MAE180+Laboratory+Project+1 - MAE180 Laboratory Project 1...

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1 MAE180 Laboratory Project 1 DAQ analog input, half wave rectifier and bandpass filter EXPERIMENT #1: using a DAQ card for recording voltage signals. Differential versus Single- Ended Analog Inputs. DESCRIPTION A voltage signal is transferred to the A/D converter of a DAQ card through a buffer (e.g. a very high input impedance operational amplifier wired as a voltage follower). Thus, to record the voltage of point A in a circuit (voltage referred to some circuit common voltage, or circuit ground), all we need to do is connect point A and the input of a DAQ buffer (i.e. any one of the 8 analog input terminals in the LabJack), and make sure that the voltage references of the circuit and the LabJack are the same (i.e. connect the circuit ground and any GND terminal of the LabJack). It seems simple, but it is not: both the current and the resistance through the grounds connection can be significant, which can result in an undesired voltage difference between the LabJack and circuit references. Furthermore the ground connection can pick up noise. All this may cause a poor recording of the circuit signal, especially if the signal of interest is low (a fraction of a Volt). The solution to this problem is to use two LabJack analog input terminals to record an analog signal: we connect A to an analog input terminal, and the circuit reference to the second terminal. The difference between the buffer outputs is equal to the circuit’s voltage difference (voltage of A minus voltage of the circuit reference) we want to measure. If we configure LabJack via software, LabJack will know that we are using the analog inputs in this way, and will compute the difference for us. This analog configuration is called “Differential Analog Input”, while the first one is referred to as “Single-Ended Analog Input”. The Differential Analog Input configuration works better because no current flows between the circuit into the LabJack (the input impedance of a buffer is very large), and therefore the signals we want to measure are less distorted.
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