calibration testing

[mgd]

Ok i understand for mordax and disting, do you know a better solution for making a good calibration ? 
Voltmeter, multimèter or other solution ? Modèle ?

I'd get me a good Multimeter. You could go for a Voltmeter but unless you try to be extremely precise, a good multimeter should do.

Next question to answer is required precision.
That depends on what you wish to use it for, aka what is it that you wish to measure. The NerdSEQ uses 12bit DACs for a range of 10V, i.e. about 2.5mV is the difference between adjacent voltages. For that a precision of 1mV in the range of 0-5V seems good enough. That is about 4 digits or something like 10000-20000 counts (the later being the measure of precision given on many online websites).
If you wish to calibrate 16bit DACs found in some quantizers you probably should have a multimeter with at least 5 digits precision, i.e. 100000 counts or better.
There's a limit to what makes sense in the context of modular analog synths though (your patch cables will have a resistance of 0.1-0.2 Ohm and the impedance of inputs is in the range of 10-100kOhm which results in the loss of 0.001-0.01mV per Volt via your cable - small enough that you can ignore it but big enough that calibrating to e.g. 8 digits precision would be pointless).

Models:
There are lots of decent vendors. Fluke, Brymen and quite a few others.
I own a Brymen BM869s (about 180/190€ a few years ago) which IMO is at the upper end of the scale that makes sense in the context of modular synths.
Basically everything with enough precision (see above) for your intended purpose should be fine.
If you happen to own a decent oscilloscope that should be fine as well. You then would not need a seperate multimeter though having both is convenient as well.

I still don't understand why you think you should calibrate in the first place though.

HTH,
Michael