voltage input range of Kinetis M ADC

svenmoeller — Thu, 22 May 2014 13:46:20 GMT

Hello,

i want to use a Halleffect sensor(Pololu ±30A ACS714 Current Sensor - RobotShop) in combination with the TWR-KM34(Kinetis M3). According to the Ddatasheet the differential inputs accept a voltage range of 1V (+-500mV).

But the sensor exclusively has positive output: centered at 2,5V +-400mV

Does the firmware of TWR-KM34 properly work if my input signal is 0 to +500mV(so i could simply use a voltage divider)?

Or is it possible to change the input rane of hte ADCs from +-500mV to 2V+-500mV?

thank you

Re: voltage input range of Kinetis M ADC

egoodii — Fri, 23 May 2014 14:19:13 GMT

Without being familiar with the KM34 in question, I will just state that by definition a differential input to an ADC is 'oblivious' to the common-mode voltage on the signal pair -- this is 'subtracted' directly by a front-end differential amplifier -- so in your case, put your positive signal on the '+' input, and put VCC-to-the-module/2 to the '-' input to properly subtract the chip's DC offset. What you DO need to know in that regard is the allowed signal-range for said diff-amp. If the inputs are 'fully operational' over the 2 to 3V range, then you will properly net +/-400mV into the ADC directly from your sensor.

Some further investigation into the matter indicates you are looking at the sigma-delta ADC, and the common-mode voltage spec there is 0 to 0.8V -- so too low for my 'direct connection' assumption (which was more based on the differential functions available in some other Kinetis parts to the SAR ADC).

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voltage input range of Kinetis M ADC

Re: voltage input range of Kinetis M ADC