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/etc... Data Converters
/etc... Data Converters Forum
use of ads 1247 to interface two pt100 sensors
i am varun. i am developing a temperature controller in which i wanna use ads1247 adc to get a resolution of 0.001C. as i know frm datasheet ads1247 adc can b interfaced with two pt100 sensors...can u suggest some circuit configuration to use two pt100 sensors (in 4 wire configuration) with this ADC. my main application is to show the temperature of two different positions simultaneously. so is this can be done by a single Ads1247 adc? i am using atmega16l micro controller for this application.i wanna give 1mA current source externally and internal clk sources is in use. plz help ...weting for ur reply
Attached below is an application note that you will find helpful. It describes in detail the ADS1248/7 measuring RTD's in the two-wire; three-wire and four-wire configuration. The device offers the programmable IDAC current sources that can be used to excite the RTD.
The ADS1248 and ADS1247 is a single ADC with a multiplexer in the front. The ADS1248 provides more input channels in the multiplexer and in addition the IEXC1 and IEXC2 current output pins. The user could connect two or three RTD's to the ADS1248 device; and change the multiplexer settings to measure the different RTD's in order to measure one RTD at a time. If measuring one RTD at a time is acceptable in your configuration , you could use the ADS1248 with up to (3) RTD's as shown on the slide provided. If you need to measure two different RTD's simultaneously, you will need (2) ADS1247 devices.
Please find the application note below showing the ADS1248 /7 measuring RTD's and a block diagram showing the 4-wire RTD connections to (3) RTD's using the ADS1248 (measuring one-RTD at a time).
Thank you and Best Regards,
ADS1247/8 Application Note measuring RTD's:http://www.ti.com/lit/an/sbaa180/sbaa180.pdf
(3) Four-Wire RTD's connected to the ADS1248 (measuring one RTD at a time)
thanks for quick and valuable reply. design looks nice but i have a doubt that if this internal current source is stable enough to measure temperature of a far position. cable length is approx 8 m long.wat if i use external current source with opamp amplification hence the input to the ADC is single ended.
i also want to know the calibration proceess of temperature, i know one method of adjusting the offset voltage and gain of the opamp as to make the input of the adc temperature equivalent voltage. OR i have to use RTD equation to in programm of MCU.
weting for nw ideas.....
The ADS1248 provides an internal PGA (programmable gain amplifier) with gains of x1 to up to x128; therefore, you probably will not need an external amplifier.
The device offers different calibration options detailed on pages 32-33 of the datasheet. The offset calibration can be performed using the self-offset calibration command (SELFOCAL). The PGA gain error of the ADS1248 is very small, typically 0.005% (max 0.02%). Most users do not perform a gain calibration and rely on the factory trimmed gain precision built in the device. Regarding the voltage to temperature conversion; some users perform the conversion using the RTD equation; or use a look up table programmed in the MCU. The slides attached below provide an overview of the different methods to perform the voltage to temperature conversion and the trade offs between the different methods.
3823.Foruml RTDUsing Delta-Sigma ADC.pdfI will go ahead and verify the IDAC current stability.Best Regards,Luis
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