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Original question:

ADS131E06: PT100/PT1000 (4-wire) input interface with ADS131E06IPAG

ADS124S08: ADS124S08 : 24-bits resolution for the range +/-100mV

Sadashiva Kamath
Expert 1175 points
Part Number: ADS124S08
Other Parts Discussed in Thread: ADS131E06, ADS1261

Hi,

We're planning to use ADS124S08 in new product development.

I'm having one question -  
1) Whether ADS124S08 gives 24-bits resolution for the range +/-100mV? If not, How many bits resolution it gives for the range +/-100mV?

  • 0
    TI__Guru** 113765 points

    Hi Sadashiva,

    You can connect two 4-wire RTDs to the ADS124S08.  A would recommend reviewing the information in A Basic Guide to RTD Measurements and in particular section 2.8.  This approach uses 2 RTDs in series, but there are other methods as well.  For example you could use the circuit in section 2.6 twice and just use different analog inputs and the REF1 as the reference.

    The question about resolution is a bit more complicated to answer.  Generally you would be measuring the RTD in a single direction as the current will flow in a single direction when exciting the RTD.  You can adjust the combination of gain and reference voltage to achieve the full-scale range of +/-100mV, but as I just mentioned will reduce the measurable range to only the positive range and 23 bits excluding any noise.

    As the RTD measurement is ratiometric, the bigger question to ask is what resistance resolution can be achieved?  I presume you have a desired temperature resolution which will correspond to the resistance measured.  Again I would recommend reviewing the guide on how best to achieve the desired resolution.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Expert 1175 points

    Thanks Bob for prompt response. AN and those 2 sections are quiet useful..!

    I want to know whether section 2.11 Multiple Four-Wire RTD Measurements in Parallel is better OR "use the circuit in section 2.6 twice and just use different analog inputs and the REF1 as the reference."?

    The following question is not related to 4-wires RTD. The following question is for pressure sensor. Different applications.

    In the existing design, we're having ADS131E06. I'm just exploring to use ADS124S08 (2 numbers) for both 4-wires PT-100 & pressure sensors applications.

    1) Whether ADS124S08 gives 24-bits resolution for the range +/-100mV? If not, How many bits resolution it gives for the range +/-100mV?

  • 0 in reply to Sadashiva Kamath
    TI__Guru** 113765 points

    Hi Sadashiva,

    As I initially said, there are other methods that can be used beyond the 2 RTDs in series.  I just used the one example, and the one you mentioned in section 2.11 of the RTD guide is another.  Is one method better than another?  The answer would depend on the application.  The design in 2.11 is less complicated and uses only a single reference and reduces the number of components required.  The duplication of 2 separate RTD measurement paths offers complete separation of the measurements, but increases the costs and requires both reference inputs to be used.

    For additional clarity I must also say that the ADS124S08 uses a single ADC with a muxed input.  This differs from the ADS131E06 which contains multiple ADCs for simultaneous conversions.  So when using multiple sensors to the ADS124S08 you must consider the time required to make all measurements as the mux reduces total throughput.

    Now for the resolution question.  The ADS124S08 is able to resolve to a 24-bit level for all values of reference and PGA gain settings for the device.  So what does this actually mean?  This becomes difficult to answer as the effective resolution and noise-free resolution may differ significantly from the converter resolution depending on data rate due to quantization and reference noise.  So even though the ADC can resolve to a certain level doesn't mean that your measurement can achieve a particular level.

    If I understand you correctly, you want to know if you can resolve to 12nV.  I would suggest that you look at the noise tables in the ADS124S08 datasheet.  For example, Table 3 shows the noise for a 2.5V reference and various data rates and PGA settings.  Take a look at the gain of 16 (for 2.5V and gain of 16 the FSR = +/-156.25mV).  Here we see that the best level of noise at this setting is 51nV of noise for a 19nV LSB (or 1 code = 19nV).  Table 4 shows the effective resolution at 22.4 bits.  You will also notice that as the gain increases the total noise decreases while the effective resolution decreases.  But number of bits only tells part of the story as the required voltage resolution is the ultimate goal.  All that said, you will not achieve 12nV resolution with this device.  19nV effective resolution may be possible using the sinc3 at the highest gains and lowest data rates.

    If you really need to achieve better voltage resolution you could try looking at the ADS1261.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Expert 1175 points

    Thanks Bob for details.

    I see that ADS1261 takes max. sample rate of 40 kSPS while ADS124S08 takes max. sample rate of 4 kSPS.

    1.  What makes ADS1261 better voltage resolution?
    2. Can we achieve better voltage resolution with ADS1261 as compared to ADS124S08 for 4-wires PT-100 application?

    Based on previous discussion, I'm shortlisting the following ADC for our application -

    1. ADS124S08  for dual port 4-wires RTD application
    2. ADS131E06  for legacy Pressure/Temp Sensor 

  • 0 in reply to Sadashiva Kamath
    TI__Guru** 113765 points

    Hi Sadashiva,

    The ADS1261 has lower noise due to the larger over-sampling ratio and bandwidth of the PGA at the sacrifice of increased power as compared to the ADS124S08.  For example, the ADS1261 Electrical Characteristics table shows current in the mA region whereas the ADS124S08 is shown as uA.  This leads to a typical power consumption of 20mW for the ADS1261 and 1.75mW for the ADS124S08. So the tradeoff is power for performance.

    Best regards,

    Bob B