually coming from the source or DMM measuring the source?

Regarding the reference voltage issue...
Are you measuring the reference voltage across the REFOUT/AVSS capacitor?
How is the breakout board laid out, and how is the board wired to the power supplies and MCU? I'd be interested to see these connections if you happen to have any images of the board - that would be really helpful to me!
Also, from looking at the schematic, how close is R6 (the resistor that connects the analog and digital grounds) to the ADS1262?

...there could be many different issues here, especially when wiring to a breakout board. If there is any significant resistance or current flowing between the analog and digital ground, its quite possible to see a shift in the reference voltage. Also, long jumper wires are usually inductive and any current transients may result in large voltage spikes; though I would expect these types of transients to be momentary and that the reference voltage would settle back to the 2.497V over time. Do you see any settling, or is the reference voltage from then on fixed at the higher potential?

Best regards,
Chris

Dear Chris;

Thank you for prompt reply.

You can find the layout design in below.

In your reply, you mainly underlined the measurement errors and the errors due to the imperfections of the PCB. In my opinion, this is not the case. In order to clarify the situation, I want to summarize the problem as follows:

• We measure the ref voltage @C3 capacitor’s pads but the problem is not due to the measurement problems as it is explained in the next item.
• Cabling between ADC and MCU boards are less than 10cm.
• Our results are exactly in line with the “ADS1262-3 Calulator.xlsx” excel’s results if the reference voltage is entered to the file according to the measured value.
  • In the case reference voltage is measured as 2.497V and we enter 2.5V to the excel, by assuming a 3mV error which may be acceptable, we get the expected result without 70mV offset.
  • In case the ADC, somehow, changes its reference voltage to 2.585V, then 70mV error occurs. By changing the value we entered to the excel to 2.585V we again get the expected result.
  • The problem is simply due to ADC’s non-deterministic changing of reference voltage i.e. it behaves as either providing Vref of 2.497 or 2.585V. There is no any value between them, either 2.497 or 2.585V just like as if ADC state changes.

Best regards

ilhan

Hi ilhan,

Have you tried probing the REFOUT pin with an oscilloscope? It's possible that the reference output could be oscillating and a DMM would only show a DC offset. I'd also recommend probing the power supplies to make sure they look clean as well. If you do see an oscillating reference, then you could try increasing C3 to 10uF.

Another think to check would be if the power supplies are current limited and choking the device of current. During power-up or reset, there may be some current transients that require the supplies to source more than the typical operating current.

Best regards,
Chris

Hi ilhan,

Typically a voltage reference is made up of a band gap reference followed by an output buffer, to prevent loading the band gap circuit. The buffer uses feedback to ensure that the output voltage matches the input voltage; however, when not compensated correctly that feedback network can become unstable. In the case of the ADS1262's REFOUT pin, there is the expectation that the REFOUT pin will have a 1 uF load capacitor to ensure a stable output voltage.

Since you have a 1 uF capacitor (C3) connected to the REFOUT pin and you still see oscillations, my guess is that the ESR or ESL between the REFOUT pin and AVSS pin is just large enough to cancel out this capacitance and make the reference buffer oscillate. In future designs, try to keep the REFOUT capacitor as close to the IC as you can and keep the trace and ground loop between these pins as small as possible.

Also, it should be fine to use a slightly larger capacitor on REFOUT (10 uF instead of 1 uF), as we've found that this also provides a slight noise performance improvement. Just keep in mind that when powering up the ADS1262, you will see a larger inrush current due to this capacitor charging up. Make sure that the analog power supply is capable of sourcing extra current (at least 100 mA or more) in order to provide for this current.

Best regards,
Chris