# [FAQ] Instrumentation

## Questions

How do the units on the B&K 2635 charge amplifier workThe knobs on the front of the B&K 2635 charge amplifer are not intuitive. Here is how to calculate the effective calibration factor in units of V / Engineering Units,`Sp = ( A * 0.001 * B ) / C`

where

• `A` is the sensitivity selected on the dial in mV / Unit Out
• `B` is the exponent which is the multiplier between the actual sensitivity of the transducer and the sensitivity that was entered on the three dials on the top of the charge amplifier
• `C` is the Unit out selected with the Acc – Vel – Disp knob.

For example, if you have a transducer with a sensitivity of 67.2 pC / m/s^2. You enter a sensitivity on the charge amplifier of 0.672 (with the switch on the left hand side in the up position indicating 0.1 – 1). The gain dial is set to 1000 mV / Unit Out. You are measuring acceleration and the units out are 1 m/s^2 and high pass of 0.2Hz. Entering this data into the equation for `Sp` yields

`A = 1000` (gain setting on the mV/UO knob)
`B = 100` (exponent for sensitivity of transducer)
`C = 1` (Units Out setting from lower left knob)

Hence

`Sp = 1000 x 10^(-3) x 100 / 1`
`Sp = 100`

The level in engineering units is obtained by dividing the voltage by Sp

`EU Level = (Measured Volts) / Sp`