VTI Instruments EX1000A, User Manual

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VTI Instruments Corp.
280 EX10xxA Theory of Operation
D-sub Connector Inputs
Each D-sub connector provides sixteen input channels which are grouped around the perimeter of
the connector (see Table 2-3). Two pins on these connectors are allocated to the connection of an
thermistor, typically used with an external cold plate. The center row of each D-sub is connected to
chassis. When using shielded test wires or thermocouples, these pins allow the shield to be
connected to chassis using very short wires.
S
IGNAL
C
ONDITIONING
C
IRCUITRY
Each input channel is conditioned by the block labeled “Per Channel Circuitry” in Figure 7-1. The
input protection circuitry ensures that only a safe level of current will flow during over-voltage
conditions.
Open transducer detection (OTD) is provided by nanoamp-level current sources (~5 nA to 8 nA) on
each input path. These current sources are small enough not to affect measurement accuracy, but
large enough to drive the high-impedance input of the instrumentation amplifier (IA)
deterministically into saturation in the event of an open condition. When enabled, they provide
continuous monitoring of the input and will generate an open indication event when the open is
intermittent in nature. Moreover, these current sources are provided on both inputs so that an open
condition is registered even in the case where only one lead of the input is open and the other is
connected to earth ground. An important feature of the OTD is that the current sources can be
enabled or disabled on a per channel basis. This permits the user to choose which of their input
signals require open circuit detection.
The core of this block is the IA that provides amplification of the differential input signal and
rejection of common mode input signals. The IA provides excellent CMRR (common mode
rejection ratio), especially at dc and (50/60) Hz, but its rejection characteristic decreases as the
interference frequency increases. Moreover, IAs have the tendency to shift their dc offset in the
presence of very high frequency signals. This effect would be particularly problematic, as the filters
that follow the IA would not attenuate it. However, the presence of the differential and common
mode filter preceding the IA attenuates high frequency interference before it reaches the IA. This
decreases the possibility of dc rectification and provides the system with excellent CMRR
characteristics at all frequencies.
For optimum noise performance, the IA output is routed through one of six user selectable filters.
Five of the six filters are two-pole, Bessel response (constant delay) with cutoff frequencies of 4 Hz,
15 Hz, 40 Hz, 100 Hz and 500 Hz. The sixth filter is a single pole, Butterworth response, with a
1000 Hz cutoff frequency. Each channel may have any one of the six filters applied to its signal
path. These filters form a powerful application tool, as it allows the bandwidth of the signal
conditioning path to be matched to the characteristics of the attached sensor. Of particular note, the
filters used are continuously connected.
Contrasting approaches utilize one filter whose characteristics are modified based on user selection.
The disadvantage to that approach is that there is an inherent latency when the filter changes state
before the data is valid. This requires the user to impose a potentially long delay in their
measurement system. The per channel filter approach used in the EX10xxA has no such
characteristic and requires no delay. Figure 7-2 shows the typical CMRR data for the EX10xxA at
each filter setting (4 Hz, 15 Hz, 40 Hz, 100 Hz, 500 Hz, to 1000 Hz) when the common mode input
is 20.118 V p-p, 60 Hz.