The quintessential Dewetron invention was a high performance, high isolation, SMALL and affordable signal conditioning front-end: that's what the DEWE-RACK is all about.
Differential Output Converter
Having a great distance between your DEWE-30 chassis and your data acquisition system can cause some problems if high bandwidth and low distortion/noise are required. As with all high-speed digital signal layer protocols (LAN, IEEE1394, USB, PCI-Express), twisted pair cables should always be used. The RACK-DIFF-OUT converts the standard single-ended DEWE-RACK output to a balanced differential output with high output current, which is needed to drive cables up to 500 m without losing the intrinsic accuracy of the DAQ modules and DEWE-30 system. The RACK-DIFF-OUT therefore is an option available for the DEWE-30-16 and above, which provides these unique characteristics and capabilities:
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Specifications
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Cable configuration
The use of shielded twisted pair cables is recommended for the connection of the DEWE-30 chassis and the data acquisition unit. Please note that at the data acquisition unit, differential input amplifiers are needed. The shield of the cable must be connected to the GND pin of the 37-pin SUB-D connector (female) of the DEWE-RACK-xx. In the typical system configuration, the GND of the DEWE-30 and the GND of the data acquisition unit are tied together via the earth connection of the AC power supply. Therefore, the cable shield should not be connected to the GND of the data acquisition system to avoid GND loops. But if the GND of both devices are nowhere tied together (at power supply without protected earth) it is also recommended to connected the shield to the GND of the data acquisition device. |
Bandwidth and cable length
Although the bandwidth of the RACK-DIFF-OUT amplifier itself is greater than 1 MHz, the total available bandwidth also depends on the length, i.e., the distance between the DEWE-30 and the data acquisition system. The longer the cable the lower the efficient bandwidth will be. The reason for this is parasite cable capacitance, which is typically around 30-200 pF per meter. Therefore a 100 m cable can have a capacitance of around 10 nF. This capacitive load have to be driven by the RACK-OUT-DIFF amplifier. The current needed for driving this load depends on the signal level as well as the signal frequency, since:
Therefore, the current needed to drive a 100 m cable at 5 Vpp and 100 kHz is already around 30 mA. The maximum output current of the RACK-DIFF-OUT is specified at ±60 mA. If the signal frequency increases, or the cable is longer, it is possible (but not advisable) to exceed this specification. |
Cable termination
As in any higher frequency application (RS-485, CAN, IEEE1394 … ) the cable should be terminated at the end to avoid signal reflections. The value of this resistor should match the characteristic impedance of the cable being used. This resistor causes on the one hand additional load for the RACK-DIFF-OUT amplifier, but on the other hand, also errors because of the voltage dropout at cable. |
For example, a 100 meter 2 x 0.25 mm² cable has a line resistance of around 7 Ohms. If the termination resistor is 120 Ohms (typical value for twisted pair cable) this error is already around 5%. This influence should be recognized in the measurement result or should be rescaled in the measurement setup.
Connectors
Availability
The RACK-DIFF-OUT is an option that is available for the DEWE-30-16, -32, -48, and -64 (one is required for each bank of 16 channels that your DEWE-RACK has). It should be ordered with the DEWE-30 from the beginning, since it needs to be seamlessly integrated into the DEWE-30 mechanics and electronics.
