High impedance sensors protections - thermocouple and others - Page 1
Forum section: electronics / projects

author: Morgoroth
author_rank: Regular Contributor
timestamp: March 12, 2017, 02:07:01 am
content: Hi all,I'm on a new project and I was searching protections for high impedance sensors (thermocouples so far), but in future will be others with higher impedances and I want one design.I was thinking to use an isolated power supply for it and diode protections at the connector input, however, I couldn't found a proper ESD at nA or fA rating leaking current, plus, this device is industrial so must be protected somehow, and given this low currents involved no idea if there is others option around. If someone has a clue how to solve this I'll be graceful.Thanks in advance,JP

author: Morgoroth
author_rank: Regular Contributor
timestamp: March 12, 2017, 03:16:59 am
content: Thank guys, with this I have criteria to start, nothing like someone with experience to help to take the right desition/choice.JP

author: Marco
author_rank: Super Contributor
timestamp: March 12, 2017, 03:30:02 am
content: Quote from: Morgoroth on March 12, 2017, 02:07:01 amThat's a pretty big jump, lots of auto-zero opamp buffers with a bit of series resistance will do fine against ESD and even a couple 100V, with sub nA leakage and low offset voltage. It's the fA part which is the tricky bit if you want to zero out offset voltage.A LMC662 buffer with a couple 100k series resistance will give you robustness with fA leakage, but it doesn't give you any way to zero out the offset. A SPDT relay can solve it easy with low leakage, but mechanical so failure prone. Doing it electronically is probably going to involve a bunch of 2n7002s, with one or two bootstrapped to the buffer output during normal operation.

author: ebclr
author_rank: Super Contributor
timestamp: March 12, 2017, 04:49:36 am
content: " high impedance sensors (thermocouples so far)"From where did you take that false information ?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 12, 2017, 09:04:16 am
content: Thermocouples are usually not high impedance - more like low impedance, often less than 100 Ohms. So just normal 1N400x across the inputs are good enough for protection, as the voltage is usually very low ( < 30 mV).Thermocouple are still special in that they might need a well defined temperature or a temperature measurement.The typical low offset amplifiers often have quite some input bias and might not be suitable for really high impedance sources.High impedance sensors are something like PH sensors - still you don't need less than 10 pA leakage in most applications. This is because one get comparable leakage from normal cables.

author: Morgoroth
author_rank: Regular Contributor
timestamp: March 12, 2017, 01:59:57 pm
content: you are right, I meant to say that I want to use the design for thermocouples so far, but I have others sensors like pH sensor and I need one design for both.

author: Marco
author_rank: Super Contributor
timestamp: March 12, 2017, 07:06:18 pm
content: How about this :LMC662 buffer, 100K+ series resistance on the input, back to back unprotected gate small signal MOSFETs connected to the positive input as a low leakage switch (these need high standards by the assembly house) connected with a SPDT analogue switch to either the LMC662 output (normal operation, during which the MOSFETs are turned off, use a large negative gate voltage, you want to pinch off the leakage current caused by the couple mV offset between input and output) or ground (offset calibration, during which the MOSFETs pull the input to ground).The precision of the offset calibration depends a bit on the input voltage/impedance, because the MOSFET/SPDT on resistance forms a divider with the input series resistance and the source resistance, but for a thermocouple with 100 mV output you'd still know the opamp offset in the uV range.This measures the offset during a calibration cycle, which is less convenient than autozero ... but autozero opamps don't have fA bias current.PS. ignoring for a moment he doesn't need fA bias currents, just a thought exercise.