■ FIGURE 4. Supplying a reference voltage less than Vcc
can be done in a number of ways. The best use diodes for
better stability. Other methods are also possible.
■ FIGURE 5. A typical non-inverting op-amp. The gain, or
multiplication, is defined as: Vout = Vin (Ra+Rb)/Ra.
input divided by reference). So, if the reference is changed
to three volts (instead of Vcc which is five volts), you will
be able to use the full range of the A/D. You will need to
check on the reference specifications of your particular
A/D to see how low you can go. Ideally, you want the
reference to equal the maximum input signal.
Note that generally A/D converters are tolerant of
input voltages higher than the reference. They usually
return a maximum value but are not damaged. However,
virtually all A/Ds can be damaged by input voltages higher
than Vcc. Using a lower reference voltage provides some
safety margin should the input be larger than expected.
If your input is very small, you will need to add a
gain circuit which multiplies the signal by some factor.
Figure 5 shows a typical non-inverting op-amp gain
stage. The formula for finding the resistor ratios is: Vout =
Vin (Ra+Rb)/Ra. High resistor ratios (megohms) can result
in noise problems. Low resistor values (100s of ohms)
will cause relatively high current consumption. Gain
stages of 1,000 to 10,000 are reasonable if the
frequencies are low. For higher frequencies, multiple
■ FIGURE 6. Voltage summing is simple. Just be sure that
the resistor connected to ground is much less than the
input resistors so
that it will not limit
the current. The input
resistors do not have
to be the same. If
they are different,
the voltages will
Note that the output
is also divided by
the resistor to
■ FIGURE 3.
filtering the power
noise to the
op-amp. If the
supply is ground,
then the V-components should
and the op-amp
V- pin should be
stages will be
the slew rate
bandwidth product) of a single amplifier will be exceeded.
Adding or subtracting a voltage from your sensor is
not too difficult. Let’s assume your sensor produces an AC
wave of ±2.5 volts and you want to see zero to five volts.
Basically, you want to add 2.5 volts to the signal so that it
will always be positive.
You can sort of do that with just resistors as shown in
Figure 6. The result will be positive but not zero to five
volts. Rsum collects all the currents coming from the other
resistors and converts it to a voltage. Therefore, Rsum must
be much lower than the other resistors so that it will not
limit the current and give poor results. Typically, Rsum is
about 1% of the other resistors. The second point is that
the circuit acts as a voltage divider, as well as a summer.
So as diagramed, the output will be about 1% of the input.
The input resistors do not have to be the same. If they
are different, the lower-valued input resistor will have a
proportionally larger effect on the sum. To determine any
individual resistor ratio (see voltage dividers above to
calculate the actual voltage), assume all the other input
resistors are disconnected.
■ FIGURE 7. By adding a gain stage after the resistor
summer, the output can be scaled properly. The op-amp
gain setting resistors can be the same ratio as the
summing resistors. Many other variations are possible.