for a spec related questions a look in the spec is often very helpful:
In the datasheet of the LT1638 we state on page 1 following:
This should answer your question, correct?
Is the output impedance versus frequency plot on the LT1638 data sheet under open loop or closed loop conditions?
Help me find an analog to the LT1638 low noise operational amplifier. I'm not an electronics engineer, I don't understand the basic parameters of an op amp.