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Below are several footnotes used with the connecting-to-sensors documentation.

1Heating Considerations with Resistors
When current is pumped through a resistor, it heats up. When a resistor heats up, its resistance changes, and this can cause errors in your measurement. The current though a resistor is calculated via:

Current (Amps) = Volts Across Resistor / Resistance in ohms

The power dissipated by a resistor is:

Power Dissipated (Watts)
    = Volts * Volts / Resistance (Ω)
    = Current (Amps) * Current (Amps) * Resistance (Ω)

The amount a resistor heats up is:

Change in Temperature (Celsius)
    = Thermal Resistance (C/Watt) * Power Dissipated (Watts)

The amount a resistor changes for a change in temperature is:

Change in Resistance (Ω)
    = Change in Temp (Celsius) * Temp Coeff (ppm/C) * Resistor Value (Ω)

For example, a 100 ohm resistor with a 100 ppm/C temperature coefficient and 30C/Watt thermal resistance that is passing 50 milliAmps would enjoy the following situation:

5 Volts across resistor = 100ohms * 50mA
0.25 Watts power dissipation = 5V * 5V / 100 ohms
7.5°C temperature change = 30C/Watt * .25Watts
0.075 ohms change due to temperature change
  = 7.5°C temp change * .0001 ohms/ohm/C thermal resistance * 100 Ω

2Setting Excitation Voltage
The Vout field in the Constants settings area is used to set the excitation output voltage when working with hardware that has variable internal excitation (e.g. the i100 excitation ranges between -5V and +5V). In cases where excitation is fixed (e.g. the i42x/i43x/i60x have a fixed +3.3V excitation), one can ignore the Vout field.

Alternatively, if one applies an external excitation voltage (e.g. external +10.000V precision voltage reference box available from 3rd party), then you must enter a -Ro value in the Ro edit field (e.g.  -100 instead of 100 ohms) to tell the software that the excitation is external, and then enter the external excitation voltage in the Vout field.

If you type an unreasonably high value into the Vout field and then press the Update button, instruNet will set the output voltage to the highest possible value without allowing the internal output amplifier to saturate (e.g. ≤ 4mA for #iNet-100/100B and ≤15mA for #iNet-100HC). Setting the highest possible Vout, causes the highest possible voltage to be read by the Vin terminals, which increases the signal to noise ratio and therefore increases accuracy. The disadvantage to having a high excitation voltage is that it increases the power dissipated by the resistors, which increases their thermal heating, which increases the drift from their resistance's at ambient temperature (e.g. typical resistors offer 100ppm resistance drift per degree C change in temperature)1. Resistors with low temperature coefficients (e.g. 25ppm/C) are helpful if this is a problem.

When working with variable internal excitation (e.g. i100) and high current excitation (e.g. >6mA), it is sometimes helpful to alternate the polarity of the excitation voltages to evenly burden the ±12V supplies 11.

3Shunt Resistors and Bridge Completion Resistors
Bridge completion resistors and shunt resistors should be as accurate as possible (0.1% is often ok, 0.01% is very good), and have a low temperature coefficient (25ppm/C is often ok, 5ppm/C is very good). If you use a less accurate resistor, we recommend that you measure it with a DVM, and then type this more accurate value into the Rshunt field in the Constants setting area. To determine the effect of a resistor inaccuracy, calculate your engineering units for a typical case, and then increase the resistor value by its expected error, and note the change in the resulting engineering units output. For example, if a 100ohm shunt resistor is used to measure a 1mA current source and it changes 1%, then the measured reading would change 1%, or .01mA. For a list of typical precision resistors, click here.

4Bridge Completion Resistors in Strain Gage Bridge Circuits
In a bridge, all 4 resistors must be the same value, within 10% or so (1% is better, 0.1% is excellent), in order for the bridge to operate properly. In some bridge circuits, all 4 resistors are supplied by the sensor manufacturer; whereas in others (e.g. ¼ or ½ bridge circuits), the user must supply "completion" resistors of the same value as the gage to complete the bridge circuit. This can be done by installing precision resistors (e.g. 0.1%), or by installing fixed unstrained strain gages of the same ohmic value.

5Filter Settings for Low Level Measurements
Strain gage, thermocouple, and RTD voltages are typically very low and therefore often require low pass filtering to reduce noise. Low pass filters cause high frequencies to be rejected, while low frequencies are passed. Visually, the signal becomes "smoother" when plotted on the computer screen. instruNet provides several low pass filter options:

  • The Low Pass popup menu in the Hardware settings area can select a variety of analog low pass filter options (e.g. the i423 provides the following analog 2-pole low pass options: off, 6Hz, and 4KHz).
  • The Integrate field in the Hardware Settings area selects how long the signal is averaged before instruNet returns one number. This "averaging", in effect, is a low pass filter. Careful, this averaging fully consumes the instruNet controller, and therefore reduces the maximum possible sample rate, as noted in Sample Rate Vs Integration Vs. Noise. A 0.0001 or 0.001 second integration time is often very helpful at reducing noise and increasing accuracy.
  • The Lowpass Filter settings area provides a means by which one can digitally filter a signal, post acquisition, with tremendous accuracy.
  • If working with the i100 (not with the i420/i430/i60x 12), the user can manually place a capacitor across the Vin+ and Vin- input terminals with any bridge or voltage divider circuit to provide a 1pole low pass filter where the cut-off Frequency in Hertz is equal to 1 / (6.28 * R * C); where R is the source resistance in ohms, and C the parallel capacitance in Farads.
  • For more details on reducing noise, click here.

