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ADC Accuracy

Question asked by Kavitha Kakarla on Aug 2, 2017

Hi,

 

We are using NXP MPC5746C MicroController for one of our projects.

We have used external Multiplexers (PartNumber: 74HC4051PW) to connect to ADC_0. 

 

Below are the configurations I used for ADC_0:

ADC Bus clock(FS80) is set to 80MHz using a system clock of 160 MHz.

ADC Clock - 40MHz ( ADC_0.MCR.B.ADCLKSEL = 0 - Half of the bus clock)

Sample Time - 0.6 microseconds (ADC_0.CTR2.B.INPSAMP = 24)

Decode Signal Delay - 2.5 micro seconds (ADC_0.DSDR.B.DSD = 100)

One shot mode and Normal Conversion is enabled for all the external Channels(64 to 95) of ADC_0.

 

With this configuration I applied a voltage of 0.337V to one of the external channels and expected to observe the ADC Count as 69. But I have observed the ADC Count as 77.

 

If I change the decode signal delay (ADC_0.DSDR.B.DSD) to 4000 (which is approximately 50microseconds), only then I am receiving the ADC Count as 69.

But increasing the DSD shall increase the total conversion time which may make our system  slow.

Please suggest if we should use a delay of 4000 values in DSD register to get the accurate ADC results or is there any other solution to increase the ADC Accuracy.

 

Note:

Using this DSD delay, to convert 32 external channels (Decode Signal delay of 50*8 = 400 microseconds + sampling time of ~1*8 = 8 => Results in total of ~410microseconds)

Please correct me if above calculation is wrong

 

 

 

Thanks

 

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