Please help me to check this

[hebu]

I designed a single stage OPamp for SC filter in audio frequency, the sampling
frequency is 1MHz. The cap I use is around 2.5pF and 250fF, the input capacitance
of OPamp is around 3pF,(larger size to reduce flicker noise), The current consumption of opamp is only 120uA. The input signal should be small than 1.6V
peak to peak voltage(differential), and a 40KHz anti-aliasing filter is preceded the SCF.

My question is, do you think the 120uA current of OPamp is too small?
The technology is 0.35um.

 

[nxing]

So, what's the DC gain and bandwidth of your Opamp?

 

[ocarnu]

Assuming slew rate:
Q=CU=It => t=CU/I t=3p*1.6/120u=0.04us.
From the slew rate point of view is fast. Of course the exponential part is very important also. How much precision in settling do you need?

 

[hebu]

My Opamp cut-off frequency is at around 13MHz, DC gain is 45dB. I would like to make the settling time be the 1/9 half period. The feedback factor is around 0.3~0.4 including the input capacitance of OPamp.

 

[ocarnu]

I guess you know already that t=1/(w-3dB) where w-3dB=β*wT (wT is unity gain freq, t is actaully tau, the time constant). Now you still didn't say what do you mean by settling time: to what precision? The circuit will never settle completely as the output is exponential. I assume is 9*t.
w-3dB= 2pi*13MHz*0.3=24.5Mrad/s.
t=0.04us, 9*t<0.4, plus a little slew rate (which is fast), everything seems to settle in very well in 0.5us.

 

[willyboy19]

There is usually no definite answer to your power consumption question: it depends on many issues, but the most important requirement come from noise and linearity. The noise spec is self-evident. The linearity mostly comes from the settling accuracy of your Opamp and the gain accuracy of the SCF -- remember finite DC gain causes both gain error and pole error which affects your overall filter transfer function.

As long as you are clear what you are looking for, you can easily justify if 120uA is sufficient for your application based on simulation results.

 

[hebu]

Thanks to all you guys, I would like to get a generic specification of SC filter
design in audio application. I'm not really confident about my design because
it's my first time to design SC filter. I really got much in this time.
Thanks, everybody.