The resistors are used to cancel effects from input bias currents. The one in the feedback path should equal the source impedance.op-amps in a unity gain configuration. How do you determine the appropriate resistance values for the resistors?
If you don't match the impedances on the inputs, input bias currents can affect the output.But in a unity-gain follower that is not a factor as no resistors give the least output voltage offset effect from the bias currents.
I have 2 reasons for this, please let me know if I am incorrect here:But why are you using an analog op amp to buffer a digital oscillator signal?
Then use two buffer gates on the same chip in parallel.The buffered signal needs needs to support 27.5 mA (its 3.3V, and I have a 120 ohm resistor, so a dead short would be 27.5mA). I wanted to use a buffer gate, but they tend to be just under this mA rating.
The OPA356 has a rise and fall time of about 10ns (the delay is not shown) which is longer than a fast gate.Also regarding the buffer gates, the fastest propagation delay I could find was between 2.5nS and 5.5nS. An 80MHz signal is 6.25nS high, and 6.25nS low. So I was worried I would get a distorted signal and that these buffer gates are not fast enough?
Can you elaborate on "transmission lines" and signals that are traveling more than ~1"? What do you mean?Lots of reasons to place those resistors or not. I'll add another.
A 80MHz clock will have 800MHz+ edges. At these frequencies I've often used resistors on the output of digital gates (50-300ohm) to reduce the signal bandwidth and ringing/overshoot. This has the opposite effect in the feedback of an op-amp circuit due to a pole in the feedback loop being shifted to a lower frequency and potentially destabilizing the amplifier. Tread cautiously.
I would also likely use a digital gate. The issue here isn't usually prop-delay, but gate-to-gate phase shift which will be low (sub-ns) if using a single chip. A phase shift from the source to the gate output usually doesn't matter.
If you haven't already realized, you'll need to use transmission lines if you need to keep 800MHz bandwidth and signals are traveling more than ~1".
A comparator may also be a good solution... not an opamp.
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by Jake Hertz
by Jake Hertz