Knowledge Base:  
Tech Note 119: CMCP420VT Buffered Output
Last Updated: 05/10/2017

CMCP420VT Buffered OutputThe CMCP420VT buffered is intended for connection to a portable data collector or analyzer that is isolated (floating) relative to system ground. Connection of this type of instrument will not affect the output (Loop) current of the CMCP420VT. The buffered output is NOT intended for connection to remote instruments directly connected to system ground. However, if the buffered output is to be connected to this type of instrument, there are several restrictions / limitations that should be carefully considered:

1. The - terminal of the CMCP420VT should NEVER be connected directly to system ground as
the unit will not operate correctly.

2. The Buffered Output terminal (marked D) should only be connected to an instrument input
that provides a high input impedance (>1MOhm) differentially and relative to system ground
to minimize both signal loading and loop current errors. Generally differential inputs would
be required, with or without AC coupling.

3. The buffered output bias voltage is +2.5 Vdc relative to the - terminal of the CMCP420VT.
With a nominal sensitivity of 100 mV/g, this output will clip acceleration signals larger than
approximately 25 gs peak.

4. The buffered output has a 10 KOhm output impedance that in conjunction with the
capacitance of a connecting cable and instrument input, will limit the upper frequency
response. The -3db corner of this response can be calculated approximately by 1 / 2PI x
RC. For example: A typical twisted-pair instrument cable 100 meters in length, plus a typical
instrument input capacitance of 100 pF, would present a total capacitance of approximately
10,000 pF. This would limit the signal bandwidth to approximately 1.59 kHz.

5. Long cable runs connected to the buffered output can introduce noise back into the
CMCP420VT and affect loop-current and reading stability. Care must be taken with routing
and connection of cables. Installation in bonded metal conduit may be necessary.

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