Model i100 DRIVING CURRENT AND CAPACITANCE
WITH VOLTAGE OUTPUTS PRODUCT LIST > i100 >
The i100 has a voltage output screw terminal that is used to provide excitation
to different various sensors. Each sensor type has its own current requirements (i.e.
milliamps of Vout output current), as discussed below.
What load does my sensor require?
This varies depending on the sensor. For example, a 350ohm strain gage typically
puts out 2mV per 1 Volt of applied excitation. One might want to apply 4V excitation,
so that 8mV max comes out, and then read this with the ±10mV voltage input. And
this requires 11.4mA of drive capability from the instruNet voltage output (4V / 350ohm
= 11.4mA). The iNet-100HC, which is capable of supplying 15mA would suffice,
whereas the iNet-100 or iNet-100B would not.
What current can instruNet provide?
Vout Max Current
Vout max Cap
Adding a load to the Vout screw terminal increases the current drawn from the
instruNet ±12V power supply inputs; and the user must make sure that adequate
power is supplied to the units. For example, an instruNet box without any loads draws
80mA of ±12V power, and with 8x 15mA loads with alternating positive and negative
voltage, it would draw 80mA + (4 * 15mA) = 140mA from the +12V supply and 140mA
from -12V supply.
External Power Supply
The user has the option of supplying excitation via their own external reference
voltage, and then telling instruNet what voltage is applied by entering this value into the
Vout field of the Constants Settings group and making the Ro value negative to
indicate an external power supply. If the user provides their own power source (e.g.
5V), it is recommended that they run this voltage though a voltage reference IC (e.g.
the National Semiconductor LM4040A-4.1 device has a .1% initial accuracy). Note that
these reference IC's are very stable and accurate, yet usually only provide 15mA of
current; therefore multiple IC's might be needed when powering multiple devices.
Powering a sensor directly from a power supply is ok if accuracy of several percent is
acceptable. A precision power source that provides much current (several hundred
milliamps) at a precise voltage would be most useful with multiple sensors that require
much (e.g. >10mA) current.
Turn off Vout Calibration with the i100
With iNet32.dll version ≥ 3.4, if filename "iNet32_VoutCalOff.txt"
exists in folder "..MyDocuments\instrunet\End User\iNet32_VoutCalOff.txt",
all iNet software (e.g. DasyLab, LabView, C) will not calibrate i100 Vout channels using the
standard set/readback method, and will instead rely on default values. The advantage of this
is the Vout channels will not move when one powers up the i100 and it is easier to
calibrate w/ default values with high loads. The disadvantage is the i100 vout accuracy
will be ±40mV worse, yet this is not an issue when using the vout channels for
sensor excitation since Vout Readback voltages measured by the A/D are used to
calculate sensor values, and they are very accurate.
i100 Ch26 High Speed Digital Output
With iNet32.dll version ≥ 4.0, if filename "iNet32 Diagnostics On.txt"
exists in folder "..MyDocuments \instrunet\End User\iNet32 Diagnostics On.txt",
and the file contains the words "i100ch26 not off", then an additional channel will appear as Ch26 for all
i100 boxes. This Ch26 is the same as Ch25 Digital I/0 Group, yet is high speed digitized as an output instead of input. The previous Channels 26..33 are shifted to positions 27..34. Output is different than input, so Ch25 In and Ch26 might reflect different values.