Recommend an amplifier with lower noise at low frequency to replace OPA2348

You might look at the OPA2834, not RRin but much lower 1/f corner and better CMRR if the input is in range. 

Hi Wendy,

You can also consider the TLV2314 and possibly the TLV2313.  They should meet your requirements.  Click here for a quick comparison.

With regards to op amp speed, this will depend also on your gain.  Generally speaking, it is a good rule of thumb to leave 2 orders of magnitude in margin for the highest frequency you would like to sample.  For example, if the highest signal you will sample is 1kHz with a gain of 10V/V, then you will want an op amp with a bandwidth of at least 1MHz.  If the op amp will be placed in 1V/V buffer, then your op amp will be ok with signals of 10kHz.

Please let me know if you have any further questions.

Regards,
Daniel

Hi Wendy,

I wanted to add a little bit to my response to avoid any possible confusion.

To get a quick idea of the op amp speed you need, take the highest signal frequency of interest, find the gain in dB, and make sure that there is 40dB of margin between that point and the open-loop gain of the op amp at that frequency.  This is explained in this app note (click here) and is summarized at the bottom of page 7.

This is essentially the same comment as what I made before.  But, I wanted to clarify in case my previous comment might not be clear enough.

Regards,
Daniel