ADS1262: resistance measurement.

Hi Johannes,

Welcome to the TI E2E Forums!

I think you're idea is reasonable... The ADS1262's multiplexer wouldn't be the main cause of delay when switching between two channels; the digital filter and the programmed data rate would be the biggest source of delay. So long as your current source is stable and doesn't change amplitude between the measurements, you probably wouldn't have any issues.

However, instead of performing a second ADC measurement to determine the amplitude of the current source, I would recommend using Rshunt to create the external reference voltage to source to the ADC. By using this voltage as the ADC's reference, you create a truly ratiometric measurement that is independent from any change in current. Since the ADC compares its input voltage to its reference voltage, you're effectively measuring both inputs simultaneously. Rshunt would need to be a precision resistor on the order of 10 to 500 ohms, such that the voltage drop across this resistor exceeds the minimum 0.9V reference voltage requirement and also does not exceed the ~5V maximum reference voltage.

Your current, current source implementation would probably work, but here are some other possible implementations from another E2E thread:
https://e2e.ti.com/support/amplifiers/precision_amplifiers/int-precision_amplifiers/f/16/p/140361/506445#506445

Perhaps you could even use some combination of the ADS1262's internal IDACs and an op-amp to create a programmable current source, for example, forcing the IDAC current through R5 to change the current through Q1.

Best regards,
Chris

Hello Chris, 

Thank you for the feedback.

I changed the design a bit to do a ratiometric measurement. 

I also shifted the transitor to the top and am now using the precision reference resistor for the current feedback into the opamp.

by the way, the link in your post, to different current supply methods does not work.

With the 21.1 Ohm resistor the current range will be limited by the ADC min ref voltage of 0.9V and the max power of the resistor. 

so the minimum test current will be 0.9V/21.1ohm = 42.7 mA

and the maximum test current will be sqr(250mw/21.1) = 108.9 mA

To create the powersupply for the analog circuit I am planning to use an LDO, to convert 12V from a DC/DC converter to 5V for the analog circuit. is there a specific LDO that you would recommend.  The Digital part of the ADC will be powered from a 3.3V LDO of an Microcontroller dev board.

With Kind regards, Johannes

Hi Johannes,

That looks looks good to me!

Sorry about the previous link... I updated my last post to convert the URL to plain text.

You should be able to use the ADS1262's PGA in a gain of 4 V/V for all input ranges. However, for the smaller values of R you could increase the gain to utilize more of the ADC's input range if needed.

Do you have any idea yet of which data rate you plan to run the ADS1262 at or of how much resolution you need in your system?

For the current setting, I would recommend running it somewhere in the lower or mid-range of what is acceptable (maybe in the 50 - 80 mA range). This way, you're not likely to fall below the 0.9V minimum reference voltage requirement and you also keep self-heating of the resistors to a minimum.

Best regards,
Chris