LPC2468 ADC issue

[ancymuhammed]

We are using the LPC controller LPC2468FET208 ]. We had assembled 72 boards. Out of this 72 boards in 22 boards the ADC readings at higher voltage (above 1.6V) seems to be less than the expected value. We had checked with ADC channel 0, 1 & 6. External reference of 3.3V is given at the VREF pin. In remaining 50 boards all readings are correct. In one of the failed boards after replacing the microcontroller with new one it is working fine. What may be the possible reasons for this

[xiangjun_rong]

Hi,

I have checked the data sheet of LPC2468, the Integral non-Linearity spec is +/-2LSB at most.

As you said that just 22 boards out of 72 boards had issue, it appears that this is chip issue.

Anyway, pls check the input source impedance which is connected to ADC analog channel, if it is large, I suggest you develop a buffer based on AMP to convert the impedance.

Hope it can help you

BR

XiangJun Rong

[ancymuhammed]

We are giving the output of the pressure sensor to ADC channel . The output of the sensor is given to a voltage divider of value 10K & 20K where 20K is connected to ground and 10K is the series resistor before feeding to the ADC. Also a filtering capacitor of value 10Nf is added at the input of the ADC pin of the controller. We had tried by giving a voltage from the power supply to the ADC directly by removing 10K,20K & 10nf capacitor. But the failure is same. No dip observed in the ADC input of the controller. The input to the controller is correct.

[xiangjun_rong]

Hi,

I suggest you reduce the resistor and the filtering capacitor.

for example using 100 ohm/200 ohm as dividing resistor if possible, using 2.2nF capacitor.

Pls have a try

BTW, if you connect 3.3V to the analog channel of ADC directly, what is the result?

BR

XiangJun Rong

[ancymuhammed]

While connecting the supply 3.3V, the reading shown is around 875.

[xiangjun_rong]

Hi,

It is obvious that the ADC result is 0x3FF or 1023 in decimal if you test 3.3V voltage, but you get 875 in decimal, it is wrong.

Because only part of the boards has the issue, is it possible to contact local FAE so that they can help you.

BR

Xiangjun Rong

[ancymuhammed]

We checked the ADC reading by reducing the resistors to 100 & 200. Still we are facing the same issue