secnod harmonic distortion in sigma delta modulator

[chungming]

Hi~~~
My sigma delta modulator has second harmonic distortion like attached file
But i see papers there has no second harmonic just has third harmonic.
Is anybody know what cause second harmonic distortion.
My sigma delta is switch-capacitor with fully differential OPA.

 

[sixth]

Hi~~~
My sigma delta modulator has second harmonic distortion like attached file
But i see papers there has no second harmonic just has third harmonic.
Is anybody know what cause second harmonic distortion.
My sigma delta is switch-capacitor with fully differential OPA.

Check carefully if the output swing of the opamp within the linear range.

sixth

 

[jcpu]

Dear chungming

we happened to have similar problem with our silicon.
we have 2nd, 4th,... at ~-40dBc.
only that the main tone is not at 0dBV,
at -40dBv instead.
we are still looking for the cause...

Hi, sixth
by

Check carefully if the output swing of the opamp within the linear range.

do you mean they get saturated?

 

[chungming]

sixth:
thanks for you reply
I will check it carefully.


jcpu:
I early designed SDM not had 2nd distortion .

My spec is third order and the two and third OTA of SDM has ideal current in there,
Then i use current mirror to replace the ideal current,the 2nd appear.
The OTA's common mode output has some change , about 15mV below CMFB.
I think that is another possibility of 2nd distortion.

If you solve this problem please tell me the reson.
thanks a lot~~!!

 

[eldo]

Basically, the even order haomonics come from the mismatch on fully-differential circuits. This happens in real circuits. But in simulation the mismatch is not seen at all. I suggest you check your CMFB and the output impdeance of the tail current-source in the differential pair.

 

[Btrend]

The gain of CMFB also will influence 2nd harmonic.

 

[gingerjiang]

in silicon, if the references are coupled with input signal, there will be even-order harmonic distortion.