MC9S12XHZ256 ADC ISSUE

[rajnirajpoot]

We are using MC9S12XHZ256 – 144 pin microcontroller, In this project we are using 9 ADC channel. 

ADC channel AN13 (MCU pin no 2) voltage goes down from 4.6 to 1.8V ,when we connect with circuit and other channel are working fine with same circuit.We have tested it with multiple board but same issue found. ADC channel configured by processor expert.

circuit :-

 

Voltage observation with /Without connect MCU pin

Sr. no.	Without connect ckt with MCU pin	connect ckt with MCU pin 2 (AN13)
1	4.006V	3.992V
2	3.071V	3.052V
3	2.001V	1.988V
4	1.010V	1.006V

Thanks

Rajni

[alessandrofinez]

HI Rajni,

For what I know about ADCs from NXP you are using a series resistors (ie. R111) to much higher than the usual, (100ohm...1K as maximum), especially if you are multiplexing the single ADC circuit from many external IO pins. The input capacitance should charge and discharge continuously from a voltage to another across multiply sampling channels.

This working mode causes an high leakage current, so combined to an high input impedance, it results ia a great voltage difference between the connector input and the analogue input pins. This difference justifies the behaviour you observed: the voltage drop (i.e. the leakage current) depends from the voltage present on the other inputs, and mainly from the previous have been sampled in the chain.

Please reduce the R111 down to 100R and let me aware about it

Regards.

Alessandro Finezzo

[rajnirajpoot]

If anybody have solution, please help me out.

[danielmartynek]

Hi Rajni,

I replied to the ticket you created. Please check it.

When do you measure the voltage, after initialization?

Can you provide your code?

Please make sure test pin is tied to Vss.

Regards,

Daniel

[rajnirajpoot]

Hi Daneil,

We are measuring the voltage at the input going to AN13, using multimeter.

As you can see in table with and without connect circuit with MCU voltage is given. This voltage is noted by connecting only a potentiometer with refrence of 5V to MCU ADC Channel AN13. Then with circuit 5V is drop to 3.9 V.

ADC initialization file is attached.

[danielmartynek]

Hi,

I think the voltage drop from 5V to 3.9 is a typo, because in the table the drop is 14mV.

If you run the application, what result do you get on AN13 and other channels with the same circuit?

The serial resistance is really too high. Please keep in mind that the input signal must charge the sampling capacitor during sampling. If you repeatingly sample multiple channels with different input level, higher input current can be observed. And 14mV voltage drop / 48k resistance is only about 290nA.

Please refer to the following application notes:

Interfacing to the HCS12 ATD Module AN2429

An Overview of the HCS12 ATD Module AN2428

Regards,

Daniel

[rajnirajpoot]

We are using AD channels – 4,5,6,7,12,13,14,15. The same circuit works fine with the other channels.