[SOLVED] Conditioning circuit for microphone

[Audioguru]

The maximum output of the OPA1652 opamp is about +4.2V when it has a 5V supply and is sourcing a current of 850uA. You need it attenuated to 3.3V. The series 1k and 3.9k resistors at the output of the opamp are an attenuator: 4.2V/(1k + 3.9k)= 857uA. 857uA x 3.9k= 3.34V.

You know what? I was wrong. Its output does go to 0V in the circuit when the opamp has no load current which is what happens without a dual polarity supply.

The maximum output voltage is reduced when the load current is higher so you can play around reducing the value of a single load resistor to ground until the maximum output voltage is 3.3V (but it might cause distortion). Or you could use a buffer opamp for a lower resistance if you want.

 

[electroman2000]

The maximum output of the OPA1652 opamp is about +4.2V when it has a 5V supply and is sourcing a current of 850uA. You need it attenuated to 3.3V. The series 1k and 3.9k resistors at the output of the opamp are an attenuator: 4.2V/(1k + 3.9k)= 857uA. 857uA x 3.9k= 3.34V.

ah ok. Now I understand.

You know what? I was wrong. Its output does go to 0V in the circuit when the opamp has no load current which is what happens without a dual polarity supply.

so in the single supply mode (with ground at V- pin) there isn't rail to rail problem for lower voltage? I understand right?

The maximum output voltage is reduced when the load current is higher so you can play around reducing the value of a single load resistor to ground until the maximum output voltage is 3.3V (but it might cause distortion). Or you could use a buffer opamp for a lower resistance if you want.

so I will use a buffer

Thank you very much

 

[Audioguru]

Yes, the output voltaqe goes to ground in the single supply mode since the load also is connected to ground resulting in no load current when the output voltage is low.
The OPA1652 has two opamps inside. If your signal is mono then one can be the preamp and the second one can be the buffer. If your signal is stereo then used the quad version of this opamp.

 

[electroman2000]

Yes my signal is mono and I thought at the same solution, first OP AMP for preamp and second for buffer.
Thank you!

 

[electroman2000]

last question: for other reasons, in my circuit I must use a dual supply. So, it is convenient to convert the circuit with dual supply instead of single supply or it is indifferent?

 

[Audioguru]

Why do you need a dual supply and what voltages? We have been talking about using +3.3V for the microcontroller and since the opamps will not work with a supply less than +4.5V then we were using a +5V supply for them.

 

[electroman2000]

Because other section of my circuit needs dual supply (-+ 5 V ) so if there are any advantages using dual supply in this OP AMP, I can do it

EDIT: I attach the data sheet of my microphone. I don't think the amplifier inside is a simple pull up JFET, but a real amplifier; in this case R1 resistor is useless, right?

 

[Audioguru]

I have never seen a MEMS surface-mount microphone. Its very low sensitivity is exactly the same as all the electret microphones listed at Digikey so it needs a preamp unless you scream at it. Its datasheet shows a preamp circuit similar to mine.
It is tiny, how will you make connections to it? It has a horrible 15dB peak at 20kHz in its frequency response.
The R1 in my schematic is not used for your mic but you must make a filtered supply of 1.5V to 3.6V for it.