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Topic: EMG Signal DC Bias (Read 697 times) previous topic - next topic

dgreenheck

Hey all,

I'm building an EMG for my biomedical design class and have ran into issues with biasing the AC signal. I do all of my amplification/filter stages with op-amps powered using +/- 9V. Once all the signal processing is done, I put it through a DC-biasing circuit to get a DC offset of +2.5V before I send it to an analog pin on the Arduino Uno. The bias level is set using an op-amp voltage divider powered by the Arduino 5V connection. Unfortunately, this doesn't seem to be working as planned. The bias level seems correct (it's not quite at 2.5V but I'm guessing there is a little common mode voltage in my signal), but I get weird clipping at ~1.5V on the bottom end and ~4.5V on the top end. Below is a link to that part of the circuit.

http://imgur.com/GfwOI

I would just like to know why I'm experiencing this strange clipping. If you need any more information feel free to ask.

Dan

retrolefty

#1
Oct 22, 2012, 07:20 am Last Edit: Oct 22, 2012, 07:27 am by retrolefty Reason: 1
What is the Vcc value for the op-amp driving the arduino analog input pin? Is the op-amp rated to be a rail-to-rail op-amp? If not many op-amp's output can't go completely to their Vcc and Vdd values due to voltage drop in their output stage design.

EDIT: Just opened your drawing link and see it's a 741 op-amp, which is a very old and tired device to use and also not a rail to rail op-amp, it's output can never get to +5 or to ground before saturating it's output stage. So either run the 741 with higher voltages like +8 for Vcc and -3 for Vdd, or get a more suitable op-amp.


Lefty

dgreenheck

Thank you! This makes a lot of sense. I didn't know this about the 741, although I did know it was a very general purpose op-amp. I just started studying op-amps so I'm not too familiar with their different flavors.

Where on the datasheet does it specify that it cannot go rail-to-rail? I completely believe you, but it'd be nice to know where I could find that information in the future. I looked over it and couldn't find anything that pointed to the output voltage drop. Thank you once again, you saved me a lot of headaches!

retrolefty

Different manufacture have different ways of stating if a specific model has rail to rail capabilities. Here is a good place to start:

http://www.analog.com/en/precision-op-amps/precision-rail-to-rail-amplifiers/products/index.html

Lefty

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