Chapter 2 Analog Input© National Instruments Corporation 2-11 E Series User ManualUse Short High-Quality CablingUsing short high-quality cables can decrease several effects that decreaseaccuracy including crosstalk, transmission line effects, and noise. Thecapacitance of the cable can also effectively 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.Carefully Choose the Channel Scanning OrderAvoid Switching from a Large to a Small Input RangeSwitching from a channel with a large input range to a channel with a smallinput range can greatly increase the settling time.Suppose a 4 V signal is connected to channel 0 and a 1 mV signal isconnected to channel 1. The input range for channel 0 is 0–10 V and theinput range of channel 1 is 0–100 mV.When the multiplexer switches from channel 0 to channel 1, the input to thePGIA switches from 4 V to 1 mV. The approximately 4 V step from 4 V to1 mV is 4,000% of the new full-scale range. For a 12-bit device to settlewithin 0.012% (120 ppm or 1/2 LSB) of the 100 mV full-scale range onchannel 1, the input circuitry must settle to within 0.0003% (3 ppm or1/80 LSB) of the 4 V step. Some devices can take as long as 100 μs for thecircuitry to settle this much.To avoid this effect, you should arrange your channel scanning order so thattransitions from large to small input ranges are infrequent. Another usefultechnique is to insert a grounded channel between signal channels.In general, you do not need this extra settling time when the PGIA isswitching from a small input range to a larger input range.Insert Grounded Channel between Signal ChannelsAnother technique to improve settling time is to connect an input channelto ground. Then insert this channel in the scan list between two of yoursignal channels. The input range of the grounded channel should match theinput range of the signal after the grounded channel in the scan list.Consider again the example above where a 4 V signal is connected tochannel 0 and a 1 mV signal is connected to channel 1. The input range forchannel 0 is 0–10 V and the input range of channel 1 is 0–100 mV.
