Finally, we have found the correct calibration of Guralp and STS2 seismometers.
ADC: 216counts = 20V Hence, calibration of ADC is 3.2768e+03 counts/V.
Sensitivity of seismometer = 800 V/ms-1
Gain of the guralp breakout box (reference elog entry) = 20
Calibration = 3.2768e+03 counts/V x 800 V/ms-1 x 20 = 52428800 counts/ms-1 -----> 1.9073e-08 ms-1/count
Sensitivity = 1500 V/ms-1
Gain of the STS electronic breakout box = 10
Calibration = 3.2768e+03 counts/V x 1500 V/ms-1 x 10 = 49152000 counts/ms-1 -----> 2.0345e-08 ms-1/count