aaron_do wrote on Aug 24th, 2006, 5:14pm:It's gm-C...
so just OTA's and C's. I figured with a good bandgap reference i could keep the variation in the gm of the OTA to a minimum. I designed a simple bandgap and found that the total variation in the center frequency was about 15% from f0, mainly due to the slow-slow and fast-fast corners. So with the extra capacitor variation i expect 25% variation at worst. My Filter BW is 1 MHz around 2 MHz center, so i guess if I give a 2 MHz bandwidth i should be ok. The adjacent channel rejection is 0 dB (3 MHz) and the alternate channel rejection is 30 dB (8 MHz). What do you think?
thanks,
Aaron
It depends on your system design. Can your ADC/Demodulator tolerate 0dB adjacent channel rejection ? Will it have addition digital filtering to take it out ?
What it the specification for the power in the adjacent channel to be handled? in some specs it can be bigger than the wanted channel, so in those systems i doubt your filter is good enough. If the system isn't tough, then thats great!
Anyway, looks like you've done well to get the gm variation down to 15% with on chip resistance. Maybe you might want to take a look at your process data to ensure the "corner" simulation is actually at an appropriate sigma (3 is good, and remember normal distribution variance is additive, so 2 indepedant equal weighed 3stddev figures is √18=4.2sigma).
As fc is proportional to gm/C to get 25% means you have C at about 8% variation.... also pretty good. In this case an external R would get you down in the realms of fc variation in the region of 10%.
Filter tuning can get to you guaranteed to 1~2%, but if thats not required, then your right not to create work for yourself.