ADS125H02: Measurement of selectable different input types

Thanks.
So, what are the alternative components so that it can be considered as an alternative? Also, please share my RTD and thermocouple sample application schematics prepared with ADS125H02.

Hi elektroboard,

There is a lot that goes into designing a universal input system, so you will need to make some design choices based on the goal of your actual system

As a starter, you can look at the input structure on the ADS124S08EVM, which I have copied in the image below: https://www.ti.com/lit/ug/sbau272a/sbau272a.pdf

This EVM showcases how to setup inputs for RTDs, thermocouples, bridge measurements, thermistors, voltage, and current inputs.

The main difference with the ADS125H02 is that you will not need the resistor divider on the voltage inputs (R76 and R77 in the image below), because the ADS125H02 has a high voltage PGA

-Bryan

Hi Bryan.
Thank you for your valuable answers.
So, does this chip perform the necessary linearization process for RTD and thermocouples? Or should I do this on the microcontroller side?

Hi elektroboard,

The ADS125H02 does not perform linearization for RTDs or thermocouples, so this must be completed by the controller

However, we have a temperature sensor firmware library that you can use for this purpose: https://www.ti.com/tool/ADC-TEMP-SENSOR-FW

There are functions within this library that perform linearization and convert to temperature using look-up tables or polynomials. You would need to add in the low-level functionality for the ADS125H02, but this can be derived using the ADS125H02 example code: https://www.ti.com/tool/download/SBAC233

-Bryan

Hi.
So why should I choose which chip(ADS125H02 or  ADS124S08)?
Your answer is important to me as I am going to order an evolution board right now.

Best regards

Hi elektroboard,

Do you need high input impedance (>1 Mohm) for your application? If yes, then the ADS125H02 will likely be the best option due to the high voltage PGA. The resistor divider approach using the ADS124S08 will limit the input impedance you can achieve with your circuit

The H02 also supports higher common-mode differences between channels. This might be important for multi-thermocouple measurements where the sensors are at different common-mode voltage. This could also be beneficial for voltage or current measurements.

The H02 will likely require an external multiplexer to be able to measure all of your different signal types, so this is something else to consider

The ADS124S08 will not be able to support higher common-mode differences between channels, and will not allow for a high impedance input on the voltage channels due to the resistor divider (you could also use an external amplifier with the S08 however). But the S08 has more channels and may not need an external mux, as you can see from the EVM schematic that I shared with you

So those are really the pros and cons between these devices. I leave it up to you to decide which to choose based on your design goals

-Bryan

Hi Bryan;

Thank you for these nice answers. I would actually prefer the ASD124S08 for the multiplexer and other advantages you mentioned. But now I have some doubts.

I'm also curious about how to connect a 2-, 3- and 4-wire RTD to the ASD125H02. I would appreciate it if you could explain this with a diagram or by matching the pins to the RTD leads.

Hi elektroboard,

You can use our RTD and thermocouple design guides to help you answer these questions. These design guides provide generic examples for all of the different RTD and TC connections, which you can then apply specifically to the H02.

RTD: https://www.ti.com/lit/an/sbaa275a/sbaa275a.pdf?ts=1698958746113

Thermocouple: https://www.ti.com/lit/an/sbaa274a/sbaa274a.pdf?ts=1698958876491&ref_url=https%253A%252F%252Fwww.google.com%252F

-Bryan

Hi Bryan

When I examine the relevant documents in detail, I think I need a universal measurement method for RTD measurement.However, how can I configure this since ads125h02 has 2 analog pins? Because as far as I see from the document, I need 4 analog pins.