Constant Meter Factor (Cmf) And Buffer Pc Assembly - ABB CK-MAG 10D1477 Instruction Manual

10D1477 MAGNETIC FLOWMETER INSTRUCTION MANUAL

5.2 Constant Meter Factor (CMF) and Buffer PC Assembly

The CMF Assembly provides several functions. These include:
1. Establishes interconnections between the primary internal wiring and the secondary.
2. Permits adjustment of meter calibration factors to a fixed value for each nominal size pri-
mary.
3. Permits adjustment of primary zero.
4. Establishes proper wiring connections for integrally or remotely mounted Converters.
5. Buffers the capacitively generated electrode signals.
A simplified circuit schematic of the primary CMF/BUFFER assembly is shown in Figure 5-1. The
meter calibration factor is established by the selectable voltage division of precision binary weighted
resistors R1 through R14 applied to coil current sensing resistor Rx.
The capacitive electrodes are buffered by U1 and U2. These amplifiers both employ positive feed-
back "bootstrapping" to raise their input impedance. Their outputs provide signal guarding to offset
the effects of capacitive loading of the electrodes from internal cabling and amplifier input capaci-
tance. Amplifier outputs are also connected to driven guard "cans" placed on top and bottom of each.
System zero is provided by amplifier U4, which sums a portion of the sensed coil current with the U2
buffer amplifier signal. The outputs of U4A and U1 are directed to the signal inputs of the converter.
The reference voltage at W1-7 is also connected to the signal converter. System circuit common is
W1-13, and the coils are operated through W1-10 and W1-9.
Connections to the CMF/BUFFER assembly are made via the 16 pin receptacle W1 located near the
edge of the board. If the converter is to be remotely mounted, an adaptor plug is supplied to provide
proper signal connections at the customer terminal box. For integrally mounted converters, a second
type of adaptor plug is supplied which is characterized for the desired signal converter output
options.
5-2