VTI Instruments EX1401, User Manual

Page: 99 / 114

www.vtiinstruments.com
EX1401 Theory of Operation 99
S
ECTION
7
T
HEORY OF
O
PERATION
I
NTRODUCTION
The block diagram in Figure 7-1 illustrates the key components of the analog circuitry. Each of the
main blocks is described below.
F
IGURE
7-1: EX1401 A
NALOG
C
IRCUITRY
B
LOCK
D
IAGRAM
(
PER CHANNEL
)
V
OLTAGE
/T
HERMOCOUPLE
I
NPUT
Each input channel has a type “U” (universal, copper) miniature thermocouple connector that is
used for both temperature and voltage measurement. Each connector has a dedicated thermistor
mounted internally for cold junction compensation. Good mechanical design insures that the CJC
is thermally isolated from the active electronics and maintained close to ambient temperature to
ensure best in class performance.
Each input is protected with ESD clamping circuits that present a small capacitance to earth ground
(i.e., the metal enclosure, which is also grounded to your local earth ground by the ground cable
included.) There are also gas discharge tubes on both inputs that will arc (short to ground) for any
common mode voltage greater than 600V. Each input can have up to +/-500V common mode
working voltage with respect to earth ground.
Each input is protected from a sustained, normal mode overvoltage condition, as long as the
overvoltage does not /- 100V between the two inputs of each connector. This is done with
a series resistance in each input signal along with clamping diodes. The diodes clamp outside the