ADS114S08: What is difference between high side Rref circuit and Low side Rref circuit for 3-Wire RTD application?

Hi Kazuya,

I would suggest that you have the customer take a look at A Basic Guide to RTD Measurements.  This application guide will show various ways to make RTD measurements and explain the usage for both high-side and low-side references.

Kazuya Nakai59 said:

Q1. 
In case of ADS114S08, Rref is placed on high side of 3-wire RTD as Figure 113 on page 89 in the device datasheet.
On other hands, In case of ADS1248, Rref is placed on low side of 3-wire RTD as Figure 1-1 on page 2 in TI application Note,
RTD Ratiometric Measurements and Filtering Using the ADS1148 and ADS1248 Family of Devices (SBAA201A). 
 
What is difference between high side Rref circuit and Low side Rref circuit for 3-Wire RTD application?

The high-side measurement uses the same excitation source for both the reference and the RTD.  The error is limited to the voltage drop created from the second IDAC flowing through the wire.  In contrast the low-side measurement uses a single IDAC to excite the RTD but the reference is a combination of both IDACs to excite the reference.  Any differences in the second IDAC directly impacts the measurement as there can be increased noise and drift.

Kazuya Nakai59 said:

Q2.
For other application, we want to measure resistance of sensor which has 500ohm~4000ohm range using ADS114S08.
Could you please tell me any reference circuit?

There are a number of ways that this can be accomplished and I don't have a specific application circuit.  One method is a direct measurement where the resistance is excited by an IDAC and the ADC measures the voltage across it.  To calculate the resistance you would take the voltage measurement result and divide by the IDAC current value.

Another method is to use the reference output (2.5V) of the ADC as the excitation voltage and create a voltage divider where the known resistor value and the sensor resistance are connected in series.  This method is similar to the thermistor measurement circuit shown in the ADS114S08EVM user's guide in Figure 19 on page 25.

Kazuya Nakai59 said:

Q3.
This is different sensor resistance range from Q2.
We want to measure resistance of sensor which has 5000ohm~40000ohm range using ADS114S08.
Could you please tell me any reference circuit?

This would be the same as the response for Q2, except the value of IDAC current would need to be less 10 times less (IDAC/10) and the voltage divider resistance increased by 10 times greater (R*10).

Best regards,

Bob B

Hi Kazuya,

There are many ways to make the 3-wire RTD measurement.  Sometimes customers choose one method over another based on what was designed in previous generations of product.  It is primarily a preferred choice.

Kazuya Nakai59 said:

Q1. Is the high-side measurement is better than low side measurement for higher accuracy?

The measurement method uses a second IDAC for lead resistance cancellation.  The amount of error due to the measurement path will depend on the IDAC matching.  The reference resistor will require a larger value of resistance (at least as large is the highest temperature expected) for the high-side measurement.  A larger resistor is more thermal noise.  However, a single IDAC can be used in either use case if enough analog inputs are available to make a second measurement to measure the voltage drop created from the leads.  These circuits, the pros/cons and the design descriptions are all in the basic RTD measurement guide I referenced in my first post.  At a 16-bit performance level you most likely will not see any differences in the results using one method of measurement over the other.  It is more important to consider the temperature coefficient of the reference resistor used as the reference resistor directly impacts the accuracy of the result.

Kazuya Nakai59 said:

Q2. If answer to Q1 is yes, do you know why low side measurement is used in SBAA201A document?

This application note was developed for the ADS1248 to show how to best design the input filters for best matching of the filter response to maintain a ratiometric measurement.  The ADS124S08 (ADS114S08) are more recent devices and have better IDAC matching and drift performance.  The application note just showed one method and the calculations involved.  It was not meant to show every possible permutation of configurations or to promote one measurement method over another.

Using a high-side reference for the ADS1248 has some additional considerations which complicates the design, so in general the high-side measurement example was never shown.  These design considerations were removed for the ADS1x4S08 devices due to improvements in the IC design.

Kazuya Nakai59 said:

Q3. Why is the left top circuit of Figure 117. Do's and Don'ts Circuit Connections on page 95 of ADS114S08 datasheet incorrect. I think ADC convert step becomes one second of full step. 
However the device can convert the input voltage to digital data. Is my thought wrong?

I'm not sure what is being asked.  The picture (shown below) is showing that it is incorrect to make a single-ended measurement (AVSS connected to ground) with the PGA enabled.  With the PGA enabled, the output of the PGA cannot drive all the way to AVSS which will create an error.  If the PGA is bypassed, then the single-ended measurement will be valid.  See section 9.3.2.1 on page 32 of the ADS114S08 datasheet.

Best regards,

Bob B