Crosstalk in M series PCI-6221

[Muni]

I am using a M series PCI-6221 together with BNC-2110 box.
The rated max. sampling rate for 6221 is 250 KS/s/channel.
I am reading 8 analog inputs. Therefore, my max rate should be about
30 KS/s. But it turned out that unless I lower my sampling rate
to as low as 1 KS/s there is crosstalk of about 1% from the
channel above. i.e. 1% of the signal on, say ai0, shows up on top of
ai1 signal. This is a major problem when adjacent channels
have significantly different signal levels. Let's say I have 9V on ai0
and 0.2V on ai1. It reads correctly when one channel at a time is selected.
But ai1 reads 0.3V (and ai0 reads 9V)if I select both channels.
Reading 8 channels at a rate low as 1 KS/s is not acceptable for me.

[Muni]

I found the answer myself from the M series manual.

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 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 PGIA is switching from a small input range to a larger input range.
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