# How is the CTMU wrong

## Forum: Analog electronics and circuit technology Measurement of small currents and capacities

Hello dear community, I'm not sure if I'm at the right place here, but I hope you can help me! For a school project I would like to develop a security system that capacitively perceives people approaching a large surface (39 square meters). For this purpose, the surface is printed with a sensor anode and a sensor cathode. Now I have used the Arduino controller and solved the whole thing using an applied frequency, but this is suboptimal due to the relatively high power consumption, especially since I fear that the area will be too large (it works in the small format. The runtime of the system is also so too low. I would like to saturate the "capacitor", ie to charge it, if a person approaches now I would like to measure the current that flows when the capacitance is increased. Do you have any ideas which controller or which circuit for such a project in question comes? Also other suggestions are desired how the whole thing can be realized. Thank you and LG David!

Well There are capacitance measuring controllers that can measure 10-20pF. Now you just have to be in this area. Settling a capacity ... what's that supposed to mean? Depending on the material, that will be difficult anyway. .. and doesn't really help. How and for what would you like to use that.

Due to interference, the measurement of the current to keep the capacitor at a constant voltage will not work so well. In addition, one can only record relatively quick changes in capacity. The version with AC voltage on the capacitor and then measuring the current (or charge) for recharging (or similar) is more promising, because the measurement is, so to speak, continuously repeated and thus interference can be better suppressed. With 2 electrodes one would probably supply one electrode with alternating voltage and measure the induced voltage of the current / change of the charge on the other electrode. There are already really economical operations for something like this today. With such a large area to be monitored, consideration should be given to subdividing it so that the relative change in capacitance is greater.

So to the technology: To measure capacities (also <1pF) you can use the CTMU of PIC controllers: http://ww1.microchip.com/downloads/en/DeviceDoc/30009743b.pdf There are many PICs with CTMU, an example is the PIC24FJ128GC006. The controller has several constant current sources (550nA-550µA), which can be coupled in the chip with the ADC and timers. You can charge the capacitor over a defined period of time and then measure the voltage. 550nA are very little, and since they can be controlled precisely, the resolution is quite high. This works quite well from 100fF to 100µF. I used it to build a capacitance meter that works well. The CTMU is also suitable for detecting capacitive buttons. I could even detect a contact with a two-pole line :-) I can't tell you whether it is the cheapest solution for your system. Regarding the concept: I regret to say that I am skeptical. Reason: On the one hand you have to charge the capacity with tiny currents, on the other hand you have huge disruptions. There is the 50Hz hum, but also other electrical and magnetic fields. This will be problematic with a plate of several m². It will at least be necessary to protect the whole thing well. ESD protection diodes and series resistors come to mind.

There are ready-made chips like the AD7745, which can measure up to 20pF or so, and which measure with 50Hz suppression, 24bit. Depending on whether it is asymmetrical or symmetrical.

Thank you for the answers. Since the alarm system is used outdoors, e.g. on tarpaulin, I do not think that I will have such enormous disruptions there. I have now thought that I have a "low power measurement" that measures a current peak by increasing the capacity, which in turn triggers the "high power measurement", which works frequently in order to be able to make more precise measurements. I just don't know exactly how to do it all. For the current measurement, I will probably work with sensitive Hall sensors. Since the capacity will be very high at 39 square meters, I need a circuit that saturates the capacitor regardless of the measuring unit ... I would be very grateful for further suggestions.

David W. wrote:> Since the alarm system is used outdoors, e.g. on tarpaulins,> I do not think that I will have such enormous disruptions there. You see that wrong. It is basically possible to measure small currents and capacities, but this is typically done in the laboratory, where there is a ban on computers and mobile phones, among other things. In your concept, the interfering signals will be larger than the useful signals.

David W. wrote:> Thank you very much for the answers. >> Since the alarm system is used outdoors, e.g. on tarpaulins,> I don't think that I will have such enormous disruptions there. Well, believe it, then. I know it is. There are at least the 100V / m E-field of the earth. That is not always constant. Then there would be radio and co. At the latest with the first thunderstorm, you can worry about protection. Indirect lightning strike. As far as low power is concerned, that is no problem at all with the CTMU. The PICs with CTMU can usually run with a few µA, and are usually capable of ultra low power - i.e. sleep with <1µA is not a problem. The measurement itself is also very economical. The power source has only 550nA, and is in operation for a very short time. The low-power capacity measurement is actually the main purpose of the CTMU. Think buttons - that's what this is actually intended for. As far as the currents are concerned: If you absolutely want to measure extremely small currents, a transimpedance amplifier might be useful. This can be used to measure nA. The OPV that can do that will give you more power than the CTMU.

So, for example, I set up the tent, switch on the system, this calibrates itself to the value of the capacity that is present exactly at this location, which also includes the changed values ​​through radio, this calibration can be done again at certain intervals to change the values by taking into account the humidity, for example. Don't you think that a person who is approaching now has a greater influence on the whole system than radio waves etc.? The whole thing can be set so that the system does not react too sensitively, or am I wrong? I've already ordered the CTMU, thank you for that. Addendum: I think that with the thunderstorm is a much bigger problem. But who's camping in such weather :)

David W. wrote:> Well, I set up the tent, for example, switch on the system, this> calibrates itself to the value of the capacity that is exactly at this location> I think you shouldn't ask any further questions, just try it out . But don't be too disappointed if it doesn't work.

You are right, thank you for your help. I will post the structure and the results here on occasion.

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