2 years, 8 months ago.

How can I measure the internal impedance of LPC1768?

Currently completing a Power Analyser project which includes measuring and analysing voltage of which is being supplied to the LPC1768, however in order to check the voltage I will be measuring will be the same as what is being supplied, there has to be very high internal impedance and I can't find this anywhere. After taking advice I was told to measure this using a voltmeter but I'm not too sure how I would go about this. Thanks in advance for any help!

It's not clear what you're asking. Can you provide the exact problem/analysis you've been asked to do? You need to measure the voltage (and presumably current) for the LPC supply? It can run over a range of voltages, so you would have to define what supply voltage is to be used. Current consumption varies with clock frequency, which peripherals are used, what's connected to the pins, if sleep mode is used, etc. If you really need to measure micro impedance you would have to specify which pin you are talking about and what mode the pin is in and which peripheral is connected to that pin. Sometimes you do care about pin impedance - say if it's an ADC pin or even just a Digital Pin with internal pullup.

posted by Graham S. 21 Mar 2019

So the aim of my project is to measure the characteristics of power from the mains supply. So in order for it to be fed into the micrcontroller I have reduced the 230V AC to 5V AC. However, I was told the internal impedance should be somewhere in the region of MOhms so the voltage going into the LPC1768 is still the 5V (as with a smaller internal impedance there would be current and so the Voltage would be different thus not giving an accurate reading). But I can't find the internal impedance anywhere so I was told to measure the voltage at the LPC1768 using a voltmeter, I was just wondering if there was any way of doing this? Thanks for the reply!

posted by Luis Preston Stefani 21 Mar 2019

1 Answer

2 years, 8 months ago.

You want to know the input impedance of the ADC input pins?

Going by the datasheet, assuming no pull up/downs are active the steady state input current is typically 0.5 nA, max 10 nA.

For a varying voltage, especially if the signal source is high impedance, there are capacitance's that need to be taken into account. These capacitances will be connected and disconnected as different ADC inputs are selected and measured. Page 69 of the datasheet for the LPC1768 gives the equivalent circuit for the capacitors and selection switches.

Accepted Answer