Olympic 50 - Detailed Circuit Description; Load Cells; Instrumentation Amplifier

Detailed Circuit Description
Load Cells
The wiring diagram, Fig. 4-15, shows the load cell connec-
tions. Inside the load cells there are temperature com-
pensation networks which are not shown.
The capacity of the Scale is 25 kilograms. The sensitivity
ofeach load cellis 2mV per volt at 30 kg. The output signal
of one separate load cell is 2mV per volt of excitation at 30
kg.
The load cells are connected in parallel through a balanc-
ing pot, R9. The net effect is an output of 148.5 nV per
gram (with 5.0 Volts excitation). Since this is such a small
signal, a very stable amplifier is needed.
1
Le
Instrumentation Amplifier
Refer to the Main Board schematics, Fig. 4-16, 4-17 and
4-18.
U7 and U8 form a differential chopper stabilized instru-
mentation amplifier. The gain is fixed by R18, R19 and R20
at 328.3. These are special precision resistors whose
temperature coefficients are matched to within 3 parts per
million per degree C. The input to the amplifier is guarded
at the common mode voltage of about +2.5 V provided by
US.
U4 is a high accuracy unity gain amplifier. It changes the
differential input to a single ended output. Italso sums in
the zero offset voltage. Zero offset is provided from R5, R6
and U1. A copper electrostatic shield is situated over this
very sensitive analog section. The signal from U4 output
is fed through either filter to the input of the A/D converter,
U11. Analog switches of 010 are used to select the
appropriate filter.
Fig. 4-14 Detailed Block Diagram
SS50-N1-220V(B09)
Page 4-8