MKM33 AFE Inputs below zero.

[Edrianocarlos]

Hello.

I have been reading all the documentations regarding this micro-controller and its front end 24bits ADC.

There is a single or three Fase meter app that describe using a current sense transformer or coil. directly connected to the in+ and -.

Ok but how the converter deals with the negative input? maybe i am a little new to current or energy metering.

But at the transformer it has to be a negative voltage right?

For Voltage measure that is ok they shift the voltage input to vref/2 and its ok to read.

I would really appreciate some help. just understanding how it works.

thanks in advance.

[david_diaz]

Hello Edriano,

The analog-to-digital converter (ADC) is constructed from a ΔΣ modulator and digital filter.

Using the delta-sigma (ΔΣ) modulator, the ADC converts the input into a digital pulse stream with a 1’s density proportional to the input divided by the reference.

The following figure shows a modulator. Let’s set the input to 0.15V and the reference to 1V so that the input range is +/-1V.

I assume you are referring the to the negative reference voltage, and indeed, it is necessary in order to success with the conversion as it is shown on the last figures.

The result of the conversion would be like the following table:

Hope this information will be useful for you.

Have a great day.

Best Regards, :smileyhappy:

David Diaz.

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[Edrianocarlos]

Hello David.

Thank you for the answer what i mean is the following.

I took a picture from the Freescale example.  if i insert a current transformer without some Dc offset i will have a negative voltage at the inputs referenced to zero.

[xiangjun_rong]

Hi, Edriano,

Thank you for your interest in freesale solution to test power and energy.

As you can see that there are two capacitors C231 and C232 with the same capacitor value, the middle node of the two capacitor is connected to Ground. The current transformer outputs symmetric voltage. If we take the above two factors into consideration, the common mode voltage on the SDADP0 and SDADM0 pins will be zero.

Pls refer to the Analog Front End (AFE) in reference manual of KM family, the Input common-mode voltage range of AFE is from 0 to 0.8V,  input range for all AFE differential inputs is +/-250mV. In other words, the AFE can accept the voltage:

|(voltage of SDADP0- voltage of SDADM0)|<250mV, it means that voltage of SDADP0 pin can be 250mV greater than voltage of SDADM0 pin, while voltage of SDADM0 can be 250mV greater than voltage of SDADP0. The condition is that the common mode voltage ranges from 0 to 0.8V.

The common voltage is (voltage of SDADP0 + voltage of SDADM0)/2.

for your question, the minimum negative voltage of both SDADP0/SDADM0 pins is -250mV/2=-125mV, if we take the common mode voltage is zero.

Hope it can help you

BR

XiangJun Rong