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Apill
Apill
Associate III
September 17, 2018
Question

How in general ADC conversion takes place ? with DC bias affecting the accuracy of ADC input

  • September 17, 2018
  • 3 replies
  • 828 views

I am trying to measure the vibrations from a motor. I am using an accelerometer to measure the vibrations of motor. The output from accelerometer is in mV so i have amplified the signal using LM324 opamp and output is coming in Volts which is helpful for me but At the moment I could able to see the values of vibration data in keil( debugging window). My concerns are the following;

  1. When taking vibration data, when there are no vibrations there is a dc value that is driving to the input of ADC terminal which is affecting my acquisition data. If i add a capacitor at the ouput of amplifier then too it becomes complicated becuase of capacitor present in ADC terminal.
  2. will there be any opamp that gives only ac as output by eliminating a DC ? If there is any IC like that I will use that opamp?
  3. In general how is the ADC conversion takes place ? what are the stages in them ? How DC signal can be nullified. Is there any way I can bias the input of ADC in a way it works only after specific voltage lets say 0.3V? I am using stm32f429 discovery board.
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3 replies

AvaTar
AvaTar
Senior III
September 18, 2018

To comment in reverse order:

3. There are plenty of resources and tutorials on the basic principles of successive approximation ADCs, I see no need to repeat it here. Just try a web search.

2. All opamps have such offset currents to some extend. What you can do:

Select a opamp type with minimal offset values, and optimized lower gain and single-rail power supply.

Add offset compensation circuitry. This usually includes an adjustable resistor. Do the adjustments only after the warm-up time, about 15 minutes at least.

1.Not sure what exactly you mean with adding a capacitor. An AC coupling to your ADC input will not work.

I guess your sensor delivers a small AC signal. The opamp circuit needs shift it to the "middle" of the input range (about 1.5V), and amplify it the ADC input range (0...3V), perhaps with some reserve space to the rails. And of course lower the impedance to that of the ADC input.

The further processing (removing the introduced DC offset, scaling) is done in software.

Consult an experienced hardware developer.

Apill
ApillAuthor
Associate III
September 18, 2018

@AvaTar, thanks for the answers. It's helpful for me to gain a better understanding. Yes my signal is delivering in mV ( 200mV)

AvaTar
AvaTar
Senior III
September 18, 2018

> Yes my signal is delivering in mV ( 200mV)

Probably up to a few Kilohertz, which would suggest low-noise audio opamps as well.

If the signal is asymmetrical (0 .. 200mV), a buffer amplifier with a gain of about 15 would do.

Proper opamp circuit design is an art in itself, I highly suggest an expert.

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