Raco Verbatim, Owner'S Manual

Page: 106 / 246

Analog Signal Input
B-12 Verbatim Owner's Manual
In practice it is not unusual to have some element of the loop in fact tied to
ground or to some other voltage source away from ground -- or if not directly
tied, at least limited in its ability to depart from the ground or other voltage. As
long as only one element in the loop is so committed, there is no problem since
the other elements can freely accommodate as needed.
The Verbatim has its own limitations in this respect. It can only accommodate
a departure from ground voltage potential, of 8 volts nominal, before its
protective tranzorbs begin to conduct and clamp the signal. Such clamping
when in direct conflict with some other voltage commitment in the loop, will
not only cause incorrect readings by the Verbatim, but also cause the other
elements in the loop to read and respond incorrectly.
This ability to accommodate departures of both sides (positive and negative) or
the analog signal input, is called the common mode input voltage range. A truly
isolated input would have as much common mode input voltage range as the
voltage limitation of the isolation, typically over 1,000 volts.
The reason we do not provide isolated inputs is because it is bulky, and
expensive to achieve accurate translation across the isolation barrier. Also,
these days there has been a large shift to transformer and capacitive coupling
schemes to achieve DC isolation, but these provide almost zero protection
against the fast rise time transients induced by lightning. So, we need to be able
to troubleshoot when a customer places one of our analog inputs into a current
loop where there is another conflicting voltage commitment.
When this problem occurs, the customer will typically report that his loop
works but is thrown off when our analog input is placed in the loop. Sometimes
the disturbance takes the form of not just altering the DC current but causing
parasitic oscillations in the loop. It may not be easily discernible whether the
disturbance is or is not taking the form of a parasitic oscillation. Regardless,
temporarily ungrounding the dialer or unplugging the analog card, will usually
eliminate the disturbance.
The procedure for troubleshooting and correction of this problem is generally as
follows: First we need to find out as much as we can about any preexisting,
conflicting voltage commitments. To do this, have the customer unplug the
card or unground the dialer so that the loop is not disturbed, and then use a
voltmeter to check both the AC and the DC voltage readings at each node
around the loop, with respect to electrical ground.
We hope there is not much AC signal present. If there is a strong enough AC
component on top of the DC voltages, there will be disturbance to the extent
that the peak level in the AC waveform exceeds the common mode input
limitation of our analog input. In such a case the cause of the AC component of
the signal needs to be found and eliminated, if the following procedure does not
lead to a good result.