Figure 6-1. Voltage Divider
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.
Dropping the voltage proportionally is often called attenuation. The formula for attenuation is:
| The variable Attenuation is the proportional |
Attenuation = R1 + R2 | difference between the signal voltage max and the |
R2 | full scale of the analog input. |
| For example, if the signal varies between 0 and 20 |
2 = 10K + 10K | volts and you wish to measure that with an analog |
10K | input with a full scale range of 0 to 10 volts, the |
| Attenuation is 2:1 or just 2. |
R1 = | For a given attenuation, pick a handy resistor and |
| call it R2, the use this formula to calculate R1. |
Digital inputs also make use of voltage dividers, for example, if you wish to measure a digital signal that is at 0 volts when off and 24 volts when on, you cannot connect that directly to the CIO-DAS16 digital inputs. The voltage must be dropped to 5 volts max when on. The Attenuation is 24:5 or 4.8. Use the equation above to find an appropriate R1 if R2 is 1K. Remember that a TTL input is 'on' when the input voltage is greater than 2.5 volts.
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