LPC824, loading effect of ADC really that high?

[lpcware]

Content originally posted in LPCWare by rayhofmann on Wed Jun 03 05:19:48 MST 2015
I am not sure if there is something wrong in my circuit or the software because the loading of the ADC looks a bit strong.

The loading effect varies with what other channels it is sampling, it may be coming from low or high voltages. The example is with no other channel active. It does two identical conversions in sequence on one channel only, both loading effects look the same.


The source impedance is about 2KR, the added probe capacitance maybe 10pF plus 5pF stray on the board.

Or did i damage my part? There definitely was one incident where i shorted some power supply with the probe, but not directly the 3.3V supply of the LPC, so it was a bit isolated behind a regulator that obviously survived.

Maybe i just need a really low source impedance lower than 500R?

[vojtechhavlicek]

Any idea what is the reason of this beahvior?

[vojtechhavlicek]

I´ve just tried to measure voltage on my own board with LPC824 and I´ve found out, that I have same problem. 

I tried to eliminate possible problem with my board, so I tried it on development board and example from LPCopen 2.15.

Voltage has been connected to P0_6 through 3,9k resistor (graph1) and then 820R resistor(graph2).

Measuring was done from the 0V to 3V (0,5V steps), ther are results in graph - each column is ADC bits / 0,5 volt, so there is very high nonlinearity, which is same on the development board, my board, different pins, different sampling speed (very slow)

column 1    0V - 0,5V

column 2    0,5V - 1V

column 3    1V - 1,5V   

column 4    1,5V - 2V   

column 5    2V - 2,5V

column 6    2,5V - 3V

My question is how to reach a linearity? I can use just a half of ADC range and measure only to 1,5V but this is no solution...

[lpcware]

Content originally posted in LPCWare by IanB on Wed Jun 17 23:46:49 MST 2015
It will work with a high source impedance - there is no loading. (For a high source impedance you will have to leave it longer to settle, but there is no loading effect).

Check for a second time that the input REALLY is set to A/D mode,  then it's time to buy a new LPC824 and get out the hot-air-rework-tool.

[lpcware]

Content originally posted in LPCWare by jdesbonnet on Tue Jun 16 03:52:36 MST 2015
I'm not sure if this is the same thing, but I notice a strong pull up to Vdd at the ADC input of the LPC824. I initially thought a pullup resistor was enabled somehow, but after checking IOCON, it seems the pull up is disabled on the ADC pin (the datasheet states that this happens automatically when an analog function is selected, but I checked anyway).

For example a Vdd/2 Vref I created with a two 100k resistor divider was pulled right up to almost Vdd.

Anyhow, it seems not to be a problem when driven by an opamp.