Sometimes, when we’re interfacing an analog signal with a microcontroller,
the analog voltage doesn’t match the range of the analog-to-digital
converter. For instance, the signal might swing 100 mV, but the converter is
referenced to 5V - the converter would only use 2% of it’s range.
To match the signal to the converter, we can build an amplifier circuit that
boosts the incoming voltage. This circuit is known as a “conditioning
amplifier” or “pre-amplifier” (“preamp” for short).
One simple preamp is an operational amplifier wired in non-inverting mode.
The gain of the amplifier is configured with a pair of resistors.
The gain factor for the amplifier is
1+(Rf/Rs)
For a given pair or resistors, we can calculate the gain factor - for example,
if both are 10K
Ω
, the equation works out to
1+(10,000/10,000)
Which simplifies to 2 - the output of the amplifier will be the input voltage
multiplied by 2. To continue the example above, if 100mV go in, then 200
mV come out.
Sometimes when we start designing, we don’t know the gain factor we
need for an application. When building an amplifier to suit a specific
application, it can be useful to have a preamp with adjustable gain. We can
build one that uses a decade box for the feedback resistor, and selecting a
shunt resistor that gives us a reasonable gain range.
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