Unformatted text preview: function as debouncing filters to smooth noise on digital trigger input signals, thus enabling the triggerdetection circuitry of the DAQ device to understand the signal as a valid digital trigger. Volts (V) TTL Logic High TTL Logic Low Time (t) Figure 516. Digital Trigger Input Signal with a HighFrequency Component National Instruments Corporation 515 SCB68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Apply a lowpass filter to the signal to remove the highfrequency component for a cleaner digital signal, as Figure 517 shows. Volts (V) Time (t) Figure 517. Lowpass Filtering of Digital Trigger Input Signals Note Due to the filter order, the digital trigger input signal is delayed for a specific amount of time before the DAQ device senses the signal at the trigger input. Measuring a 4 to 20 mA Current
Since DAQ devices cannot directly measure current, this section describes how to add components for measuring current when transistors output a current value ranging between 4 and 20 mA. Theory of Operation
The conversion from current to voltage is based on Ohm's Law, which is summarized by Equation 57, where V is voltage, I is current and R is resistance: V = IR (57) Thus, you must multiply current by a constant to convert the current to a voltage. In an electrical circuit, current must flow through a resistor to produce a voltage drop. This voltage drop then becomes the input for a DAQ device, as Figure 518 shows. SCB68 Shielded Connector Block User Manual 516 ni.com Chapter 5 Adding Components for Special Functions I + + Transducer Input R Vm Figure 518. CurrenttoVoltage Electrical Circuit The application software must linearly convert voltage back to current. Equation 58 demonstrates this conversion, where the resistor is the denominator and Vin is the input voltage into the DAQ device: Vm I = R (58) Selecting a Resistor
For best results when measuring current, you should choose a resistor that has the following characteristics: Low wattage of approximately 1/8 W Precision of at least 5% Temperature stability Tolerance of 5% 232 (suggested) AXL package (suggested) Carbon or metal film (suggested) If you use the resistor described above, you can convert a 20 mA current to 4.64 V by setting the device range to either (5 to +5 V) or (0 to 5 V). National Instruments Corporation 517 SCB68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Adding Components
Caution Do not exceed 10 V at the analog inputs. NI is not liable for any device damage or personal injury resulting from improper connections. You can build a oneresistor circuit for measuring current at the singleended or differential inputs of the SCB68. SingleEnded Inputs
To build a oneresistor circuit that measures current at the singleended analog inputs of the SCB68, add the resistor to position B or D depending on the channel being used. Leave the jumpers in place for channel positions F and G, respectively. Calculate the current according to Equation 59 or 510. Vm I = RB Vm I = RE (59) (510) Differential Inputs
To build a oneresistor circuit that measures current at the differential inputs of the SCB68, add the resistor to position E for each differential channel pair that is used. Leave the jumpers in place for positions F and G. Calculate the current according to Equation 511: Vm I = RE (511) Attenuating Voltage
This section describes how to add components for attenuating, or decreasing the amplitude of, a voltage signal. Transducers can output more than 10 VDC per channel, but DAQ devices cannot read more than 10 VDC per input channel. Therefore, you must attenuate output signals from the transducer to fit within the DAQ device specifications. Figure 519 shows how to use a voltage divider to attenuate the output signal of the transducer. SCB68 Shielded Connector Block User Manual 518 ni.com Chapter 5 Adding Components for Special Functions + R1 + Vin R2 Vm Figure 519. Attenuating Voltage with a Voltage Divider Theory of Operation
The voltage divider splits the input voltage (Vin) between two resistors (R1 and R2), causing the voltage on each resistor to be noticeably lower than Vin. Use Equation 512 to determine the Vm that the DAQ device measures: R2 V m = V in  R 1 + R 2 (512) Use Equation 513 to determine the overall gain of a voltage divider circuit: R2 Vm G =  = V in R1 + R2 (513) The accuracy of Equation 513 depends on the tolerances of the resistors that you use.
Caution The SCB68 is not designed for any input voltages greater than 42 V, even if a userinstalled voltage divider reduces the voltage to within the input range of the DAQ device. Input voltages greater than 42 V can damage the SCB68, any devices connected to it, and the host computer. Overvoltage can also cause an electric shock hazard for the operator. NI is not responsible for damage or injury resulting from such misuse. National Instruments Corporation 519 SCB68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Selecting Components
To set up the resistors, complete the following steps: 1....
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