Is it possible to connect 6 two-wire RTDs to ADS124S08 Analog to Digital Convert - Page 1
Forum section: electronics / projects

author: ashmit_jaiswal
author_rank: Newbie
timestamp: October 25, 2024, 03:32:44 am
content: I have connected 6 two wire RTDs to a ADS124S08 Analog to Digital Converter in series and parallel connection. My requiernments are to connect as many RTDs as possible to the chip with least amount of extra equipment and to achieve a mix between accuracy and number of RTDs.The attached image is a schematic of 6 Two Wire RTDs connected to a ADS124S08 Analog to Digital Converter. I would like to know the following things about my idea:1) Will the AINCOM pin acting ac IDAC1 comply with the load of 6 RTDs?2) Will the readings of the RTDs be distorted? If yes, to what extent, and will it be filterable by hardware or software?3) Is there any chance of any unexpected error I have not predicted yet?4) Is there any other way/schematic to achieve this objective?Also, the specifications of my circuit and components are mentioned below:1) RTD Used: Pt1002) ADS Specificationsa) Analog Power Supply Voltage (-0.3 to 5.5 V)please let me know if any other specifications of the

author: coppercone2
author_rank: Super Contributor
timestamp: October 25, 2024, 04:05:57 am
content: of the what??its like the transmission was terminated.

author: Eheran
author_rank: Regular Contributor
timestamp: October 25, 2024, 05:17:39 am
content: I recommend you switch to something like the LTC2983, a dedicated chip to measure all sorts of analog thermometers (RTDs, diodes, thermocouples, PTCs, custom). It also provides a current source to measure those. And an open circuit detection. And and and...If you read through the datasheet, you will also find things it avoids to reduce error, like parasitic thermocouples.I highly recommend the LTC2983 over a simple ADC based on my experience.

author: Poroit
author_rank: Frequent Contributor
timestamp: October 26, 2024, 03:55:42 am
content: G'day Ashmit,Are you only monitoring only one temperature point with all these RTD's grouped together?What accuracy are you aiming for?

author: Terry Bites
author_rank: Super Contributor
timestamp: October 26, 2024, 10:08:32 am
content: Not all PT100's are created equal. The tolerance of the sensor affects the precsion of the measurments.You need to keep switching resistances to a max of 10-20 ohms.I dont get the serial parallel bit. RTDs in serial parallel?? Explain further.A mux like MAX14661 with spi/ i2c control might be a good choice here. About 10 bucks ea.https://www.mouser.co.uk/c/?q=max14661In SPI mode you can daisy chain these mux's. Build any matrix configuration that tickles your fancy.See attached on reducing switch and sensor lead errors.

author: Eheran
author_rank: Regular Contributor
timestamp: October 28, 2024, 07:00:35 am
content: Quote from: Terry Bites on October 26, 2024, 10:08:32 am20 Ohms would be an error of >50K. With a On-Resistance of up to 8 Ohm for the MAX14661 that would still be an error of 20 K with Pt100 (so 20 °C measured instead of actual 0 °C for example). Not sure in which application such errors would be acceptable. Even the fairly low 0.25 Ohm On-Resistance Match between Channels would introduce a 0.64 K error.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 28, 2024, 02:57:29 pm
content: One could use a MUX to switch between channels. However for this one should do the MUX like a 4 or 3 wire sensor. So separate switched for the current and voltage sensing. One side could still be connected all the time (like a common ground with no switching). A minimal solution could use something like 2 x 74HC4051 for the switching, maybe with 1 channel as a reference 100 ohm resistor.