OPA134: Useful opamp for audio tone from microcontroller

Below diagram

below diagram

Thanks, Samuel,

I would relocate the volume control after the OPA134, I think. Right now, your low-pass cutoff will vary from ~100Hz with the volume control set to minimum all the way up to ~80kHz with the volume set to it's highest setting. I'd suggest the following circuit:

This keeps the same 80kHz cutoff frequency as before, but makes a few changes. C10 was added to block the DC component of the microcontroller's square wave output (2.5V for a 0-5V square wave). This is important because DC potential across a potentiometer can cause pops and clicks as the volume is changed.

With C10 blocking DC from the input of the OPA134, R11 was added to provide a path to ground for U1's input bias current.  

I relocated the volume control after the OPA134 to provide some isolation between it and the input, though it could be installed where R11 is, instead. Beyond that, the DRV135 configuration is exactly as you have in your schematic. 

A typical line-level signal in pro audio would be +4dBu, which works out to ~3.47Vpp. This would correspond to a volume setting of around 10% on your potentiometer. I assumed that your microcontroller would output a 5Vpp waveform, and assumed that a ~10dBu output signal should cover most use cases. In order to achieve a reasonable gain range, I had to add an additional resistor, R10, which knocks the maximum gain of this circuit down to ~0dB. Since R10 needed to be equal to P1, I dropped both down to 20k to help reduce noise from large resistors. I reduced the value of R6 to 100 ohms in order to set a lower limit on the volume, though R6 could be omitted entirely if full muting was desired. As drawn (and assuming that the microcontroller output is 0-5V), this gives a volume range of ~-30dBu to +10dBu. If the microcontroller outputs 0-3.3V, R10 could be decreased to achieve the same output range.

If you want to test this out in simulation, I'm including my TINA-TI file here. 

Hi Samuel,

to make the low pass filtering (R7 + C5) more immune against the volume settings (P1), I would increase R7 in your circuit to about 10k and decrease P1 and R6 by a factor of 10. Eventually C5 should be decreased, if R7 is increased.

Then, the gain setting resistors R8 and R9 are too small. I would choose R8=R9=10k, or so.

The two signal grounds "GND" and "GND_A" should be connected together to make the DRV135 work properly. Is this a problem? Do you use the +/-15V switcher only to generate higher supply voltages, or also to provide a galvanical isolation?

Do you really need the DRV135? The OPA134 is strong enough to drive long cables. Or do you need a balanced output signal?

Kai

Hi Alexander,

Thank you for your partial response. Due to microcontroller and opamp don't share the same common ground, I will use an isolate digital or optocoupler, Could you recommend any device?.

You said that I'd need to use a DC bloking capacitor, Could explain how can protect op amp common mode range agains different ground potentials?.

Thanks

I improved the design adding Single-Channel Digital Isolator ISO7710F for avoid problem for use no common ground, increased value of R6 to 100 ohm in low pass filter in order to obtain cutoff frequency of 16 kHz about and decreased R9 to 10ohm.

I attach new circuit where I renumbered components. I hope this work fine, please verify it.