MM1089 View Datasheet(PDF) - Mitsumi
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MM1089 Datasheet PDF : 9 Pages
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MITSUMI
Sensor Amplifier MM1089
is superposed the input difference voltage Rs/Rg.
The common mode level of the input signal does
not appear in the basic equation, meaning that an
amplifier with an inherently very high CMR
characteristic can be obtained. Of course in the
basic circuit considered here, because of the Early
effect of the transistors the current mirror circuit
operation will not be ideal, and the CMR
characteristic values are as yet insufficient. In the
actual circuit, cascade connections suppress the
Early effect, and a current mirror circuit with an
extremely small voltage dependence was adopted.
Further, a differential amp was not used as the
input buffer amp; instead, a simple configuration
was used to obtain the required characteristics.
Through these circuit designs, a CMR
characteristic in excess of 100dB using standard
values was achieved.
(4-2) Means to obtain a broad input voltage range,
and circuit operation
One unavoidable problem is the transistor VBE
voltage, so that an input circuit which can handle
all voltages from GND to VCC is inherently
impossible. If a resistance is used to attenuate the
input, a broad range can be achieved; but then the
input impedance cannot be kept high, deviating
from the original development goals.
In actual environments of use there are likely to be
extremely few signal sources with signals varying
continuously from GND to VCC, and so a design
was adopted in which it is possible to switch
between a mode with an input voltage range of
-0.3V to VCC-1.8 V, and a mode with input
voltages ranging from 1.8V to VCC+0.3V. A
switching pin was provided for this purpose.
Specifically, the NPN emitter follower has an input
circuit to shift the voltage in the negative direction,
and the PNP emitter follower has an input circuit to
shift the voltage in the positive direction; these are
switched during use. Because the input offset
voltage is different for the NPN and PNP inputs, in
applications requiring switching during operation
some special measures may be required for offset
correction.
(4-3) Output circuit and operation
A standard op-amp circuit configuration with a B-
class output stage is adopted. The potential
applied to the+input (O.COM pin) is output as the
reference potential.
Fig.3. Pinout
To summarize the block diagram of Fig.3 and pin
functions,
1. There are two circuits in a SOP-18P package.
2. Rg and Rs are external resistances. Rg is used
to set the sensitivity, with smaller resistances
yielding higher sensitivity. Rs is used to set the
output scale; to obtain a larger output range,
choose a higher resistance.
Rs/Rg is the total gain. In actuality, there is a
degree of error, and so a coefficient K is
included (cf. Fig. 6).
3. The common voltage of the output circuit is
applied to the O.COM pin; when the differential
input is zero, the common voltage is output
without modification (of course the offset
voltage is added).
When there is a differential input, the output
voltage VO is
VO=Vd Rs/Rg K+VC+Vof
where Vd is the differential input voltage, VC is
the common voltage applied to the O.COM pin,
and Vof is the offset voltage. On startup the
offset amount is determined automatically, and
when adding correction VCOM less the offset
voltage is applied.
4. Input range switching pins
When the low potential is applied, the input
range from -0.3V to VCC-1.8V is covered,
when high, the input range is +1.8V to
VCC+0.3V.
Switching at TTL level is possible. However, it
should be noted that the input offset voltages
are different for the two ranges. Of course if the
IC is to be used fixed at one range, the
switching pin can be shorted to GND or to VCC.
5. Major performance parameters
1. Differential gain vs CMR characteristic
Differential gain vs CMR appears in Fig. 4.
When the gain of an ordinary instrumentation
amp is lowered the CMR generally falls, but in
this IC there is almost no change.
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