Connecting Signals
4-9
Each DAS-TC/B board provides sixteen differential inputs. These inputs
are designed especially for thermocouple signals, but are also suitable for
general-purpose analog signals. Signals from all sixteen inputs are
amplified by one input amplifier, an instrumentation amplifier, and are
digitized by one voltage-to-frequency (V/F) converter. The multiplexer
allows a time-sharing arrangement, in which inputs are scanned and
connected intermittently to the input amplifier and V/F converter. The
multiplexer is effectively a solid-state 16-pole, double-throw switch.
The entire input section of the DAS-TC/B is isolated to reject most
unwanted signals from your measurements. Refer to “Isolated Input
Section” on page 2-3 and see Figure 2-1. Additionally, the differential
High and Low input terminals of the input amplifier reject all but a tiny
fraction of the common mode voltage—the voltage that they “see” in
common relative to the analog ground (B GND). The High and Low input
terminals respond almost exclusively to the voltage difference between
them. This limits response to noise, power supply voltages, and other
unwanted signals that are common to the terminals within the common
mode rejection ratio (CMMR) of the amplifier.
Note:
The CMRR, when expressed in decibels, may be defined as shown
below:
However, the common mode voltage must be within specified limits. If no
resistance path is defined between the Low input terminal and the analog
ground (B GND), amplifier bias currents attempt to flow to the analog
ground across a high insulation resistance, driving the common-mode
voltage close to the positive supply voltage of the amplifier. The amplifier
saturates and data measurements are unusable.
CMRR = 20 log
10
Common mode voltage
Common mode error, the part of
common mode voltage not rejected
