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</head><body style=""><div>Hi Mattias,<br> <br>I never used the SSM2164, but figure 15 in the datasheet suggests a capacitor of<br>10pF is viable (presumably in the example circuit with an OP275).</div>
<div>Note that the required capacitance depends on the GBW of the opamp used, see the formula for C2 in<br>www.analog.com/media/en/training-seminars/tutorials/MT-059.pdf</div>
<div>For example, the TL074 has a GBW of one third that of the OP275, resulting in a corresponding capacitance</div>
<div>of 17pF.</div>
<div>According to the same formula the corresponding capacitance is proportional to the square root</div>
<div>of C1, and therefore to the square root of the number of VCA outputs summed. You can also use it to </div>
<div>calculate the capacitance depending on the value of the summer resistor.</div>
<div> </div>
<div>Best regards<br>Simon<br><br>> Op 25 november 2015 om 17:43 schreef Mattias Rickardsson <mr@analogue.org>:<br>><br>><br>> Hi fellow 2164 addicts, ;-)<br>><br>> the typical application circuit in the SSM2164 datasheet shows a 100<br>> pF capacitor in parallel with the 30k feedback resistor on the op-amp,<br>> saying<br>><br>> "The 100 pF capacitors ensure stability and reduce high frequency noise.<br>> They can be increased to reduce the low pass cutoff frequency for<br>> further noise reduction."<br>><br>> [...]<br>><br>> "If additional SSM2164s are added, the 100 pF capacitor may<br>> need to be increased to ensure stability of the output amplifier.<br>> Most op amps are sensitive to capacitance on their inverting<br>> inputs. The capacitance forms a pole with the feedback resistor,<br>> which reduces the high frequency phase margin. As more<br>> SSM2164’s are added to the mixer circuit, their output capacitance<br>> and the parasitic trace capacitance add, increasing the<br>> overall input capacitance. Increasing the feedback capacitor will<br>> maintain the stability of the output amplifier."<br>><br>> The problem is that they already lowpass filter the sound a little bit<br>> - more than ½ dB rolloff at 20 kHz - and sometimes you might need a<br>> bigger resistor than 30k in order to optimize other aspects of the<br>> circuit, resulting in even more HF rolloff. Not awfully much, but you<br>> might have several of these stages in an audio path adding up to some<br>> decibels.<br>><br>> But is 100 pF really needed?<br>> Is TL07x sensitive to capacitance on their inverting inputs?<br>> How about other common op-amps?<br>><br>> Using the bread-and-butter op-amp TL074, has anybody tried out any<br>> variations on the 100 pF theme and run into problems?<br>><br>> I've used as low value as 33 pF & 47k after *two* summed 2164s,<br>> withouth known problems yet. The datasheet pole recommendation would<br>> be 127 pF for this scenario. Should I worry? :-)<br>><br>> (I'm sorry if this has been discussed already)<br>><br>> /mr<br>> _______________________________________________<br>> Synth-diy mailing list<br>> Synth-diy@dropmix.xs4all.nl<br>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy</div></body></html>