Electronic – Connecting op amp circuit output to ADC makes signal asymetric

adcdistortionoperational-amplifier

I'm connecting an electric guitar to amplifier circuit and the output of the circuit to ADC. I've checked the output of amplifier with the osciloscope. The signal seemed fine, but after connecting it to ADC (maybe because of capactive load?) it become asymetric. The output signal doesn't go below ~500 mV when the output is connected to 'something' (I've tried also to connect it to hifi stereo system, with the same result) so it gets badly distorted. I need it to be in range 0 – 3 V. How to get rid of this effect?

Used circuit

  • Op amp: LM324
  • ADC: MCP3202
  • R1, R2 give DC offset by 1,5V. R1+R2 = 1M
  • gain close to 1

EDIT
I've tested the circuit with square wave to illustrate the problem.

On the first plot below is the unloaded version of the circuit (op-amp output isn't connected to anything), Vmin is 80mV which is almost what I want.
On the second one we can see the loaded version, output connected to hifi, however conneting it to ADC results in the same effect, Vmin doesn't go below 500mV:Plots

Best Answer

If you look at the equivalent circuit of an LM324 you will see why it cannot pull down to Ground under load. Output transistor Q13 is connected in Emitter Follower configuration, so when sinking current the Emitter will be ~0.6V above the Base (which at best can almost reach ground potential through Q12). There is a current sink in parallel with Q13 which can pull down further, but only at very low current (<50uA).

enter image description here

This behavior is confirmed in the output current sink graph,

enter image description here

An opamp with rail-to-rail outputs would solve the problem. If you want to continue using the LM324 then you have a few choices:-

  1. Provide the LM324 with a negative supply of at least -0.6V (might not be practicable if you don't already have a suitable negative supply).

  2. Connect a load to the output which pulls down to Ground, eg. a 150Ω resistor (probably still won't get the voltage low enough).

  3. AC couple the op amp's output to the input of the A/D converter, and use a voltage divider to set its bias point at 1.5V. To maintain accuracy the input impedance of the MCP3202 needs to be 1K or less, so a voltage divider using two 1.8k resistors should be OK. You will also have to change R1/R2 so that the LM324 is biased at >=2.1V for sufficient output voltage swing.

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