DAC output filtering with Sallen-Key opamp configuration connected to audio power amplifier RCA in

Thread Starter

Luca Luca 1

Joined Jul 8, 2017
8
Hello Everyone!

This is my first post and I want to thank you in advance for your help! I am a firmware engineer not so expert in electronics design.
I am trying to develop an audio application using an Atmel SAM4S Xplained Pro board and its microcontroller DAC output.
In the proto board I designed, the DAC output is connected to a second order low pass filter implemented with a Sallen-Key opamp configuration. The opamp used is a OPA344PA rail to rail single supply. Then I remove the DC component with a 10uF capacitor and connect the output to an RCA input of a car audio power amplifier. When I set the gain of the power amplifier to high value, the sound is very bad with high distortion.
This not happens if I directly connect the DAC output directly to the amplifier (using the 10uF capacitor only) and removing the opamp filter.

My question is: the OPA344PA is not able to drive the RCA input of the amplifier?

Many Thanks for your replies!
 

Thread Starter

Luca Luca 1

Joined Jul 8, 2017
8
Thank you very much for your answer...you are very kind!
Attached you can find a rough schematics of my circuit. The issue is also present if I use the opa in the simple buffer configuration.
The opa is powered using the 3.3V out of the atmel board but I also tried to power it using a linear voltage regulator.

Thank you!
 

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AnalogKid

Joined Aug 1, 2013
8,377
Note the input voltage range on the datasheet. What is the peak-to-peak amplitude of the audio signal out of the DAC?

ak
 

Thread Starter

Luca Luca 1

Joined Jul 8, 2017
8
Measure the voltage at all the op amp pins and post here.
Hi guys! Thank you again!

Attached you can find the measures of DAC out (blu) and OPAMP out (red). You can see the opamp out drop down during distortion.

During the measure, I tried to also acquire the IN+ pin of the opamp: as soon an I connect the probe to the pin, distortion disappears with a very very very low volume down.

Any idea?

Thanks!
 

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Thread Starter

Luca Luca 1

Joined Jul 8, 2017
8
Hi guys,

looking around on Internet, I found a possible solution on the Analog site:

Missing DC Bias Current Return Path When AC-Coupled
One of the most common application problems encountered is the failure to provide a dc return path for bias current in ac-coupled operational- or instrumentation-amplifier circuits. In Figure 1, a capacitor is connected in series with the noninverting (+) input of an op amp to ac couple it, an easy way to block dc voltages that are associated with the input voltage (VIN). This should be especially useful in high-gain applications, where even a small dc voltage at an amplifier’s input can limit the dynamic range, or even result in output saturation. However, capacitively coupling into a high-impedance input, without providing a dc path for current flowing in the + input, will lead to trouble!

Here, a resistor is connected between the op-amp input and ground to provide a path for the input bias current.

What do you think?

Many thanks again!
 

Thread Starter

Luca Luca 1

Joined Jul 8, 2017
8
Hello again guys!

I think you were right...the problem is the input range! I tried to decrease the voltage reference of the DAC using the potentiometer on the Atmel board and the distortion disappears!
 

crutschow

Joined Mar 14, 2008
24,716
Here, a resistor is connected between the op-amp input and ground to provide a path for the input bias current.

What do you think?
I think I don't see a capacitor in series with the input, so without a capacitor there's no problem from that.
Attached you can find the measures of DAC out (blu) and OPAMP out (red). You can see the opamp out drop down during distortion.
What part of the output do you consider "distortion"?

Measure the DC voltages at all the op amp pins and post here.
 
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