I am using multiple LP2950s in the same circuit. Can the Cin capacitor be the same for all of them (the same physical device)? I wanted to reduce parts count by using the same physical capacitor for all three LP2950s for Cin. Is that ok to do?
As stated in the datasheet, a 1 uF capacitor is recommended to be used locally, meaning located nearby the regulator. Here you can take "nearby" as around 1 cm away as a rule of thumb. Hence, if the regulators are located far away from each other, you should use separate capacitors for each regulator. Otherwise, a single capacitor would be sufficient since the impedance is low between the capacitor and the regulator.
The regulator does have a common emitter current source to drive the regulated voltage but that base current comes from an Op Amp which have excellent at PSRR but declining towards BW limit.
This is also a low current and a linear LDO and thus both criteria make this part non-critical to input Cap ESR. This makes the design less critical for the location of Cin but if measuring AC output noise that must be 60 dB down and you have step load currents, then a local input cap improves performance.
Not convinced Cin needs to be close.
But the input current slew rate can amplify input voltage noise with long line inductance. They suggest 10 " is long which at 10nH/cm is ~ 250nH
e.g Est. step load input spike dV = L dI/dt . 250nH * 200mA/1us = 50mV
Yet a 1uF std Alum Cap has an ESR*C = Tau of 200us or ESR=200 ohms compared to SMPS that often need ESR of 10 mohm for Cin.
Thus I really wonder what criteria was used here.
A 200mA step load with an inductive spike of 50mV will not be suppressed using a 1uF cap with 200 Ohm ESR no matter how close it is.
Depending on your output noise specs and step load current, you can do better with 1uF shared and 0.01uF near each input. I suggest if your application is critical, verify the AC noise performance. But if your Vin input is very noisy with Rf TX bursts and motor noise, then use a low ESR shared cap near the noise source , rather than don't care on ESR cap near the LDO.