noise in ADc chanel

[babidesai123]

Posted on November 26, 2003 at 16:59

noise in ADc chanel

[babidesai123]

Posted on May 17, 2011 at 11:56

hi,

we are trying to sense temperature using upsd 323x. But what i have observed is in noisy enviornment. the reading are differed by more than 5-6 LSB. i have even tried with connecting same sensor to all 4 chanels., But all the four chanel reads the different values. Can anyone plz tell me the input impedence of ADC chanel. so that we can put suitable filter across the pin. Purvi

[neil2]

Posted on May 17, 2011 at 11:56

Hi,

What is the physical layout? are you using a development system or do you have a PCB? The layout can be critical to analogue performance. If you have any common ground paths in your analog and digital circuits, this can result in common impedance coupling between the analog and digital circuits, and Microcontroller circuits are noisy! on the 3234, the pin next to Vref should be used as analog ground and the other ground pins for digital ground. It doesn't say this in the data sheet, but I was told this by an ST engineer.

[babidesai123]

Posted on May 17, 2011 at 11:56

thanks for the reply.

We are trying to use our own PCB. But unfortunately we haven't provide the next to vref pin as analog ground. Will try to do the same. Plz let me know if u know the ADC impedence.. It should be specified in datasheet as Radc . But it's not... Thanks purvi

[neil2]

Posted on May 17, 2011 at 11:56

Just to clarify, the pin on the uPSD3234 that should be analog ground is pin 45 on the 52 pin device and pin 69 on the 80 pin device.

pin 71 on the 80 pin device is NC (not connected) so should be left open. I can't find the input impedance in the data sheet either.

[jinguanghua]

Posted on May 17, 2011 at 11:56

Do you have a RC filter circuit on the board?

[joseph2399]

Posted on May 17, 2011 at 11:56

When the port1.4 ~ port1.7 are used for ADC inputs, the ADC input impedance is almost infinity.

ADC input is connected to the internal capacitor that is used for sample-and-hold. There is no DC current path in the capacitor when an input signal is sampled. Therefore, the datasheet does not specify the input impedance for ADC.