Thermocouple and RTD AFE Chips  - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 07, 2021, 08:04:31 am
content: Hi,I want to design a new product, I want to know what chips are there to be able to interface different Sensors to them, do you know any chinse part too?

author: ebclr
author_rank: Super Contributor
timestamp: January 07, 2021, 09:07:14 am
content: https://www.ti.com/sensors/overview.html

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 07, 2021, 09:19:40 am
content: You don't need an AFE for an RTD - you just use an ADC which has a differential reference input, and feed it the voltage across a reference resistor in series with the RTD supply.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 07, 2021, 10:39:14 am
content: Thanks for the tips, what about combining RTD and thermocouples like PLC's that would accept different types of sensors with 2,3 or 4 connections?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: January 07, 2021, 01:42:42 pm
content: I hope You know LTC2983

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 07, 2021, 01:53:36 pm
content: QuoteThanks for the info, I did not know about it, But now I will never use it, because the price just do not make any sense!

author: girishv
author_rank: Regular Contributor
timestamp: January 07, 2021, 02:53:57 pm
content: Quote from: ali_asadzadeh on January 07, 2021, 01:53:36 pmDisclaimer: I do not have much experience in Analog electronics.From TI MUX36D08 datasheet.I am working on something similar to your requirement using TI MUX36D08 which offers differential 8:1 channels feeding a discrete INA built on 3 discrete TI OPA189 (max 4µV offset). I paid about $4 for MUX and $2 for op-amp (totaling to $10) in small quantities.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 09, 2021, 08:13:08 am
content: Is it possible to use HX711, since it's super cheap and has PGA and use an external low cost Temp sensor for cold compensation?

author: girishv
author_rank: Regular Contributor
timestamp: January 09, 2021, 10:08:04 am
content: Quote from: ali_asadzadeh on January 09, 2021, 08:13:08 amI could not see the input Vosin the specification of HX711. Usually ADC's have high input Voswhich is not ideal for Thermocouple amplification.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 09, 2021, 10:54:40 am
content: Quote from: girishv on January 09, 2021, 10:08:04 amQuite a lot of the modern SD based ADC chips actually have a pretty good offset drift, as they use some kind if input chopping. The HX711 is kind of specially made for DMS bridges - so it may wok with RTD too. For voltage measurement the specs may be lacking a little, but it may still work. Themocouples are usually not that accurate anyway.A point with directly using the ADC input for a thermocouple is more with EMI effects and ESD protection, if the inputs are externally acessable. It should have at least some filtering.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 10, 2021, 10:48:53 am
content: thanks for the feedback, what about using discrete low cost op-amps? do you suggest any circuit configuration? and use the MCU internal ADC, STM32 have 12-16bit internal ADC

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 10, 2021, 11:23:24 am
content: Quote from: ali_asadzadeh on January 10, 2021, 10:48:53 amThat's very dependent on what level of accuracy you need

author: Nominal Animal
author_rank: Super Contributor
timestamp: January 10, 2021, 12:12:47 pm
content: I've foundMAX31865nice to work with Pt100 temperature sensors, interfacing to a microcontroller of some sort. The same chip can work with Pt100 to Pt1000, and in 2-, 3-, or 4-wire configurations. Adafruit and others provide break-out boards for use with e.g. Arduino microcontrollers.I cannot attest to their absolute accuracy, because I don't have a lab setup to do precise temperature measurements.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 10, 2021, 05:40:20 pm
content: For thermocouples a low drift OP is a real option, when the signal goes to a µC. The cold junction compensation can be done numerically with a separate simpe temperature sensor.For the RTD a ADC with differential reference inputs can really simplify things. A bridge circuit with 1 OP is possible, but not that easy and for higher accuracy can fast become more expensive than the solution with a high res. ADC with differential inputs. These days it can make sense to use a cheap 24 Bit ADC to save 2 expensive resistors.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: January 10, 2021, 06:56:34 pm
content: Depending on your budget and need for flexibility, you could also consider the PSOC5, which in some configs has four 12-bit SARs, a 20-bit delta-sigma ADC, DACs, MUXes, opamps and a CM3 core in a single package. The eval boards are stillfairly cheap.

author: girishv
author_rank: Regular Contributor
timestamp: January 11, 2021, 02:08:52 am
content: Quote from: blueskull on January 10, 2021, 12:39:48 pmI was also looking for a design as suggested but was not sure how to go about building one. If you can point us to an example schematic or share one, it would be very helpful.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 11, 2021, 08:18:27 am
content: QuoteThanks for the idea,Can this bird be killed with this stone CS1237-SO, it has a built in Temp sensor and 24Bit ADC @ low priceAny Ideas?