Any opinions on PCB design for thermocouple interface? - Page 1
Forum section: electronics / projects

author: miceuz
author_rank: Frequent Contributor
timestamp: May 23, 2013, 09:05:19 pm
content: Hai,I'm building a thermocouple interface that could be accessible via RS485 modbus protocol. I've done one thermocouple interface before and it works ok, but I want to do this one right.I'm currently stuck at the PCB level - I'm trying to avoid splitting the ground and coupling analog and digital grounds. The final pcb is 55mm in diameter and i'm attaching a screenshot of current layout. At 12 o'clock there's an inst amp, termosensor and opamp, to the right, at 1 o'clock it's ferite bead and caps, then linear regulator and analog reference at 2, then RS485 transceiver at 4 ant MCU at 7.The thing should work not far away from heaters driven by triac, maybe chopping the sinewave, maybe switching on zerocrossing.I don't know, something feels not right with it for me.. any enlightments?Schematic is in attached pdf orhttps://github.com/Miceuz/thermocouple-modbus

author: Paul Price
author_rank: Super Contributor
timestamp: May 23, 2013, 09:44:45 pm
content: Looks pretty good, my only concern is at the thermocouple input to the instrumentation amp, there could be some RF or EMI pickup to rattle your chain. Perhaps a bypass cap across the input would be in order to cool it down RF wise.

author: free_electron
author_rank: Super Contributor
timestamp: May 23, 2013, 10:17:45 pm
content: when you say thermocouple . do you mean a real thermocouple where two dissimilar metals produce a voltage ?in that case :1) get a real thermocouple amplifier chip with cold juntion compensation. the connector should be as close as possible to the amp so it can sense the temperature of the connection and compensate for it ( that's why it is called the cold junction )use a proper thermocouple connector with screws made from the right material. omega has pcb mountable connectors you can screw the wires in. you don;t wan to creat additional thermocouples in the connector ....2) flick those tantalum capacitors out of your design and go to all ceramics. longevity and the absence of suicidal tendencies will be your benefit.3) common mode choke on the input. rf bypass caps ( as already stated ) but if you use a good thermocouple amp those already have great common mode rejection.

author: miceuz
author_rank: Frequent Contributor
timestamp: May 24, 2013, 06:25:25 am
content: Quote from: free_electron on May 23, 2013, 10:17:45 pmI went for separate inst amp and temperature compensation in software as those integrated chips seem to be quite pricy, at least for now I'm still below the price for the integrated solution. Point taken about connectors.Here I was wondering particulary about PCB layout, the design itself probably will have it's drawbacks, but it's a bit of an educational excercise - what accuracy and precision can I achieve by designing an analog circuit from grounds up (well, not exactly, as AD8237 has enought of complicated hitech going inside)Quote from: free_electron on May 23, 2013, 10:17:45 pmAD8237 I'm using seems to have sufficient RF filtering on the input, so I didn't go too much on this side.http://www.analog.com/static/imported-files/data_sheets/AD8237.pdf

author: Dreso12
author_rank: Contributor
timestamp: May 24, 2013, 05:40:43 pm
content: Have you tried the AD8495?, it is not so expensive and it will make you life much more easier. If not less expensive the AD597.

author: free_electron
author_rank: Super Contributor
timestamp: May 24, 2013, 11:10:12 pm
content: i'd go for the maxim chip . directly digital out...

author: miceuz
author_rank: Frequent Contributor
timestamp: May 25, 2013, 09:06:22 am
content: Quote from: Dreso12 on May 24, 2013, 05:40:43 pmI have considered AD8495, have some samples of them. The main problem with it for me is that I want my temperature range to extend up to 1100C, and AD8495 would give me Vout 4.9V max with 5V supply, equalling to 980C max. This makes the life not that much easier as I would have to add another regulator on board and deal with the consequences of that.

author: poorchava
author_rank: Super Contributor
timestamp: May 25, 2013, 11:04:41 am
content: Wouldn't you rather use something like LMP90100? It gives you mix, amplifier and a good adc in one package. It's not absurdly cheap, but might prove to be cheaper than multiple ICs doing that job.I would also consider switching to some more digital friendly reference voltage, like 2.56, 2.048, 4.096 etc. Less operations to be done by mcu, and less complicated calculations for you.

author: miceuz
author_rank: Frequent Contributor
timestamp: May 25, 2013, 06:46:50 pm
content: Quote from: poorchava on May 25, 2013, 11:04:41 amgood point. I went for 5V reference as I want to measure temperature up to 1100C, this makes about 4.95V for K thermocouple with 101 gain. Might change the gain a bit so it fits into 4096. But again, I was going to put temperature tables for every ADC value into MCU flash as I want to perform linearity correction and don't want to solve polynomials in MCU. It's 4Kb of flash for the tables and then the program is dead simple - decode modbus, perform ADC conversion, lookup, respond, wait.

author: Rasz
author_rank: Super Contributor
timestamp: May 27, 2013, 06:51:50 am
content: This might be a good thread to ask my question.This is a chinese PID. Its build using:4051 multiplexertl082 opamp74hc74 flipflopmpc89e52a 8051 micro _with no ADC, only timers_As I understand this is Voltage-to-Frequency converter. It is extremely cheap (this mcu is ~$1, rest is jellybean). What are the drawbacks versus ADC inside mcu?

author: SeanB
author_rank: Super Contributor
timestamp: May 27, 2013, 05:42:28 pm
content: Nothing, it is capable of a very good resolution at the expense of speed, taking a second to do a measurement, but it is good at getting rid of noise in the signal. Some use voltage to frequency, others make a dual or single slope converter to do it with discrete components.

author: senso
author_rank: Frequent Contributor
timestamp: May 27, 2013, 07:59:34 pm
content: Sorry for the off-topic, but how fast can your circuit be read?I would like to read a K-type TC at 1 to 2Khz, up to 700ºC with 1ºC resolution?Would it work, and if so, could I borrow it for my personal project?