(help!!)32.768kHz crystal oscillator

how to realize the 20M ohm feedback resistor???
if i use the MOST works in linear region, it's res won't be stable, used a opamp as a buffer?
thx

buckaroo said:

how to realize the 20M ohm feedback resistor???
if i use the MOST works in linear region, it's res won't be stable, used a opamp as a buffer?
thx

Click to expand...

hi
i think there is no need to set such a large resistor, saying 1-5mhohm can be work well too. besides, a long length pmos transisor working in linear region can also help.
good luck
jeff

when use the MOST in linear region, its value varies with the VGS……it's a tough problem
does anyone has good idea?
thx

jfyan said:

buckaroo said:

how to realize the 20M ohm feedback resistor???
if i use the MOST works in linear region, it's res won't be stable, used a opamp as a buffer?
thx

Click to expand...

hi
i think there is no need to set such a large resistor, saying 1-5mhohm can be work well too. besides, a long length pmos transisor working in linear region can also help.
good luck
jeff

Click to expand...

32K Hz Xtal can be find in Chipset RTC cell ..
as I know 32K is more difficut than 1M Hz ..

and you need more large gain
some XTAL have gate control (use AND gate) for disable /enable ..
hspice simulation maybe OK you can enable/disable XTAL clock , but real chip test , when power on is work but disable XTAL -> restart will fail

if textbook said invert gain need > 7
real chip will fail

we use 15 is better

by the way , 32KHz Osc need more start_up time
some time have test cost issue .. if many XTAL need
warm up 30ms .. test cost is High
we use "impedance"

you can real measure > 500 sample XTAL and
measure I_current
Pass chip maybe I ~ 1u
Fall chip maybe I > 10u (gain too low ??)