Working Voltage Range - National Instruments NI 6115 User Manual

Working Voltage Range

Note All inputs are protected at up to ±42 V.
© National Instruments Corporation
Like any amplifier, the common-mode rejection ratio (CMRR) of the PGIA
is limited at high frequency. This limitation has been compensated for in the
design of the NI 6115/6120 by using a common-mode choke on each
channel.
♦
NI 6115
The purpose of the 10 nF capacitance on the ACH<0..3>– connection of the
NI 6115 is to provide an impedance for this choke to work against at high
frequency, thus improving the high-frequency CMRR. Depending on your
application and the type of common noise at your source, further
common-noise rejection might be gained by placing a 0.1 µF ceramic
bypass capacitor between ACH– and ACH0GND.
The PGIA operates normally by amplifying signals of interest while
rejecting common-mode signals as long as the following three conditions
are met:
1. The common-mode voltage (V
ACH<0..3>GND from ACH<0..3>– and which is shown in Figure 4-2,
must be less than ±2.5 V. This V
2. The signal voltage (V
ACH<0..3>– from ACH<0..3>+ and which is shown in Figure 4-2,
must be less than or equal to the range selection of the given channel.
If V is greater than the range selected, the signal clips and information
s
is lost.
3. The total working voltage of the positive input, which can be thought
of as (V + V
cm
ACH<0..3>+, must be less than ±11 V for ranges ≤ ±10 V or less than
±42 V for ranges > ±10 V.
If any of these conditions are exceeded, current limiters limit the input
current to 20 mA maximum into any input until the fault condition is
removed.
), which is equivalent to subtracting
cm
is a constant for all range selections.
cm
), which is equivalent to subtracting
s
) or simply as subtracting ACH<0..3>GND from
s
4-13
Chapter 4 Connecting Signals
NI 6115/6120 User Manual