The sensor generates a DC output signal. In this case, a DC
coupled, 2-stage amplifier is used. The output voltage swing
of the second op amp should me matched to the input voltage
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 drive of the op amp.
Example:
Assume; the output voltage range of the sensor is 0 to 90 mV.
The available op amp is a LMP7702 (Dual LMP7701 op amp)
that can be used for A
1
and A
2
. The op amp is using a 0/+5V
supply voltage, having an output drive of 50mV from both
rails. This results in an output range of 50 mV to 4.95V for
each individual amplifier.
Let choose two resistors values for R
G1
and R
F1
that result in
a gain of 10x for the first stage (A
1
) and a gain of 5x for the
second stage (A
2
) The output of the A
2
in the LMP7702 should
swing from 0V to 4.5 Volt. This swing is limited by the 2 dif-
ferent factors:
1.
The high voltage swing is no problem; however the low
voltage swing is limited by the output saturation voltage
of A
2
from the LM7702 and won’t go below 50mV instead
of the desired 0V.
2.
Another effect has more impact. The output saturation
voltage of the first stage will cause an offset for the input
of the second stage. This offset of A
1
is amplified by the
gain of the second stage (10x in this example), resulting
in an output offset voltage of 500mV. This is significantly
more that the 50 mV (V
DSAT
) of A
2
.
When using a 12 bit ADC, and a reference voltage of 5 Volt
(having an ADC step size of approximate 1.2 mV), the output
saturation results in a loss of the lower 400 quantization levels
of the ADCs dynamic range. This will cause a major non-lin-
earity in the sensor reading.
Dual Supply, True Zero Amplifiers
The limitations of the output stage of the op amp, as indicated
in both examples, can be omitted by using a dual supply op
amp. The output stage of the used op amp can then still swing
from 50 mV of the supply rails. However, the functional output
range of the op amp is now from ground level to a value near
the positive supply rail. Figure 10 shows the output drive of
an amplifier in a true zero output voltage application.
20173042
FIGURE 10. Amplifier output drive with a dual supply
Disadvantages of this solution are:
•
The usage of a dual supply instead of a simple single
supply is more expensive.
•
A dual supply voltage for the op amps requires parts that
can handle a larger operating range for the supply voltage.
If the op amps used in the current solution can’t handle
this, a redesign can be required.
A better solution is to use the LM7705. This low noise negative
bias generator has some major advantages with respect to a
dual supply solution:
•
Operates with only a single positive supply, and is
therefore a much cheaper solution.
•
The LM7705 generates a negative supply voltage of only
−0.23V. This is more than enough to create a True-zero
output for most op amps.
•
In many applications, this “small” extension of the supply
voltage range can be within the abs max rating for many
op amps, so an expensive redesign is not necessary.
In the next section a typical amplifier application will be eval-
uated. The performance of an amplifier will be measured in a
single supply configuration. The results will be compared with
an amplifier using a LM7705 supplying a negative voltage to
the bias pin.
TYPICAL AMPLIFIER APPLICATION
This section shows the measurement results of a true zero
output amplifier application with an analog to digital converter
(ADC) used as back-end. The biasing of the op amp can be
done in two ways:
•
A single supply configuration
•
A single supply in combination with the LM7705, extending
the negative supply from ground level to a fixed -0.23
Voltage.
Basic Setup
The basic setup of this true zero output amplifier is given in
Figure 11. The LMP7701 op amp is configured as a voltage
follower to demonstrate the output limitation, due to the sat-
uration of the output stage. The negative power supply pin of
the op amp can be connected to ground level or to the output
of the negative bias generator, to demonstrate the V
DSAT
ef-
fect at the output voltage range.
20173043
FIGURE 11. Typical True Zero Output Voltage Application
with/without LM7705
The output voltage of the LMP7701 is converted to the digital
domain using an ADC122S021. This is an 12 bit analog to
digital converter with a serial data output. Data processing
and graphical displaying is done with a computer. The nega-
tive power supply pin of the op amp can be connected to
ground level or to the output of the negative bias generator,
to demonstrate the effect at the output voltage range of the
op amp.
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LM7705
