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Lena/Oct 2016: Difference between revisions
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[[File:20161024LargeImbalance.png|300px]]
== 10/26/2016 ==
Measured a pregnant patient in WIMR. Although all channels worked properly and probe sensitivity was 1 fT/sqrt(Hz), we didn't get any good data, and couldn't see the fMCG signal at all. Maybe there were large field gradients that we didn't properly get rid of, like from wires in a bra or other clothes. The patient returns on November, 14th.
Added a real-time fMCG preview into the FPGA so that we don't end up in this situation again.
== 10/27/2016 ==
Implemented delta-sigma PDM (pulse-density modulation) for the FPGA. Now the FPGA can output "analog" control signals from the digital outputs, which our FPGA has plenty of. This would allow us to control every sensor shell coil on all three axis, and permit us to feedback on the field to keep all the sensors in average zero field and thus extend the dynamic range of our measurements. The signal to noise ratio in 0-1 kHz range is surprisingly good, and is around 100 dB. The VI is configured to output the "analog" levels with 16 bit precision. The switching rate is 20 MHz, with the CLK output.
We want to drive the atoms with the high-frequency signal and have them act as the low-pass filter. Mike built a current supply for the coils that reverses the polarity of the coil current for 0 and 1 logical levels.
It would be great to replace the analog inputs with digital signals as well. This would permit us to use a bigger FPGA (NI PXIe-7971R) that could drive as many as 25 sensors. Texas Instruments has Verified Designs section, that explains how to design and layout ADC circuits. [[File:TI ADC design ECG.pdf]]
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