8
ANALOG ELECTRONICS
8.1
VOLTAGE DIVIDERS
If you need to measure a signal whos span is greater than the input span of an analog or digital input, use a voltage divider to drop the
voltage of the input signal to fall within the range values of the analog or digital input.
A voltage divider takes advantage of Ohm's law, which states,
Voltage = Current * Resistance ( V = I * R)
and Kirkoff's voltage law which states,
The sum of the voltage drops around a circuit will be equal to the voltage drop across the entire circuit.
Thus, any variation in the voltage drop for the circuit as a whole
will have a proportional variation in all the voltage drops in the
circuit.
In a voltage divider, the voltage across one of the resistors in a
circuit is proportional to the voltage across the total resistance in
the circuit.
The object in using a voltage divider is to choose two resistors
with the proper proportions relative to the full scale of the analog
or digital input and the maximum signal voltage.
The action of dropping the voltage proportionally is often called
attenuation. The formula for attenuation is:
For a given attenuation, pick a suitable resistor and call it R2, the use this formula to
calculate R1.
R1 = (A-1) * R2
For example, if the signal varies between 0 and 20 volts and you wish to measure that
with an analog input with a full scale range of 0 to 10 volts, the attenuation is 2:1 or
just 2.
2 = 10K + 10K
10K
The variable Attenuation is the proportional difference between the signal voltage max
and the full scale of the analog input.
Attenuation = R1 + R2
R2
Digital inputs often require the use of voltage dividers. For example, suppose you wish to measure a digital signal that is at 0 volts
when OFF and 24 volts when ON. You cannot connect such a high voltage directly to the CIO-AD digital inputs. The voltage must be
dropped to 5 volts maximum when ON. The attenuation must be 24:5 or 4.8. Use the equation above to find an appropriate R1 if R2
is 10K. Remember that a TTL input is ‘ON’ when the input voltage is greater than 2.5 volts.
R1 = (4.8- 1) * 10K
R1 = 38 Kohms
4.8 = (38K + 10K)
10K
IMPORTANT NOTE: The resistors, R1 and R2, are going to dissipate all the power in the divider circuit according to the equation
Current = Voltage / Resistance, and Power = Current squared * Resistance (W = I2R). The higher the value of the resistance (R1 +
R2) the less power dissipated by the divider circuit.
For attenuation of 5:1 or less, no resistor should be less than 10K.
-20-
SIGNAL HIGH
SIGNAL LOW
R1
R2
A/D BOARD
HIGH INPUT
A/D BOARD
LOW INPUT
SIGNAL
VOLTS
V1
V2
Vout
Vin
=
R1 + R2
R2
SIMPLE VOLTAGE DIVIDER
Vin
Vout
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