SCB-68 Connector Block

Special consideration for analog output signals

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Unformatted text preview: function as debouncing filters to smooth noise on digital trigger input signals, thus enabling the trigger-detection 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 5-16. Digital Trigger Input Signal with a High-Frequency Component National Instruments Corporation 5-15 SCB-68 Shielded Connector Block User Manual Chapter 5 Adding Components for Special Functions Apply a lowpass filter to the signal to remove the high-frequency component for a cleaner digital signal, as Figure 5-17 shows. Volts (V) Time (t) Figure 5-17. 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 5-7, where V is voltage, I is current and R is resistance: V = IR (5-7) 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 5-18 shows. SCB-68 Shielded Connector Block User Manual 5-16 ni.com Chapter 5 Adding Components for Special Functions I + + Transducer Input R Vm Figure 5-18. Current-to-Voltage Electrical Circuit The application software must linearly convert voltage back to current. Equation 5-8 demonstrates this conversion, where the resistor is the denominator and Vin is the input voltage into the DAQ device: Vm I = -----R (5-8) 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 5-17 SCB-68 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 one-resistor circuit for measuring current at the single-ended or differential inputs of the SCB-68. Single-Ended Inputs To build a one-resistor circuit that measures current at the single-ended analog inputs of the SCB-68, 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 5-9 or 5-10. Vm I = -----RB Vm I = -----RE (5-9) (5-10) Differential Inputs To build a one-resistor circuit that measures current at the differential inputs of the SCB-68, 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 5-11: Vm I = -----RE (5-11) 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 5-19 shows how to use a voltage divider to attenuate the output signal of the transducer. SCB-68 Shielded Connector Block User Manual 5-18 ni.com Chapter 5 Adding Components for Special Functions + R1 + Vin R2 Vm Figure 5-19. 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 5-12 to determine the Vm that the DAQ device measures: R2 V m = V in ----------------- R 1 + R 2 (5-12) Use Equation 5-13 to determine the overall gain of a voltage divider circuit: R2 Vm G = ------ = ----------------V in R1 + R2 (5-13) The accuracy of Equation 5-13 depends on the tolerances of the resistors that you use. Caution The SCB-68 is not designed for any input voltages greater than 42 V, even if a user-installed voltage divider reduces the voltage to within the input range of the DAQ device. Input voltages greater than 42 V can damage the SCB-68, 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 5-19 SCB-68 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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This note was uploaded on 05/19/2012 for the course ELEN 3030 taught by Professor Joshi during the Spring '12 term at Marquette.

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