6Selecting a Voltage Divider Shunt Resistor
Shunt resistor values are typically chosen to cause a large voltage (several volts maximum) to be measured by instruNet, without heating up the resistor significantly to cause its resistance to change or causing the excitation voltage source to over shoot its maximum output current (e.g. 4mA on the #iNet-100/100B and 15mA on #iNet-100HC). If the Rshunt value is low, then the voltage across it is low, and this decreases the signal to noise ratio of the measured signal. Also, Rshunt must be selected such that the voltage across Runknown does not exceed the instruNet maximum input voltage (e.g. ±10V with the i420/i430/i60x). Due to these limitations, instruNet might not let you set some of the fields too high or too low.

7Voltage Range Settings For Strain Gages
Since strain gage voltages are often very small, a small input range (e.g. ±10mV) works best for most measurements. Increasing the voltage range increases the range of strain that can be read, while sacrificing accuracy with small voltages (e.g. instruNet can read 5mV more accurately with a ±10mV range, than with a ±100mV range). Please refer to your equation for details on how strain relates to voltage measured.

8Balancing your Bridge with the Vinit Correction Voltage
Vinit is the voltage measured across the intermediate nodes of the bridge when the strain gage(s) are unstrained in a bride circuit. For details, click here.

9The Strain Gage "GF" Factor
All strain gages are manufactured with a specific Gage Factor (GF), which relates a change in resistance, to strain. The GF is often printed on the strain gage package, and must be correctly entered into the instruNet GF field within the Constants settings area. This is used to calculate the "strain" value returned by instruNet.

10Accuracy of Measurements
Accuracy measurements are affected by the noise pickup on the leads, the accuracy of the sensor itself (i.e. thermocouple devices themselves are typically accurate to ± 1 to 3°C) and the instruNet measuring system. A noisy environment and long sensor leads are often the worst threat to accuracy. Integrating (via the Integrate field) a signal over a period of time will give a more accurate measurement by filtering out noise at the expense of a lower maximum sample rate.

Here's an example. Suppose you are doing a current measurement where the current is calculated as the voltage drop across a shunt resistor divided by the resistance in ohms of the resistor.

Current (Amps) = Volts across shunt resistor / shunt resistance in ohms

Suppose the measured voltage is accurate to 1mV and the 1K ohm shunt resistor is accurate to 1%. Subsequently, the accuracy of the measured current would be

Max Current Error = 1mV / (.01 * 1K) = 100 microAmps

11Alternating i100 Positive and Negative Excitation Voltages
To reduce the burden on one side of an i100 power supply (e.g. +12V or -12V), excitation voltages often alternate positive and negative. For example, when powering 350 ohm strain gages, the excitation voltages are typically set to {+4V, -4V, +4V, -4V...}. The alternating polarity evenly burdens the ±12V supplies. Please note that in low current cases (e.g. <2mA), this is not necessary.

12External Capacitors with the i420/i430/i60x are NOT Recommended
In order to reduce noise in some setups, an external capacitor is added between VinScrew+ and GND, and/or VinScrew+ and VinScrew-. For example, this is often helpful with the i100 when working with a low level signal and a source impedance > 100 ohms (e.g. strain gage, load cell, not thermocouple). However, with the i420/i430/i60x, the external capacitors will lead to small offset errors and are therefore not recommended. This is due to multiplexor that charges up the external capacitor and results in a small offset error. The size of that error relative to your expected accuracy depends on several factors such as voltage input range, source impedance, capacitor size (μF), and integration time. The i420/i430/i60x are less likely to be effected by RFI than the i100 due to internal RFI filters used with low level measurements. In the case of the i423 module, external capactors will not hurt, yet also are not likely to help either due to internal filters that do the same as an external filter (i.e. i423 has 4KHz or 6Hz analog low pass filter in series with 120KHz rfi filter).

13Setting up Senors via a dialog box Interview process
When in the Channel Setup dialog, one can setup a sensor with an easy interview process that asks the user several questions and then sets all fields based on the responses. For details on the interview process, click here.

14Creating i4xx Digital Output RESET Signal
If you want an i4xx digital output bit that is low when instruNet device power is off, low when instruNet power is first applied, and low during and after software reset; then you need an external circuit. An example circuit would be a logic Exclusive OR gate that combines two instruNet digital outputs; and outputs a 0 if they are the same and a 1 if different. Then, in order to get a 1 out, the user would need one output to be 0 and the other to be 1. And when instruNet power was off, then both output bits would be the same and cause a 0 to output from the EOR gate. When the instruNet outputs first turn on, it is possible that one might rise faster than the other, which might create a small spike on the output of the EOR gate. i60x digital i/o are automatically low upon reset and power off; and are therefore easier to work with than i4xx digital i/o. For details, see Outputs After Reset.