MM1089 View Datasheet(PDF) - Mitsumi
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Manufacturer
MM1089 Datasheet PDF : 9 Pages
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MITSUMI
Sensor Amplifier MM1089
1. Summary
An instrumentation amp is often used as a sensor
amp to amplify weak signals. Among the
advantages of such an amplifier are
1. Good CMR characteristics
2. High input impedance
3. Means of gain adjustment which does not
affect the CMR characteristic
However, in practice an extremely high resistance
precision is demanded, making it difficult to
implement such an amplifier at low cost. In order to
eliminate these problems, Mitsumi developed the
MM1089 sensor amp, with a circuit configuration
providing the above advantages using ordinary
monolithic IC precision.
2. Aim of development
The I/O environment in which this IC will be used
was expected to include input sources ranging
from GND to VCC, while devices receiving the IC
output were anticipated to consist mainly of
microcomputers with integrated D/A converters. In
addition to a high CMR characteristic, the offset
voltage must be kept low; here it was judged that
the output voltage with no input signal could be
easily read in advance and used in the
microcomputer to correct measured values, so that
no measures are taken to force down the offset
voltage unnecessarily. Of course even if a
microcomputer is not used, a potentiometer can be
used to shift the reference voltage applied to the
O.COM pin by the amount of the offset. Emphasis
was placed on a high CMR characteristic and the
ability to accommodate a wide range of input
voltages.
3. Features of the MM1089
1. CMR characteristic of 100dB and higher
2. Input impedance of 10MΩ and above
3. Broad recommended operating power supply
voltage range (4.5V to 20V using a single power
supply)
4. Broad input voltage range (-0.3V to VCC+0.3V)
5. Range can be set freely (between 10 and 40dB)
using two external resistances
6. Reference voltage applied to O.COM pin can
be set over a broad range (1V to VCC-1.5V)
4. Configuration and summary of operation
4-1. Means to achieve high CMR characteristic and
circuit operation
As explained above, the machining precision of
ordinary monolithic ICs (with a resistance precision
of 2%) is such that a high CMR characteristic
cannot be easily obtained in an instrumentation
amplifier.
Fig. 1. Ordinary instrumentation amp
In Fig.1, in a circuit configuration with a gain of
40dB, a resistance precision of 0.1% is necessary
for a CMR of 100dB; for a gain of 20dB, the
precision must be 0.01%. Hence in this IC a circuit
configuration based on an entirely different
operating principle was employed. The approach is
simple: the transistor IC vs. VCE characteristic is a
constant-current characteristic not readily
dependent on the voltage. Hence the input signal
voltage is converted into a current signal in the
input unit, and the current component is passed to
the output circuit.
Fig.2. Basic circuit illustrating operation
Figure 2 shows the basic circuit.
A simple buffer amp is used to generate a
difference voltage for the input signal across the
resistance Rg, which determines the gain, and the
current I flowing in this resistance is passed
through a current mirror circuit before reaching Rs
of the output amp, to obtain an output voltage
I Rs.
The overall gain is Rs/Rg. The output from the amp
acting as an input buffer depends on two PNP
transistors. The first transistor is connected to one
end of the resistor Rg, a constant-current power
supply, and a feedback loop; the second PNP
transistor has an emitter area only one-half that of
the former transistor, and is connected to a current
mirror circuit and an output circuit.
By this means, an output VOUT is obtained
consisting of the reference voltage VCOM applied to
one of the input pins of the output amp, on which
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