National Instruments NI 6232 - Manual (Page 34)

Chapter 4 Analog Input
© National Instruments Corporation 4-7 NI 6232/6233 User Manual
Settling times increase when scanning high-impedance signals due to a
phenomenon called charge injection. Multiplexers contain switches,
usually made of switched capacitors. When one of the channels, for
example channel 0, is selected in a multiplexer, those capacitors
accumulate charge. When the next channel, for example channel 1, is
selected, the accumulated charge leaks backward through channel 1. If the
output impedance of the source connected to channel 1 is high enough, the
resulting reading of channel 1 can be partially affected by the voltage on
channel 0. This effect is referred to as ghosting.
If your source impedance is high, you can decrease the scan rate to allow
the PGIA more time to settle. Another option is to use a voltage follower
circuit external to your DAQ device to decrease the impedance seen by the
DAQ device. Refer to the KnowledgeBase document, How Do I Create a
Buffer to Decrease the Source Impedance of My Analog Input Signal?, by
going to
ni.com/info and entering the info code rdbbis .
Use Short High-Quality Cabling
Using short high-quality cables can minimize several effects that degrade
accuracy including crosstalk, transmission line effects, and noise. The
capacitance of the cable also can increase the settling time.
National Instruments recommends using individually shielded,
twisted-pair wires that are 2 m or less to connect AI signals to the device.
Refer to the
Connecting Analog Voltage Input Signals section for more
information.
Carefully Choose the Channel Scanning Order
Avoid Switching from a Large to a Small Input Range
Switching from a channel with a large input range to a channel with a small
input range can greatly increase the settling time.
Suppose a 4 V signal is connected to channel 0 and a 1 mV signal is
connected to channel 1. The input range for channel 0 is –10 V to 10 V and
the input range of channel 1 is –200 mV to 200 mV.
When the multiplexer switches from channel 0 to channel 1, the input to the
PGIA switches from 4 V to 1 mV. The approximately 4 V step from 4 V to
1 mV is 1,000% of the new full-scale range. For a 16-bit device to settle
within 0.0015% (15 ppm or 1 LSB) of the ±200 mV full-scale range on
channel 1, the input circuitry must settle to within 0.000031% (0.31 ppm or