How to increase sensitivity of audio-frequency pickup coil
I just homed-in on your detection-limit problem for AC magnetic fields in the higher audio frequency range. I would be highly interested to maybe add some useful suggestions, but before doing so (and say just stupid things) may I ask a few short questions.


Am I right to see from the picture showing your coil with added numbers of 2.8 Ohm and 290 uH, that the underlying paper relates to NMR-type of experiments?
Why are you so keen to use a very low DC (!) drift opamp like the ADA4528? For me 'only' AC properties matter here.
Added to that: The ADA4528 is such low DC drift because it employs a chopper stabilizing scheme, running on an internal ~200 kHz oscillator.  This is approximately the worst thing one can choose if AC detection is prime, as many interference products will turn up like Nxf_chop -/+ Mxf_sign.


Please look up the application note of the opamp, denoted as AN-1114: this could be very helpful.



Thanks for replying.


Please notify my concern of the chopper 'rattler' contained in the ADA4528, and get rid of it a.s.a.p. In that respect I forgot to mention your beautiful adstruction to the interference issue: the FFT spectrum between 26020 and 26060, showing a nice regular beating of ~2Hz. I bet this is chopper-induced!  So my first suggestion is to change to an opamp like LT1115 (powered by two 9V batteries), with &lt; 2 nV/sqrtHz and &lt; 1 pA/sqrtHz, starting from the adapted scheme which gave you 24 kHz res. and 100 pT/sqrtHz sensitivity. 
Secondly, please also add (as mentioned by others) a non-shorting Cu-foil sheet screen.
Further you may remove quite some components in the latest scheme, i.e. R3 and R4, as well as R9 and R10 as bias is provided via the feedback resistors R7 and R11. Also capacitor C1 can be removed, as C7 and C9 ensure the DC isolation. All this should boost your system considerably. 


Regards, and hope to hear from you.