5
5
SCHEMATIC & FUNCTIONAL DESCRIPTION
SCHEMATIC & FUNCTIONAL DESCRIPTION
The idea of this pre-amplifier is to enable oscilloscopes with low resolution to measure smaller
voltages by amplification of the original signal. Within the output range of the pre-amplifier the
original signal can be amplified in steps of 10 up to x1000 times. Additionally, a low-pass is
implemented, where several corner frequencies can be selected. If two similar signals need to be
compared, the pre-amplifier allows subtraction of the two signals at equal pre-amplification
factors.
As shown in Fig. 2, the coupling of each input +IN and -IN can be selected between AC, GND and
DC coupling. A fuse protects the input of the 1
st
operational amplifier against too high currents. In
case of a x1000 amplification, this fuse is bridged.
For single ended signals, the not used input needs to be switched to GND. An overload after the
first amplifier stage is detected for each input separately and displayed by a red LED. In order to
adjust for too large signal differences bIN and -IN in the higher amplifications (x100 and
x1000), an offset can be added manually with a 10-turn potentiometer onto the pre-amplified -IN
signal prior to the differential amplifier. The selectable bandwidth filters are located directly at the
output.
6
6
REVISION
REVISION
HISTORY
HISTORY
2020, the specifications on page 4ff. have been loosened in order to achieve a more robust long
term compliance of the same.
Manual AFT400B – Rev. 1.02 dated 30/09/20 Page 9 (10)
Figure 2: Schematic diagram of the AFT400B low noise differential amplifier.
