National Instruments AT-MIO-16D, User Manual, Page: 51 / 291

Configuration and Installation Chapter 2
AT-MIO-16D User Manual 2-30 © National Instruments Corporation
A resistor from each input to ground, as shown in Figure 2-28, provides bias current return paths
for an AC-coupled input signal. This solution, although necessary for AC-coupled signals, lowers
the input impedance of the analog input channel. In addition, the input offset current of the
instrumentation amplifier contributes a DC offset voltage at the input. The amplifier has a
maximum input offset current of
±
15 nA and a typical offset current drift of
±
20 pA/
°
C.
Multiplied by the 100-k
Ω
resistor, this current contributes a maximum offset voltage of 1.5 mV
and a typical offset voltage drift of 2
µ
V/
°
C at the input. Keep this in mind when you observe DC
offsets with AC-coupled inputs.
If the input signal is DC-coupled, then you only need the resistor connecting the negative (-) signal
input to ground. This connection does not lower the input impedance of the analog input channel.
Single-Ended Connection Considerations
Single-ended connections are those in which all AT-MIO-16D analog input signals are referenced
to one common ground. The input signals are tied to the positive (+) input of the instrumentation
amplifier, and their common ground point is tied to the negative (-) input of the instrumentation
amplifier.
When the AT-MIO-16D is configured for single-ended input (NRSE or RSE), 16 analog input
channels are available. You can use single-ended input connections when the following criteria are
met by all input signals:
1. Input signals are high-level (greater than 1 V).
2. Leads connecting the signals to the AT-MIO-16D are less than 15 ft.
3. All input signals share a common reference signal (at the source).
If any of the above criteria is not met, using DIFF input configuration is recommended.
You can jumper configure the AT-MIO-16D for two different types of single-ended connections–
RSE configuration and NRSE configuration. Use the RSE configuration for floating signal
sources; in this case, the AT-MIO-16D provides the reference ground point for the external signal.
Use the NRSE configuration for ground-referenced signal sources; in this case, the external signal
supplies its own reference ground point and the AT-MIO-16D should not supply one.
Single-Ended Connections for Floating Signal Sources (RSE Configuration)
Figure 2-29 shows how to connect a floating signal source to an AT-MIO-16D board configured
for single-ended input. You must configure the AT-MIO-16D analog input circuitry for RSE
input to make these types of connections. Configuration instructions are included under the
Analog Input Configuration section earlier in this chapter.