The USB-6421 can scan multiple channels at high rates and digitize the signals accurately.

Settling time affects accuracy in dual scanning applications. 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.

When the USB-6421 switches from one AI channel to another AI channel, the USB-6421 configures the NI programmable gain instrumentation amplifier (NI-PGIA) at the same range only. The NI-PGIA then amplifies the input signal with the same gain.

The USB-6421 is 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, do the following (in order of importance):

  1. Use low-impedance sources.
  2. Use short, high-quality cabling.
  3. Carefully choose the channel scanning order.
  4. Avoid scanning faster than necessary.

Use Low-Impedance Sources

Your signal sources should have an impedance of <1 kΩ to ensure fast settling times. 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.

If your source impedance is high, you can decrease the scan rate to allow the NI-PGIA more time to settle. Another option is to use a voltage follower circuit external to the USB-6421 to decrease the impedance seen by the USB-6421.

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 can also increase the settling time. NI recommends using individually shielded, twisted-pair wires that are 2 m or less to connect AI signals to the USB-6421.

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 NI-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 1/50 LSB) of the ±10 V range. Some devices can take many microseconds for the circuitry to settle this much.

To avoid this effect, you should arrange your channel scanning order so that transitions from large to small input ranges are infrequent.

In general, you do not need this extra settling time when the NI-PGIA is switching from a small input range to a larger input range.

Insert Grounded Channel between Signal Channels.

Another technique to improve settling time is to connect an input channel to ground. Then insert this channel in the scan list between two of your signal channels. The input range of the grounded channel should match the input range of the signal after the grounded channel in the scan list.

Consider again the example above where a 4 V signal is connected to channel 0 and a 1 mV signal is connected to channel 1. Suppose 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.

You can connect channel 2 to AI GND (or you can use the internal ground; refer to Internal Channels below). Set the input range of channel 2 to -200 mV to 200 mV to match channel 1. Then scan channels in the order: 0, 2, 1.

Inserting a grounded channel between signal channels improves settling time because the NI-PGIA adjusts to the new input range setting faster when the input is grounded.

Minimize Voltage Step between Adjacent Channels

When scanning between channels that have the same input range, the settling time increases with the voltage step between the channels. If you know the expected input range of your signals, you can group signals with similar expected ranges together in your scan list.

For example, suppose all channels in a system use a -5 V to 5 V input range. The signals on channels 0, 2, and 4 vary between 4.3 V and 5 V. The signals on channels 1, 3, and 5 vary between -4 V and 0 V. Scanning channels in the order 0, 2, 4, 1, 3, 5 produces more accurate results than scanning channels in the order 0, 1, 2, 3, 4, 5.

Avoid Scanning Faster Than Necessary

Designing your system to scan at slower speeds gives the NI programmable gain instrumentation amplifier (NI-PGIA) more time to settle to a more accurate level.

Example 1

Averaging many AI samples can increase the accuracy of the reading by decreasing noise effects. In general, the more points you average, the more accurate the final result. However, you may choose to decrease the number of points you average and slow down the scanning rate.

Suppose you want to sample 10 channels over a period of 20 ms and average the results. You could acquire 250 points from each channel at a scan rate of 125 kS/s. Another method would be to acquire 500 points from each channel at a scan rate of 250 kS/s. Both methods take the same amount of time. Doubling the number of samples averaged (from 250 to 500) decreases the effect of noise by a factor of 1.4 (the square root of 2). However, doubling the number of samples (in this example) decreases the time the NI-PGIA has to settle from 4 μs to 2 μs. In some cases, the slower scan rate system returns more accurate results.

Example 2

If the time relationship between channels is not critical, you can sample from the same channel multiple times and scan less frequently. For example, suppose an application requires averaging 100 points from channel 0 and averaging 100 points from channel 1. You could alternate reading between channels—that is, read one point from channel 0, then one point from channel 1, and so on. You also could read all 100 points from channel 0 then read 100 points from channel 1. The second method switches between channels much less often and is affected much less by settling time.