ADS124S08: Driving ADS124S08 with THP210

Hi Metin Enes Yildirim,

Of course we can help, but can you tell me why you want to drive the ADS124S08 with the THP210? The ADS124S08 has an integrated differential instrumentation amplifier, so the use of an FDA seems redundant.

If you can tell us more about what you are trying to do, perhaps we can offer a different solution

-Bryan

Hi Bryan, 

The main reason for using THP210 instead of internal PGA is to provide a higher resolution and accuracy, I can share my circuit. I want to build a structure suitable for the FDA and ADC I will use.

Hi, Bryan,

Metin and I are working on this project together and he will be away for a few days. In our application we will need not 1 but 4 differential channels (there are 4 in the circuit below).

Here we will take differential readings so that the ADC readings are not affected by input noise. The critical thing is that the input range is about 0 - 80 mV (sometimes negative values), so we need very low offset voltage and offset drift.

In this application we match the 0 - 80 mV range to the 0 - 1600 W/m2 range. Our measurement accuracy needs to be better than 1% and our resolution better than 0.1 W/m2. So we also need high accuracy at values close to 0.

With the following application, we think that we will be minimally affected by the noise and offset voltage at the input.

We need a minimum cost solution to meet these requirements. If it would be correct to use the THP210 and ADC pair, can you help us to determine the ADC you will recommend and the filtering elements for it?

Hello again Bryan

All resistor and capacitor values in the schematic are examples. Not calculated for actual gain and filters. Therefore, ignore them.

Hi Bekir KÖR,

Thanks for providing this additional information

Is the plan then to have 4x OPA2378 + 4x THP210 + 1x ADS124S08, since you said you need 4x differential channels? 

Can you also tell us how quickly you need to sample all 4x inputs? For example, "I need to sample all 4x channels in 1ms". The faster you sample, the more noise you let into the system, so this has a direct relationship to the system precision

Also, does your system bias the sensor, or is the sensor self biased? A self-biased sensor might be floating relative to your system (like a thermocouple for example), you would need some way to keep the sensor voltage within the range of signal chain. Since you are showing +/-2.5V supplies in your amplifier schematic, I assume the sensor absolute output voltage always fall within this range, and there is no common-mode difference between channels? 

-Bryan

Hi Bryan

The sensors are solar cells that are self-biased, floating with respect to the system ground and act as a current source. To convert the current into voltage, we place a shunt resistor across each solar cell.

The system calculates the irradiance value (0 - 1600 W/m2) from the absolute output voltage of the shunt resistors (in the range 0 - 80 mV). All irradiance data is sent to the receiver 1 time per second using MODBUS RTU over RS485, so high-speed sampling is not necessary.

Thanks Bryan
Can you help with the filter elements and schematics for the application with ADS124S08 and its built-in PGA?

Hi Bekir KÖR,

As mentioned, we have an EVM that is a built and tested solution. I would recommend getting an EVM and try measuring the signals to see if it meets your performance. The EVM user's guide includes the schematic, so you can just copy the schematic if the EVM suits your needs - there is no need to do any extra design work, unless of course you are not getting the necessary performance

Here are some guidelines for selecting the anti-aliasing filter components for a low speed delta sigma ADC: https://e2e.ti.com/support/data-converters-group/data-converters/f/data-converters-forum/955466/faq-delta-sigma-adc-anti-aliasing-filter-component-selection

-Bryan