Kinetis MicrocontrollersのトピックRe: Calculating voltage from AFE differential value

Calculating voltage from AFE differential value

Larbohell — Fri, 11 Sep 2020 12:41:56 GMT

I'm having a very basic problem with the AFE module (Sigma-delta ADC) on the TWR-KM34Z50M devkit (KM34 MCU). I simply cannot find the right way to calculate voltage from the measured ADC value.

What I'm doing is to input some differential voltage between the SD_ADP0 and SD_ADM0 input pins, say 500 mV, supplied from a basic power supply. I then take a bunch of ADC readings and use this forumla:

V_calc = (val/0x7FFFFF) * V_ref

One part of my problem is determining what V_ref should be. I find the datasheet and reference manual very confusing in explaining this aspect, as there are internally generated VREFH (1.2V) and VREFL (0.4V) signals, as well as two pins called VREFH and VREFL which seem to always be connected to these two signals. Then there is also a VREF pin, which can optionally be used as an external reference input instead of the internal VREFH.

Now, I've tried to use VREFH, VREFL and (VREFH-VREFL) as the V_ref in my calculation above, and I've tried supplying an external reference on the VREF pin as well. I've used the S0, S1 and S2 switches to set the reference going to the AFE. No combinations of this lead to me calculating the correct voltage.

What am I doing wrong here?

Re: Calculating voltage from AFE differential value

nxf56274 — Mon, 14 Sep 2020 01:36:35 GMT

Hi,

Hope this link will give you some ideals.

https://community.nxp.com/t5/Kinetis-Microcontrollers/Resolution-AFE-MKM34Z256/m-p/847175

Have a great day,
TIC

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Re: Calculating voltage from AFE differential value

Larbohell — Mon, 14 Sep 2020 07:46:27 GMT

Hi,

Scaling the voltage measurement val using the given +/- 500 mV input range actually produces okay-ish results:

V_calc = (val/0x7FFFFF) * 0.5V

but I don't think that's the correct way to do it. First of all, the 500 mV is only a "typical" value given in the datasheet (6.4.4.1), and may therefore vary enough to make V_calc useless. Secondly, what would then be the point of the reference voltages, VREFH, VREFL and VREF?

Re: Calculating voltage from AFE differential value

nxf56274 — Mon, 14 Sep 2020 08:47:20 GMT

Hi,

It is a limitation.

This device includes a voltage reference (VREF) to supply an accurate 1.2 V voltage
output. VREFH and VREFL use for output.

Have a great day,
TIC

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Re: Calculating voltage from AFE differential value

Larbohell — Mon, 14 Sep 2020 08:51:40 GMT

Hi,

I know and understand all that. My question is simply: How do I calculate the input voltage from the digital output value?

Say I use the AFE and get a measurement of 4538388 / 0x454014 from channel 0. How do I calculate the actual voltage value?

Re: Calculating voltage from AFE differential value

nxf56274 — Tue, 15 Sep 2020 06:02:37 GMT

Hi,

V_calc = (val/(2^24)) * 0.5

If the value smaller than 0, the digital value should convert. Because the result is 2's complement.

Have a great day,
TIC

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Re: Calculating voltage from AFE differential value

Larbohell — Thu, 17 Sep 2020 13:31:19 GMT

I'm still confused about the voltage references. The VREF pin can be used for giving the AFE an external voltage reference, right? So say I input 2.00V on the VREF pin, and set S3=L to use the external voltage reference for the AFE. How do I now calculate the voltage? The value should be different if I use for instance 1.50V and 2.00V on the VREF pin, so surely I can't just use the simple formula V_calc = (val/(2^24)) * 0.5 in all these cases.

Also, what is the allowed voltage range on the VREF pin? I can't find any information on that.

Re: Calculating voltage from AFE differential value

nxf56274 — Fri, 18 Sep 2020 01:12:03 GMT

Hi,

VREF is used to generate fixed voltage for VREFH(1.2V) and VREFL(0.4V). VREF should be around 1.2V.

Have a great day,
TIC

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