16bit ADC Questions

[judgeless]

We are using a Kinetis K64FN1MOVLQ12

The on board ADC is used along with a Difference amplifier, to scale 15V and/or 24V signals to 3V.  The ADC is calibrated with a  liner ‘ mX+b’ gain and offset equation.

The question;

1. What is the Vin linearity to measure specification?
2. We are seeing a ‘sawtooth’ offset error, repeating approximately every 450mV with a very steep offset change.  See attached.

Are there setting which can be controlled to achieve better performance?

• Frequent use of the ‘Auto-zero’ command?
  • Presently Auto-zero is only implemented on power initialization.
• Faster or slower clock speeds,
  • presently using 6.125MHz.
• A more readily available ADC Kinetis version?
  • Possible medical device with increase ADC capability?
  • Drop in replacement?

[xiangjun_rong]

Hi, Sean,

I am interested in the phenomenon, I want to know how you get the offset curve?

If you do not use the calibration, does the offset curve appear or not? I want to know if the calibration leads to the phenomenon.

if you use average feature of ADC, does the offset curve exist?

In differential mode of ADC, if you connect ADC_DMx and ADC_DPx together and connect to  analog GND, what is the ADC result?

BR

Xiangjun Rong

[judgeless]

Here are some answers

I am interested in the phenomenon, I want to know how you get the offset curve?

The curve is generator with an external supply sweeping from 10mV – 15V, with the K64 returning scaled readings.

If you do not use the calibration, does the offset curve appear or not? I want to know if the calibration leads to the phenomenon.

I’m unsure of the question of which calibration.  The zero offset calibration is only run at power up.

External Calibration is used, sourcing 0.8V for Zero V and 10.8V for Full-scale.  Source values are measured and stored into the K64 using a  7-1/2 digit DMM.

if you use average feature of ADC, does the offset curve exist?

32-bit Averaging is used.  A sweep with lower averaging is similar, but noisier.

In differential mode of ADC, if you connect ADC_DMx and ADC_DPx together and connect to  analog GND, what is the ADC result?

We have not tried this experiment. 

Presently the ADC1_DP0/ADC0_DP3 is the difference Amp positive (pin 29) and ADC1_DM0/ADC0_DM3 (pin 30) for difference amp negative are used.