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I need to buffer a 0-5V signal that is coming from a resistor network and being fed into an IC. When I feed it directly to the IC there is a loading effect and the value is changed specifically at 0V input the IC will source current out the input pin and produce a ~0.3V value.

My first thought was to use an Op Amp as a voltage follower. I have a TI-082 available and tried to use. I have a +24V and +5V sources available and using a single 24V supply for the op-amp works great for values from ~1.5-5V but anything under that and the output swings to the 24V supply voltage. I understand this is due to getting to close to the negative rail.

I next set up a +/-12V supply from my 24V source and everything in the 0-5V range was properly buffered, but the problem was I could no longer input it to the intended IC since the IC is powered by 24V and GND and the input is references to GND so to the IC the 0-5V was appearing as 12-17V.

Is there a way to make the op-amp go to 0V when hitting the negative rail? Forcing the bottom 1.5V to GND would be acceptable but output 24V is not. Alternatively, is there anyway to configure the +/-12V configuration to work despite the input being referenced to what would be the -12V line from the op-amp's perspective?

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    \$\begingroup\$ Use a Rail-to-Rail I/O op-amp. \$\endgroup\$ – Majenko Jun 27 '14 at 20:53
  • \$\begingroup\$ If it's a +/-12V supply why is the opamp powered from 24V and Gnd? It would be normal to power it from +12V and -12V; in other words connect the 5V -ve to the 0V centre tap on the PSU and consider THAT not the -ve rail, as GND. (Having chosen it as GND you have to be consistent about it, use it as GND for input signals, output signals, CPU, etc) \$\endgroup\$ – Brian Drummond Jun 27 '14 at 20:58
  • \$\begingroup\$ It's not a +/-12V supply, its a 24V supply. I split it to create the +/-12 while trying to find a solution. The voltage followed properly but then I couldn't connect it as needed because as you mentioned there were multiple "GND"s at that point \$\endgroup\$ – user1593858 Jun 27 '14 at 21:08
  • \$\begingroup\$ You need a supply that goes a little bit negative below your 0V and then use a rail-to-rail op-amp connected to this neg rail. I'd try and manufacture a +6V rail too - then it gives you more options with choosing the R2R op-amp. \$\endgroup\$ – Andy aka Jun 27 '14 at 21:11
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Here is a link to an article I wrote a while back about this class of problems.

A device such as the LM7705 or an inverting charge pump such as the LM2661 will generate the negative voltage needed for the amplifier to output 0 volts with 0 volts input

These devices will generate the negative voltage with reference to your circuit common.

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  • \$\begingroup\$ This describes exactly the problem I am trying to solve and unfortunately it looks like I will have to buy more hardware or settle for the resistive loading effect. \$\endgroup\$ – user1593858 Jun 30 '14 at 20:08

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