ngkdc
11-03-03, 07:41 AM
A while ago, someone had a question regarding A/D input protection. Needing to scale AND protect my A/D inputs, I came up with the attached schematic:
http://www.newmicros.com/isopod/appnotes/atodin.bmp
The ICs are LM-6134s that will operate on a supply voltage as low as 2.7 volts, and have rail-to-rail (VCC-.01 to VDD +.01) output drive capability. Further, they will withstand inputs higher than the supply voltage without failing.
The resistor networks are made up of a 2.32 K resistor and a 3.48K resistor. These resistors are connected in series, the 0-5 volt input connected to the 2.32 K resistor, the junction of the two goes to the op-amp input, and the bottom of the 3.48 K resistor is tied to ground (either VDD, or the Analog Input ground reference, depending on your circuitry). With 5 volts applied to the resistor network, you will develop 3 volts across the 3.48K resistor.
In those rare cases where the input might rise above 5 volts, the op-amp will prevent destruction of the processor chip (yes, I did test this ... accidentally, with no ill effects!).
The op-amp is powered from the 3.3 volt regulated supply. I had tried running it from the A/D reference, but this dragged the A/D reference down in an unpredictable manner ... so I changed it to the 3.3 volt supply. Bypass capacitors (0.1 uF) are not shown, but exist across each op-amp's power terminals.
Hope this helps.
Rick
http://www.newmicros.com/isopod/appnotes/atodin.bmp
The ICs are LM-6134s that will operate on a supply voltage as low as 2.7 volts, and have rail-to-rail (VCC-.01 to VDD +.01) output drive capability. Further, they will withstand inputs higher than the supply voltage without failing.
The resistor networks are made up of a 2.32 K resistor and a 3.48K resistor. These resistors are connected in series, the 0-5 volt input connected to the 2.32 K resistor, the junction of the two goes to the op-amp input, and the bottom of the 3.48 K resistor is tied to ground (either VDD, or the Analog Input ground reference, depending on your circuitry). With 5 volts applied to the resistor network, you will develop 3 volts across the 3.48K resistor.
In those rare cases where the input might rise above 5 volts, the op-amp will prevent destruction of the processor chip (yes, I did test this ... accidentally, with no ill effects!).
The op-amp is powered from the 3.3 volt regulated supply. I had tried running it from the A/D reference, but this dragged the A/D reference down in an unpredictable manner ... so I changed it to the 3.3 volt supply. Bypass capacitors (0.1 uF) are not shown, but exist across each op-amp's power terminals.
Hope this helps.
Rick