Original question:
Hello,
I want to use an OPA350 as a Low noise voltage source to drive a Quartz crystal oscillator.
The quartz crystal oscillator I plan to use requires a 10nF capacitor
According to the datasheet, the OPA350 should work ok, however, what about overshooting ?
Is there any risk for the quartz crystal ? (it is specified for 1V8 up to 3,6V)
I there any oscillation risk ?
Thank you,
Jean
Hello Jean,
The OPA350 should be able to drive a 10 nF load directly in a gain of +1 V/V as indicated by the datasheet graph shown below. But you are correct to question how much overshoot can result from driving a large capacitance such as 10 nF.
We can see from the graph that for a gain of +1 V/V that a 10 nF load corresponds to an overshoot of about 65%. An overshoot of 65% is in line with a phase margin of only about 15 degrees. Even though an op amp circuit with 15 degrees of phase margin is supposed to be stable there isn't much margin remaining. Adding an isolation resistor (Riso) between the output of the OPA350 and the 10 nF load will help increase the margin. Just a matter of tens of Ohms will make a big difference in the ac transient and stability behaviors,. Just make sure adding the resistor doesn't upset the dc operating requirements of the crystal oscillator.
Do note that in your proposed circuit that the common-mode voltage comes up very slowly after the Zener because of the R1, C2 time constant (seconds). Therefore, it is as if the circuit comes up to quiescent operating conditions very quickly. I don't think the overshoot is going to be an issue, but I do think that if you can add an Riso between the OPA350 output and the 10 nF load the phase margin will be increased and the overshoot will be reduced. The latter should help any concerns regarding exceeding the crystal oscillator 3.6 V max.
Regards, Thomas
Precision Amplifiers Applications Engineering
