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How to estimate current drawn by microcontroller(on the ADC port) when interfaced to a module/IC/sensor theoretically?

MCU manual: https://nxp.com/docs/en/data-sheet/LPC2131_32_34_36_38.pdf

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The parameters of the pin you are interested in are the "LOW-level input current" and "HIGH-level input current". In case of that MCU it is 3 µA.

For the ADC you also have to factor in the sample and hold capacitance which is the "analog input capacitance" (1 pf). Also look at figure 12 "Suggested ADC interface", it shows the ADC input in more detail.

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  • \$\begingroup\$ will it be right to say that this the load current if we are measuring the power consumed by the sensor? \$\endgroup\$ – Megh Dec 9 '18 at 16:29
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    \$\begingroup\$ Sensor power consumption and sensor output current are (almost) totally independent. You can have a sensor that consumes 5W to run a heater and still outputs just 1 mA signal. \$\endgroup\$ – filo Dec 9 '18 at 16:33
  • \$\begingroup\$ So now if I need to calculate the power consumed by a sensor(specifically analog type temp sensor) interfaced to my controller, I need to find P=VI, where V is the supply voltage and I is the sum of load current and quiescent current at that very temperature. So what should I take as the load current? \$\endgroup\$ – Megh Dec 9 '18 at 16:39
  • \$\begingroup\$ Which sensor? Thermocouple? Pt100? LM35? Provide a datasheet. \$\endgroup\$ – filo Dec 9 '18 at 16:40
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    \$\begingroup\$ @Megh, I showed how in my answer. Given the specs filo pointed out, though, I am not sure the datasheet is 100% self-consistent, and to be conservative you should probalby use the 3 uA figure. \$\endgroup\$ – The Photon Dec 9 '18 at 16:59
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This isn't specified directly in your datasheet, but there are a couple of clues in there.

First, there's a spec for the maximum resistance of the sensor or signal conditioning circuit that drives the ADC:

enter image description here

where Note 8 refers us to Figure 11:

enter image description here

The limit on the source resistance tells us that the maximum input leakage current, when run through a 40 kΩ resistor, won't cause a voltage drop big enough to disturb the ADC reading (by more than the specified absolute error of counts). Taking the minimum VDDA value of 2.6 V, divided by the resolution of the ADC of 1024 levels, this implies

$$I_l < 4 \frac{2.5\ {\rm mV}}{40\ {\rm k\Omega}}$$

or about 252 nA.

From this we can infer an input resistance of 10 MΩ or greater.

But note in the input equivalent circuit, you must also consider the input capacitance if your signal is varying somewhat quickly over time.

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