Input Isolation
Three benefits of input isolation are circuit protection, noise reduction, and the rejection of high common
mode voltage.
• Circuit protection. Input isolation separates the signal source from circuits that may be damaged by
the signal. (Voltages higher than about 10 V can distort data or damage chips used in data
acquisition.) High-voltage signals or signals with high-voltage spikes should therefore be isolated.
The protection can also work the other way—to safeguard a sensitive signal conditioner from a
failing device elsewhere in the system.
• Noise reduction. Isolation eliminates ground loops for high-gain systems and multi-unit systems that
are grounded together. The chassis for each device can rest at a ground potential slightly different
from the other devices. These irrelevant currents and the spikes they may have picked up by
induction can thus be kept out of the measurement circuit.
• Rejection of high common-mode voltage. There is a limit to the voltage applied to a differential
amplifier between ground and the amplifier inputs. Fortunately, the differential amplifier rejects high
common-mode voltage signals. High common-mode voltage and noise spikes are rejected (canceled
out) in in-phase signals (same amplitude and frequency) that are present in both the high and low
inputs at the same time.
References for Differential Modes
There are three basic types of measurement configuration related to differential mode; these are ground-
referenced, shunt-referenced, and floating.
Differential Mode, Ground-Referenced
In ground-referenced configurations, the signal voltage is
referenced to a local common ground. In most cases, the local
ground will be at a different voltage potential from the PC’s
ground.
Differential inputs provide attenuation of the common-mode
noise. When in this mode, the amplifier sees the voltage
differential between the high and low inputs (see figure).
Common-mode noise reduction occurs because noise in the
high input signal is typically the same as the noise in the low
input signal. Because of this phenomena, the voltage
difference between the 2 signals remains essentially unaffected
by noise spikes, since these spikes appear at the same instant
and at the same magnitude in both the high and low input
signals. In other words, the noise spikes cancel each other out.
As noted earlier, even if these signals were out-of-phase, the
noise would still cancel out since the spikes in both signals
would be of the same magnitude and polarity.
Note: In the simple example (shown in the figure), the
differential between the high and low signals would result in a straight line because the signals are
equal in frequency, phase, and magnitude.
Differential Mode, Shunt-Referenced
There are situations in which small voltages need to be measured and the currents flowing in the power
supply common will cause measurement errors. As shown in the figure, using the analog common as the
reference point will result in errors. These errors are the result of variations in current flow along the
common line [(I3 * Z3) + (I3 & I2) * Z2].
1-2 Signal Management 886995 DBK Option Cards and Modules