Chapter 4 Analog InputM Series User Manual 4-8 ni.comADC. The specifications document for your DAQ device lists its settlingtime.M Series devices are designed to have fast settling times. However, severalfactors can increase the settling time which decreases the accuracy of yourmeasurements. 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 NecessaryUse Low Impedance SourcesTo ensure fast settling times, your signal sources should have an impedanceof <1 kΩ. Large source impedances increase the settling time of theNI-PGIA, and so decrease the accuracy at fast scanning rates.Settling times increase when scanning high-impedance signals due to aphenomenon called charge injection. Multiplexers contain switches,usually made of switched capacitors. When one of the channels, forexample channel 0, is selected in a multiplexer, those capacitorsaccumulate charge. When the next channel, for example channel 1, isselected, the accumulated charge leaks backward through channel 1. If theoutput impedance of the source connected to channel 1 is high enough, theresulting reading of channel 1 can be partially affected by the voltage onchannel 0. This effect is referred to as ghosting.If your source impedance is high, you can decrease the scan rate to allowthe NI-PGIA more time to settle. Another option is to use a voltage followercircuit external to your DAQ device to decrease the impedance seen by theDAQ device. Refer to the KnowledgeBase document, How Do I Create aBuffer 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 CablingUsing short high-quality cables can minimize several effects that degradeaccuracy including crosstalk, transmission line effects, and noise. Thecapacitance 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 Input Signals section for more information.
