I have ICL7136 MQFC (the square one) chip driving an LCD display. it's measuring and displaying temperature data from a thermometer probe. I want to grab the analogue readings an log them with an Arduino as well. How what type of setup would yield something usable. Would I just measure from the IN HI or IN LO to the analogue in of the Arduino?
How to measure the analogue in voltage of a ICL7136 using an Arduino
analogarduinomeasurement
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These themocouples have a tiny spot weld between two thin wires to make the active sensor element, which results in a very low thermal mass. Hence they change temperature quickly, which is the same thing as reacting to temperature changes quickly. As soon as you mount them in a larger substrate or coating (eg to provide a sterile insulation, to prevent breakage, to electrically isolate) you increase the thermal mass, so you have to wait longer for stabilisation.
This seems to be very similar to Nick's schematic, was probably busy drawing it when he posted :-).
First why you can't use the N-FET on the high side: it needs a gate voltage a few volts higher than the source, and the 4.2 V is all you have, nothing higher, so that won't work.
I have a higher value for the pull-up, though a value of 100 kΩ also will do. 10 kΩ will cause an unnecessary extra current of 400 µA when you're measuring. Not the end of the world, but it's 1 resistor in both cases, so why not use a higher value.
For the MOSFETs, there are a variety of parts to choose from given the requirements are not so strict; you can consider inexpensive ones such as, e.g., Si2303 for the P-channel and BSS138 for the N-channel.
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Best Answer
It looks to me like the chip has reference voltage pins and then sensor voltage pins. So it is reading voltage changes from a sensor that reports back in that way, massaging those levels (amplification, scaling, whatever), and then outputting to the display.
Yes, I would think logically reading the same sensor outputs that the chip is reading on IN HI and IN LOW would allow you to access the data. I am assuming the mere act of connecting to those pins and reading them won't change the voltage, but I would defer on that to those more experienced as I could be wrong.
I think your issue, however, is going to be doing the scaling and amplification and conversion of the signals coming in so that the Arduino and the chip both think a given voltage represents the same (in your case) temperature reading. Once they both say that particular voltage level is that many degrees then it becomes a data logging issue.