Page 429 - Op Amps Design, Applications, and Troubleshooting

Instrumentation Amplifiers 405

The open-loop voltage gain increases as the programming current increases. It
is reasonable to expect as much as a 100:1 change as the programming current is var-
ied over its operating range. This characteristic can be considered an advantage
(e.g., programmable voltage gain) or a disadvantage (e.g., unstable voltage gain)
depending on the nature of the application.
Input bias current also increases as programming current increases. In this
case, variations as great as 200:1 are reasonable over the range of programming
currents. The minimum input bias current is often in the fractional nanoampere
range.
The slew rate of a programmable op amp increases as programming current
increases. It can also be increased by using higher DC supply voltages. As the pro-
gramming current is varied over its operating range, the slew rate can be expected
to vary as much as 1500:1. The upper limit is typically greater than that for the
standard 741 op amp (i.e., greater than 0.5 \^juS).
The unity gain frequency (or gain bandwidth product) increases as the pro-
gramming current increases. Ranges as great as 500:1 are reasonable changes to
expect as the programming current is varied over its operating range. The highest
unity gain frequency is typically higher than the standard 741 op amp rating (i.e,
higher than 1.0 MHz).
The input noise voltage of a programmable op amp decreases as the pro-
gramming current increases. Ranges of as much as 200:1 are reasonable as the
programming current is varied throughout its control range.
In general, the programmable amplifier can be used for most of the applica-
tions previously discussed for general-purpose op amps, provided the appropriate
specifications are adequate for a given application. The data sheets are interpreted
in the same manner as for other amplifiers, with the exception that the effects of
programming current are included. These effects may be shown by including mul-
tiple data sheets for different values of programming current and/or by presenting
graphs that show the effects of programming current. These devices are often
selected for low-power applications or for applications that require a controllable
parameter. Representative devices include the MC1776 and MC3476, manufac-
tured by Motorola, Inc.

11.2 INSTRUMENTATION AMPLIFIERS

An instrumentation amplifier is essentially a high-gain differential amplifier that
is internally compensated to irdnimize nonideal characteristics. In particular, it
has a very high common-mode rejection ratio, meaning that signal voltages that
appear on both input terminals are essentially ignored and the amplifier output
only responds to the differential input signal. In general, the instrumentation
amplifier is designed to achieve the following:

1. Offset voltages and drifts are minimized.
2. Gain is stabilized.
3. Nonlinearity is very low.