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Re: ADS1247 RTD Measurement
Hi, I'm a newby about ADC measurements....
I would like to use an RTD system: if I use an RTD as voltage divider with internal reference voltage ADC I have a measure depending of RTD value (temperature) and power supply.
So I need to measure also power supply.
So, to avoid this (measure not affected by power supply value) do I need to power RTD through Vref or constant current generator, similar to application report "Temperature measurement applications using the ADS1247 and ADS1248", Figure1?
I split your post from the original older thread since this is a different question.
In general, when designers measure a RTD or thermistor with an ADC on a voltage divider configuration, they use the excitation voltage (voltage supply) as a voltage reference for conversion. The user is then required to know the value of the series resistor in the voltage divider in order to solve for the RTD value. Since the voltage supply is used as a voltage reference and also to excite the RTD sensor, this configuration creates a ratiometric measurement, where any noise and/or drift of the excitation voltage creates a proportional change in the reference signal path and the input signal path. Since the change is seen both at the reference path and signal path, any errors due to drift of the excitation and some of the low frequency noise tends to cancel. In the ratiometric configuration, where the excitation voltage is used for reference, the user does not need to measure the excitation voltage, however, is required to know accurately the value of the series resistor used in the voltage divider with the RTD.
-- In the case of the ADS1247/ADS1248, the device is optimized to be used on the recommended configurations for the 2-wire, 3-wire and 4-wire shown in the application note "Temperature measurement applications using the ADS1247 and ADS1248" as you have mentioned on your post above. The device provides programmable current sources (IDACs) that are used to excite the sensor. These are also ratiometric configurations, where the IDAC current is used to excite the RTD and to generate the voltage reference accross RBIAS resistor. Similar to the case with the voltage divider configuration above, the user is not required to measure the excitation current, however, in order to solve for the RTD value, the user is required to know the value of the RBIAS resistor. Therefore, the RBIAS resistor must be an accurate/low drift resistor.
The easiest way to configure the ADS1247/48 to measure RTD's will be to use the IDAC currents as recommended on the application note, since this configuration is also ratiometric, where both inputs of the device are biased at the optimal common-mode voltage range and allows the use of additional gain by using the ADS124x internal Programmable Gain Amplifier (PGA) to increase resolution...
Thank you and Best Regards,
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