Adding an anti-aliasing filter to op-amp before ADC
It is always good practices to use anti-aliasing filter before digitizing a signal. Although your target signal does not contain frequency components above the Nyquist rate, there might be other sources of noise which do. 

First of all you need to decide which bandwidth you want to cover. If your ADC samples at 75kHz, then there should not be any frequencies above 37.5kHz. Next, we calculate the needed attenuation and order of your anti-aliasing filter. For this consider following figure:



This figure presents two cases one with a sampling rate fs and one with K * fs. Due to the sampling of the input signal (digital mixing), all frequency components higher than fs/2 will be "folded" back. Frequency components higher than fs-fa will then be aliased into the signal of interest (red).
In figure (A), we assume you want to sample a signal with a bandwidth (fa) close to the Nyquist rate (fs/2). 
To guarantee a certain dynamic range (DR) we need a steep roll-off e.g. a high filter oder which attenuates any noise with frequencies higher than fs-fa. In figure (B) we use a higher sampling rate (K * fs) which relaxes the required order of the filter and simplifies circuit design.  

As you mentioned, your ADC has a resolution of 13dB. Your ideal SNR (Signal to Noise Ratio) or in this case your DR is then:

$$SNR=N \cdot 6.02 + 1.76[dB] = 80dB $$ 

So, in the ideal case your want an attenuation of at least 80dB at fs-fa. A basic first order low-pass filter has an attenuation of 20dB/dec. If you restrict your signal bandwidth to say 20kHz, your ideal sampling frequency lies then at 200MHz. 

$$f_{-80dB} = f_a \cdot 10^{\frac{80dB}{20dB}} = 200MHz$$

To satisfy this restriction with your sampling rate of 75kHz you would need an low-pass filter 8th order. This is certainly a lot but all this calculations assume noise equal in amplitude as your signal of interest. In practice a second or third order filter is most likely sufficient. 

For additional information see:
W. Kester, Data conversion handbook: Analog devices. Amsterdam u.a.: Elsevier Newnes, 2005.