Mega4809 ADC

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Greetings Freaks -

 

Not much discussion, yet, on the ADC in this new generation of chips. One new feature that will be VERY useful to me is "convert and accumulate". It can sample in code-defined burst and at a code-defined rate within the burst:

The ADC supports sampling in bursts where a configurable number of conversion results are accumulated into a single ADC result (Sample Accumulation). Further, a sample delay can be configured to tune the ADC sampling frequency associated with a single burst. This is to tune the sampling frequency away from any harmonic noise aliased with the ADC sampling frequency (within the burst) from the sampled signal. An automatic sampling delay variation feature can be used to randomize this delay to slightly change the time between samples.

The ADC still "likes" a signal source resistance of 10K or less. But, it no longer appears to require a bypass cap when using the internal reference. You DO need to be careful to disconnect the internal digital circuitry on analog input pins; this is done through the PORT peripheral.

 

There is an obscure requirement on the internal reference setting when using an external reference. The spec sheet asks that the internal reference be set to the next larger value than the external reference voltage. <EDIT> Presumably, this does not apply when AVDD is used as the reference (or, does it?). </EDIT>

 

When using the temperature measurement feature of the ADC, you have to use the factory-provided calibration values to get the specified accuracy. These values are in the Signature Row. And, it gives a result in Kelvin!

 

The ADC clock range is 50KHz to 1.5MHz for full accuracy. A normal conversion takes 13 clock cycles so this translates into continuous conversion rates of 115K to 3.8K samples per second (with 10 bit conversions and no delay between conversions). It does appear that you can select the number of conversion bits (with a maximum of 10). You CAN select a delay time between samples when continuously sampling.

 

Triggered conversions can be driven by an analog comparator. This allow a conversion to be started if the input voltage is below a voltage threshold, above a voltage threshold, outside of a voltage range window, or within the voltage range window. It appear that a dedicated comparator is used for this, not the standard "comparator peripheral". The threshold(s) (two of them for the window function) at 16 bit settable values. The spec sheet does NOT say, as far as I can tell, what these values are referred to or if they depend on the Vref setting.

 

<EDIT> The voltage reference has a number of settings: 0.55V, 1.1V, 1.5V, 2.5V, 4.3V. It is presumed that settings above VDD are not useful. Within the ADC, you can select internal reference, external reference, or VDD. There is some confusion because AVDD is given as the power supply reference in some places and VDD in others. Given the purpose of AVDD (clean analog power source). it is likely to be AVDD. There are difference reference voltage settings for the Analog Comparator and the ADC; it appears that they do NOT have to be the same.</EDIT>

 

Hope these points help get someone started with the M4809-family ADC.

 

Jim

 

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Sun. May 23, 2021 - 06:38 PM
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Thanks.  I had dabbed into this but not to that extent.

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I skipped the mega4809 ADC, now playing with the AVR-DB ADC.

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Is AVR-DB ADC significantly different from M4809 ADC?

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Mon. May 24, 2021 - 07:26 PM
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Operationally it's similar, but it has 12 bit resolution and can do differential measurements. We could say it's an upgrade of the mega4809 ADC.

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thanks

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!