I am measuring AC signal using an instrument amplifier AD623. This will be fed to an ADC in the 0/+5V range. Therefore the signal must be centred on 2.5V. A voltage divider will create the 2.5V using two 10k resistors.
Do I need to buffer the output of the divider with a follower circuit?
The impedance of Vref is 100k (AD623 datasheet). Is this large enough compared to the 10k?
If you want the output voltage of the Instrumentation Amplifier to be centered around VCC/2 as precisely as possible then yes, you will need to buffer the VREF voltage.
If you care less about the exact level, for example, you calibrate that level in the ADC (apply zero input, measure output of ADC, store that value and use it as input = zero). Then sure, you can leave out the buffer.
Is this large enough compared to the 10k?
As explained above, there will be an error if the impedance of your VREF source is not significantly less than 100 kohm. 10 kohm is a factor 10 less so the error will be clearly present. If you want a smaller error, use a buffer. It is just a question of what you want/can tolerate.
You don't give any indication of what level of accuracy you are trying to achieve.
Adding resistance to the reference input will degrade the common mode rejection by the ratio of 50K/Rin. Your 10K/10K divider has an output resistance of 5K ohms, so it would reduce the CMRR from perhaps 100dB to 20dB or about 10,000 times worse. Even a 1\$\Omega\$ source resistance will significantly degrade the CMRR!
So, most likely, it's important to buffer your reference. If you are looking for good DC accuracy you may wish to use a better op-amp than an LM358- the output voltage of the instrumentation amplifier will change directly with the reference input (gain = 1). However since you are using Vcc to generate the reference (and presumably also the ADC reference) I assume you are not very concerned about accuracy.
Not part of your question, but you will need to keep the I+ and I- terminals within the common mode range of the amplifier or it will not work properly. See the data sheet for more information.
