Service manual

PT300, Service Manual
Rev B, October, 2000
12
Analog Section:
The analog section takes the signal from the load cells (about 10 mV) and translates
this signal into a digital representation. In our case this representation is a sixteen bit
number in a counter register.
Instrumentation Amplifier:
The signal from the load cells is about 10 mV at full load. U18 takes this signal
and amplifies it for use by the A/D converter. R21, R22, C14, and C15 filter out
RFI interference and provide some limited protection from electrostatic
discharge. The total resistance between pins 2, 3, 6, 7, and 4 and 5 select the
gain of the amplifier. Some of the amplifier's built-in resistors are used to
increase the gain stability, hence the busy connections between U18 and the
gain selection resistors. R24 and potentiometer R24 are the gain selection
resistors. In the revision 'C' and later boards R52, R53, and jumpers JP1-JP3
allow jumper gain selection. R25, R26, C10, and C11 provide low pass
frequency filtering.
Analog/Digital Converter:
U17 is a collection of integrator, comparator and analog switches. With U12,
U13, and U14 this forms a dual slope integrating analog to digital converter.
The heart of the A/D converter is R17, C7, and an integrator and comparator
internal to U17. Please examine figure 4 below for the configuration of these
parts.
+
-
+
-
C7
R17
Vin
Vref
Comparitor
Integrator
Figure 4 Integrator/Comparitor