National Instruments NI USB-621x - Manual (Page 32)

Chapter 4 Analog Input
NI USB-621x User Manual 4-6 ni.com
Multichannel Scanning Considerations
M Series devices can scan multiple channels at high rates and digitize the
signals accurately. However, you should consider several issues when
designing your measurement system to ensure the high accuracy of your
measurements.
In multichannel scanning applications, accuracy is affected by settling
time. When your NI 621x device switches from one AI channel to another
AI channel, the device configures the NI-PGIA with the input range of the
new channel. The NI-PGIA then amplifies the input signal with the gain for
the new input range. Settling time refers to the time it takes the NI-PGIA to
amplify the input signal to the desired accuracy before it is sampled by the
ADC. The specifications document for your DAQ device lists its settling
time.
NI 621x devices are designed to have fast settling times. However, several
factors can increase the settling time which decreases the accuracy of your
measurements. To ensure fast settling times, you should do the following
(in order of importance):
• Use low impedance sources
• Use short high-quality cabling
• Carefully choose the channel scanning order
• Avoid scanning faster than necessary
The following sections contain more information about these factors.
Use Low Impedance Sources
To ensure fast settling times, your signal sources should have an impedance
of <1 k
Ω. Large source impedances increase the settling time of the
NI-PGIA, and so decrease the accuracy at fast scanning rates.
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.