Chapter 3
Hardware Overview
3-4
ni.com
Settling times can also increase when scanning high-impedance signals
because of a phenomenon called
charge injection
, where the AI multiplexer
injects a small amount of charge into each signal source when that source
is selected. If the impedance of the source is not low enough, the effect of
the charge—a voltage error—does not decay by the time the ADC samples
the signal. For this reason, keep source impedances under 1 k
Ω
to perform
high-speed scanning.
Due to the previously described limitations of settling times resulting from
these conditions, multiple-channel scanning is not recommended unless
sampling rates are low enough or it is necessary to sample several signals
as nearly simultaneously as possible. The data is much more accurate and
channel-to-channel independent if you acquire data from each channel
independently (for example, 100 points from channel 0, then 100 points
from channel 1, then 100 points from channel 2, and so on.)
Analog Output
♦
NI 6014 only
The NI 6014 supplies two channels of 16-bit AO voltage at the I/O
connector. Each device has a fixed bipolar output range of ±10 V. Data
written to the D/A converter (DAC) is interpreted in two’s complement
format, where for a number
x
expressed in base 2 with
n
digits to the left
of the radix point, the (base 2) number is 2
n
–
x
.
Analog Output Glitch
In normal operation, a DAC output glitches whenever it is updated with
a new value. The glitch energy differs from code to code and appears as
distortion in the frequency spectrum.
Digital I/O
The NI 6013/6014 contains eight lines of digital I/O (DIO<0..7>) for
general-purpose use. You can individually software-configure each line for
either input or output. At system startup and reset, the DIO ports are all
high-impedance.
The hardware up/down control for general-purpose counters 0 and 1 are
connected onboard to DIO6 and DIO7, respectively. Thus, you can use
DIO6 and DIO7 to control the general-purpose counters. The up/down
