2-6
Functional Description
Low-Side Reference Selection for Single-Ended Inputs
When you use single-ended inputs, you have two ways of connecting the
low side of the amplifier: the analog ground (default) and a user-defined
common mode. The two schemes differ in how the low side of the
instrumentation amplifier is connected. In the default mode, the low side
of the amplifier is connected to analog ground (LL GND). In the
user-defined common mode, the low side of the amplifier is connected to
a pin on the connector for user-defined common mode (U_CM MD).
The user-defined common mode provides a means for eliminating ground
loops in the system by connecting the reference ground for inputs to the
U_CM MD input pin. Since the U_CM MD connection connects to the
high input impedance of the instrumentation amplifier, the signal contains
no power-supply return current.
The user-defined common mode also provides a means for making
single-ended measurements of signals referred to a voltage that is not
ground or whose output range does not include ground. For example, a
common way to perform 4 to 20mA current monitoring is to connect a
loop with a 250
Ω
resistor to ground; the resistor yields a 1 to 5V output in
this current range. This method works but uses only 80% of the input
range when connected to a 0 to 5V range. A better way is to use a 312.5
Ω
resistor and refer all measurements to 1.25V. The actual output voltage
then ranges from 1.25V to 6.25V; however, since the amplifier low side is
connected to 1.25V, the measurement range is now a span of 5V, making
the entire input range available and increasing resolution of the
measurements by 20%.
If you use single-ended input configurations, the user-defined common
mode is the recommended alternative. Use the default mode only if you
want the convenience of not having to connect a separate wire for low
input.
Unipolar/Bipolar Selection
Using DriverLINX, you can set the DAS-1800ST/HR Series boards to
operate in either unipolar or bipolar input mode (see “DriverLINX
Configuration Notes” on page C-1). A unipolar signal is always positive
(0 to 5V, for example), while a bipolar signal can swing up and down
between positive and negative peak values (±5V, for example).
Artisan Scientific - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisan-scientific.com
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com