HdrChopper
|
Hi all,
I'm having problems with the start-up of a fully differential opamp used in a simple first order switched capacitor filter. The amplifier is a two stage one, the first stage being a folded cascode one with an NMOS differential pair, and the second one a class A Miller stage (NMOS). Silicon is showing that the opamp is not properly starting up only under certain VCC ramps: for fast ramps (up to 500 us from 0 to 5v) the opamp starts-up correctly, while for ramps between 500us to ~10ms it does not. It seems to work ok again for ramps even slower than 10ms. The folded cascode stage is ideally balanced: pmos current mirrors at the top push 10ua, the differential pair 5ua and the NMOS mirrors (at which the CMFB is controlling the output CM) another 5ua. Ideally if during start-up the PMOS mirror pushes more current than NMOS the output NMOS gate will raise its voltage and output CM might not raise, so I'm think of pushing a little bit more than ideal current on NMOS cascodes to guarantee proper start up.
Any ideas and comments are welcome Best Tosei
|