Page 100 - Op Amps Design, Applications, and Troubleshooting

Inverting Summing Amplifier 83

As with previous circuits, we can utilize the data sheet supplied by the manufac-
turer if it becomes necessary to have a more accurate, or perhaps worst-case,
value.

Maximum Input Voltage Swing. The maximum input voltage swing of an
amplifier is the voltage that causes the amplifier's output to reach saturation.
Input voltages that exceed this limit will produce distorted (i.e., clipped) output
signals. In the case of the summing amplifier, the situation is more complex than
with previous, single-input amplifiers. That is, the instantaneous level of output is
determined by the instantaneous values of input voltage on all inputs. First we
will consider each input separately to determine the maximum levels of an isolated
input. The calculations, from Equation (2.1), are similar to those used with previ-
ous circuits.

.. . maximum output voltage swing
maximum input voltage swing = — -
A v

where A v is the voltage gain received by a particular input. The individual calcu-
lations are

Note that the negative and positive saturation limits were used as the maxi-
mum output "swing" for V l and V 4, respectively, since these two inputs are DC
and will only be limited by one saturation barrier.
With reference to v 2 and v 3, we may want to express them in their peak and
RMS forms to better compare them with the signals shown in Figure 2.20. These
conversions are

Since the maximum limits on all inputs (both DC and AC) are greater than
the values listed on the schematic, we will assume that no single input can cause
the amplifier output to saturate. However, two or more input signals may com-
bine at some instant to drive the output to its saturation limit. Let us determine if