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FIGURE 6. Limitation of the Output of an Amplifier
The introduction of operational amplifiers with output Rail-to-
rail drive capabilities is a strong improvement and the (output)
performance of op amps is for many applications no longer a
limiting factor. For example, National Semiconductors
LMP7701 (a typical rail-to-rail op amp), has an output drive
capability of only 50 mV over all temperatures for a 10 kΩ load
resistance. This is close to the lower supply voltage rail.
However, for true zero output applications with a single sup-
ply, the saturation voltage of the output stage is still a limiting
factor. This limitation has a negative impact on the function-
ality of true zero output applications. This is illustrated in
Figure 7.
FIGURE 7. Output Limitation for Single Supply True Zero
Output Aapplication
In the following section, two applications will be discussed,
showing the limitations of the output stage of an op amp in a
single supply configuration.
•
A single stage true zero amplifier, with a 12 bit ADC back
end.
•
A dual stage true zero amplifier, with a 12 bit ADC back
end.
One-stage, Single Supply True Zero Amplifier
This application shows a sensor with a DC output signal, am-
plified by a single supply op amp. The output voltage of the
op amp is converted to the digital domain using an Analog to
Digital Converter (ADC). Figure 8 shows the basic setup of
this application.
FIGURE 8. Sensor with DC Output and a Single Supply
The sensor has a DC output signal that is amplified by the op
amp. For an optimal signal-to-noise ratio, the output voltage
swing of the op amp should be matched to the input voltage
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range of the Analog to Digital Converter (ADC). For the high
side of the range this can be done by adjusting the gain of the
op amp. However, the low side of the range can't be adjusted
and is affected by the output swing of the op amp.
Example:
Assume the output voltage range of the sensor is 0 to 90 mV.
The available op amp is a LMP7701, using a 0/+5V supply
voltage, having an output drive of 50 mV from both rails. This
results in an output range of 50 mV to 4.95V.
Let choose two resistors values for R
a gain of 50x. The output of the LMP7701 should swing from
0 mV to 4.5V. The higher value is no problem, however the
lower swing is limited by the output of the LM7701 and won't
go below 50 mV instead of the desired 0V, causing a non-
linearity in the sensor reading. When using a 12 bit ADC, and
a reference voltage of 5 Volt (having an ADC step size of ap-
proximate 1.2 mV), the output saturation results in a loss of
the lower 40 quantization levels of the ADCs dynamic range.
Two-Stage, Single Supply True Zero Amplifier
This sensor application produces a DC signal, amplified by a
two cascaded op amps, having a single supply. The output
voltage of the second op amp is converted to the digital do-
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main. Figure 9 shows the basic setup of this application.
FIGURE 9. Sensor with DC Output and a 2-Stage, Single
11
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Op Amp
and R
that result in
G1
F1
Supply Op Amp.
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