MKS Baratron 628A - How to Use the External Zero on the Type 629 Unit; Adjusting the External Zero; Rejecting Common-Mode Voltages; Figure 8: Using Pin 7 as an External Zero Adjustment

Chapter Four: Operation How to Use the External Zero on the T
yp
e 629 Unit
37
How to Use the External Zero on the Type 629 Unit
The Type 629 transducer output is fully differential with respect to the voltage input to the
external zero (pin 7). Pin 7 allows you to connect the unit to reject common-mode voltages
between a power supply and a readout, and/or add an external zero adjustment.
Adjusting the External Zero
The external zero pin (pin 7) may be used as an external zero adjustment by summing in a
correction voltage that is equal in magnitude and opposite in polarity to the transducer’s zero
offset, at a pressure below the resolution of the Type 629 transducer. Table 6, page 26, lists the
correct pressure for each range. Use a cold cathode gauge or other high vacuum gauge capable of
reading pressures below the transducer’s resolution, to verify the pressure
before
adjusting the
zero.
±
1
7
8
Type 629
Voltage at
zero pressure
∑
0 V
- V1
+
-
V
out
V1
+
+
±
Figure 8: Using Pin 7 as an External Zero Adjustment
Note
If the external zero pin is not being used, connect it to Signal Common
(Pin 8).
Rejecting Common-Mode Voltages
The 629 transducer can be connected to reject common-mode voltages between the power supply
and readout. This arrangement minimizes the ground loops that sometimes occur when using
remote data acquisition systems and bussed power. Figure 9, page 38, shows several Type 629
transducers connected to a data acquisition system.