Reading RTD as analog

ericgibbs

Joined Jan 29, 2010
18,988
What is meant by Signal Conditioner ?
Hi engr.
This a clip from your link.
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The MAX6603 dual-channel, platinum RTD-to-voltage signal conditioner excites and amplifies the signal from two external 200Ω platinum-resistive temperature devices (Pt RTD) to achieve high-voltage, level-filtered signals for temperature measurements. The MAX6603 provides a direct ratiometric output voltage to simplify the interface to microcontrollers with integrated analog-to-digital converters (ADCs). External precision resistors and calibration processes are not needed. The MAX6603 provides the necessary signal-conditioning functions, including ratiometric excitation current, amplification, buffered voltage outputs, diagnostic fault detections, and input protection. The MAX6603 amplifies signals from two RTDs operating over the -40°C to +1000°C temperature range and provides the temperature information as two independent analog voltages. The MAX6603 features a ±6°C (max) accuracy over the +400°C to +600°C temperature range.
 

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engr_david_ee

Joined Mar 10, 2023
176
Thanks for the information. In the section of output voltage on page 6 of the datasheet. There is a relation that can be used to find the voltage across the RTD using the output voltage. In other words if the output voltage is measured, the voltage across the RTD can also be found.

But how the resistance is converted to temperature ?

The following equation can be used to compute RTD resistance given the temperature and also RO.

R(T) = RO[1 + AT + BT2+ CT3]

But how do the reverse calculation ?
 

ericgibbs

Joined Jan 29, 2010
18,988
hi engr,
Look at this simple PT100 quarter bridge example, note the a and b voltages.
These voltages would be connected to the input of an INA , then to the ADC input of a device.
The device would be programmed to convert the INA Vout to an ADC value.
E
 

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engr_david_ee

Joined Mar 10, 2023
176
How the analog output voltage from MAX6603 will be converted to temperature ?

Kindly suggest INA to read RTD.

We already have ADC and MCU solution ready.
 

ericgibbs

Joined Jan 29, 2010
18,988
Hi engr,
The Max is only a conditioner and INA, which outputs an Analogue voltage, you require an ADC integrated circuit or an MCU with an inbuilt ADC in order to convert the Analogue signal to a Digital value.

E
 

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engr_david_ee

Joined Mar 10, 2023
176
I am still not getting it. I am sorry about it.
Don't we get analog output voltage from MAX6603 ?
We can't directly connect MAX6603 to ADC/MCU ?
 

ericgibbs

Joined Jan 29, 2010
18,988
Hi engr.
This is what the D/s states.

The MAX6603 provides a direct ratiometric output voltage to simplify the interface to microcontrollers with integrated analog-to-digital converters (ADCs).

So consider that you have a 0Cdeg through 100Cdeg heat source, which the PT100 and MAX measures.

And the Max Vout is 0V through +5V, for 0C through 100C, the ADC would convert this voltage to a numeric value count value of 0 through 1023 [ for a 10Bit ADC],

The MCU program would then convert this count value into a numeric value of temperature

E
 

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engr_david_ee

Joined Mar 10, 2023
176
Thanks for reply. I understand ADC conversion of an analog signal. We have MCU with ADC input range from 0 to 3.3 V and also 0 to 5.0 V.

The question was that what analog output voltage I will get from MAX6603 for PT200 when the temperature is 0 C, 100 C and also 125 C ? Basically what is the relation between MAX6603 analog output to measured temperature.

How do we get PT200 ? I only know PT100 or PT1000 available in the market. Do we also have PT200 available ? If not can we just add two PT100 in series to make them PT200 ?
 

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engr_david_ee

Joined Mar 10, 2023
176
From the attached Figure 2, the supply voltage VCC can be the same for MAX6603 and also for MCU/ADC. Can we use VCC as 5 V for both MAX6603 and also for MCU/ADC ?

Our temperature range is 0 to 150 C and we need to find a PCB mount PT in SMD 0603/0805/1206 etc package. According to the datasheet Figure 1, they have shown PT200. Do we have PT200 available in SMD solder able packages ?

Or

Because the MAX6603 is characterized for PT200 according to Figure 1 in the datasheet. Can we just connect two PT100 in series to make them PT200 ? Connecting two PT100 in series at 0 C will result in to 200 Ohm, right ?
 

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Jon Chandler

Joined Jun 12, 2008
1,070
Pages 5 & 6 of the datasheet explain how the output voltage relates to temperature. It's radiometric, meaning the output voltage is a function of input voltage but if the ADC reference is supply voltage, this cancels out.

If this doesn't make sense, please look for a tutorial on ADCs and pay attention to the part about voltage references.

Page 6 of the datasheet also explains how to use different types of RTDs.
 

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engr_david_ee

Joined Mar 10, 2023
176
I just read that the ADC reference voltage determines the highest signal level that the ADC can convert, and all quantized digital outputs from an ADC will be some ratio of this reference voltage.

This means if the supply voltage of the ADC is 5.0 V or 3.3 V and the reference voltage is 2.5 V then the ADC will convert 2.5 V signal as highest input. The 0V at the input will corresponds to all zeros at digital output and 2.5 V will corresponds to all ones at digital output.

Now back to MAX6603. In order to keep ratiometric working, do I need to have smaller Vref compared o the supply voltage at the ADC ? How about if I powered up the MAX6603 and ADC with 5.0 V and use 2.5 V as ADC ref. Would there be still ratiometric ? meaning the output voltage would be a function of input voltage ? In other words any change in the RTD resistance due to temperature will change input voltage across RTD because 1 mA is constantly running through RTD and the output will change as a function of input, right ?

Regarding page 5 and page 6 on how the resistance and output are related to temperature.

The formula Vout = (Vcc x R(T))/(1000) can be used to find the resistance at a temperature R(T) provided the Vout and Vcc are known.

There are Callendar-Van Dusen equation R(T) = RO[1 + AT + BT2+ CT3] which calculate the resistance at a temperature R(T) provided the temperature is known.

But how do we calculate temperature T when resistance R(T) is known ?
 
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