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I am using an STMicro TL084 Opamp in a buffer format and wondering why i keep hitting the Open Circuit +12V voltage rail.

There are 2 possible connections in the below image, the first being the external 12V connector that has a 4-20mA signal. This actually works fine when its plugged in and the target voltages are met. When this was disconnected I found that the ADC-OUT would ramp up to 12V so I started playing around with a pullup resistor to see what current was required before it began to act as a buffer again. Using my resistor decade box, a 1k pullup still had the output regulating at ~12V. When I moved it to a stronger pullup (only steps I have on it), it started to regulate at 3.3V correctly.

Note that both the 3.3V pullup and 12V pullups won't be connected at the same time, the image is purely for a connection diagram. enter image description here

Larry
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  • Why are you using a pull-up, R27? for a 4 - 20 mA input R27 should not be there. Why is the output labelled "ADC-OUT"? – Transistor Oct 10 '18 at 06:11
  • Where is the 12V pull-up? – Marko Buršič Oct 10 '18 at 06:29
  • R27 is only to show what solved the open circuit current output, it is not actually in the circuit itself. The ADC-OUT label is to identify that this is to be read by an adc pin on the micro.. The "12V pullup is – Larry Oct 10 '18 at 21:14

2 Answers2

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The TL084 has an input voltage range that you are exceeding. Read the data sheet in section 6.5 and it tells you that the input common-mode range (when using +/- 15 volt rails) is typically -12 volts + +15 volts and this means any signal getting within 3 volts of the negative rail is going to cause problems and unpredictability.

Your op-amp runs from +12 volts and 0 volts: -

enter image description here

So, any input signal below +3 volts is going to give problems. Use a negative rail or a better op-amp.

Andy aka
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  • So effectively the specs (trying to translate in my mind) state that in the non-rail to rail opamp, that the input voltage must be 3V above the lowest rail. That is right now Gnd. Similary, if the Vss rail was set at -2V and I am feed in a 0.5V signal that was also be out of spec? It would require at least -3V on the Vss rail to meet the lower spec unless a Rail-Rail opamp was used? – Larry Oct 11 '18 at 03:40
  • @Larry correct. – Andy aka Oct 11 '18 at 07:30
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Like Andy says, you're outside the input voltage range, which is called common mode voltage in datasheets. This is actually extremely common mistake to make when you realize those ideal components were not in stock this week.

LM358 and derivatives (e.g. LM2904) should work fine for you, they go down to negative supply, GND in your case. Note that they have limitation on the POSITIVE rail, so you can't go above 10V or so in your case. Rail-to-rail opamps can do both but they're slower and more expensive, probably not a concern for you. These come in RRI(nput), RRO(utput) and RRIO varieties.

Your circuit does not work properly with that pull-up. You're effectively creating a bias voltage divider that makes the circuit much harder to figure out. Why do you need 12V output if your unity-gain opamp only goes up to 2.4V in normal use? You could use a 3.3V with RRIO opamp. Or you could limit the input voltage to < 1.3V by reducing the resistor size and adding amplification to the opamp to produce the same output.

Barleyman
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