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Analog Input Features
2-5
Maximum Achievable Throughput Rates
Because you can change input ranges on a per-channel basis, throughput
is likely to drop if you group channels with varying gains in sequence.
The drop occurs because the channels with low-level inputs (magnitude
of 100mV or less) are slower than those with high-level inputs and
because the channels with low-level inputs must drive out the residual
signals left by the high-level inputs. The best way to maximize
throughput is to use a combination of sensible channel grouping and
external signal conditioning. When using the channel-gain queue,
consider the following suggestions:
●
Put all channels that use the same range in the same group, even if
you have to arrange the channels out of sequence.
●
If your application requires high-speed scanning of low-level signals,
use external signal conditioning to amplify the signal to the
maximum input range of the board. This method offers the
advantages of increasing total system throughput and reducing noise.
●
In the common case where the low-level inputs are relatively
slow-speed and the high-level inputs are high-speed, you should
maintain two channel lists: one for low-speed inputs, the other for
high-speed inputs.
●
If you are not using all the channels, you can make a particular
channel-gain entry twice to allow for settling time. In this case, you
want to ignore the results of the first entry.
You must give special consideration to the direct measurement of
low-level signals with the DAS-1801HC. When using the ±20mV, 0 to
20mV, ±100mV, or 0 to 100mV ranges, measurement throughput drops
for two reasons:
●
The amplifier cannot settle quickly enough (particularly the ±20mV
and 0 to 20mV ranges).
●
Noise in the measurements is higher and thus requires
post-acquisition filtering (averaging) to achieve accurate results.
The DAS-1801HC would have better noise performance if presented with
a perfect signal in these ranges, but perfect signals are virtually
non-existent in the real world. Since the DAS-1801HC has very high
bandwidth (bandwidth for low-level signals is about 8 to 10MHz) any
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