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Programmable Gain Using Digital Potentiometers
V OUT = – R BW x V IN
Programmable Ampli?er Gain Using a
Digital Potentiometer
Many sensors require their signal to be ampli?ed before
being converted to a digital representation. This signal gain
may be done with and operational ampli?er. Since all sensors
will have some variation in their operational characteristics,
it may be desireable to calibrate the gain of the operational
ampli?er to ensure an optimal output voltage range.
The ?gure below shows two inverting ampli?er with
programmable gain circuits. The generic circuit (a) where R 1 ,
R 2 , and Pot 1 can be used to tune the gain of the inverting
ampli?er, and the simpli?ed circuit (b) which removes
resistors R 1 and R 2 and just uses the digital potentiometers
R AW and R BW ratio to control the gain.
The simpli?ed circuit reduces the cost and board area but
there are trade-offs (for the same resistance and resolution),
Equation 1-1 shows how to calcultate the gain for the
simpli?ed circuit (Figure 1-1b). The gain is the ratio of the
digital potentiometers wiper position on the R AB resistor
ladder. As the wiper moves away from the midscale value,
the gain will either become greater then one (as wiper moves
towards Terminal A), or less then one (as wiper moves
towards Terminal B).
The device’s wiper resistance (R W ) is ignored for ?rst order
calculations. This is due to it being in series with the op amp
input resistance and the op amp input impedence is very
large.
Circuit Gain Equation
R AW
Using the R 1 and R 2 resistors allows the range of the gain
to be limited and therefore each digital potentiometer step
is a ?ne adjust within that range. While in the simpli?ed
R BW =
R AB
# of Resistors
x Wiper Code
circuit, the range is not limited and therefore each digital
potentiometer step causes a larger variation in the gain.
The feedback capacitor (C F ) is used for circuit stability.
Inverting Amplifier with Programmable Gain Circuits
Generic Circuit (a)
Pot 1
R AW = # of Resistors – Wiper Code x R AB
# of Resistors
V IN
R 1
A
B
R 2
W
C F
Op Amp (1)
V OUT
Simplified Circuit (b)
Pot 1
V IN
A
W
B
C F
Op Amp (1)
V OUT
Note 1: A general purpose op amp, such as the MCP6001.
Signal Chain Design Guide
13
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