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I am having an issue with DC gain for a non-inverting op amp circuit. All I am trying to do is double the voltage from two DAC outputs (DAC range is 0 to 5V and I want it to be ~0V to 10V). The DAC output is DC (<10Hz). The op amp I am using is the TI TL974 datasheet link: http://www.ti.com/lit/ds/symlink/tl974.pdf. I am using two 1kohm resistors in my FB loop to set my gain of x2. When I input 1V and 2.5V into the op amp I get the correct values out, 2V and 5V. But when I input 5V I only get ~7.5V out. This is a rail to rail op amp and the supply voltage is 10.5V so I would expect that I should get 10V out or really close to 10V.

The output source current spec for the op amp is ~1.2mA so I thought maybe my feedback resistors are too low a value so I tried 10kohm resistors and the problem got worse, went from 7.5V out to 7V out. My load is a benchtop DMM. I attached a 10kohm resistor as the load and measured the voltage across it and things got worse again. The output voltage dropped to ~6.7V. When I just connect the DMM as the load and I set its input Z to max the voltage goes up a bit. Seems like a loading problem but if that were the case then raising the value of the feedback resistors should have helped and not made things worse.

Any help would be greatly appreciated,

enter image description here

(More traditional schematic representation shown for clarity)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ It took a couple of minutes to understand that your circuit was using two op-amps in non-inverting mode. Using op-amp symbols would have shown the schema instantly. TimWescott's answer is correct. Note that the datasheet only claims rail-to-rail operation at ±2.5 V. See also Figure 14 in that document. Edit: I see Tim has addressed this in your post. \$\endgroup\$ – Transistor Dec 30 '18 at 19:53
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    \$\begingroup\$ I answered this and then had it pointed out to me that what I said was basically what you tried in your second paragraph, and deleted it. It's certainly weird. You probably don't want to use this amplifier for this application -- the input is most certainly not rail-rail, with the common-mode input range limited to being more than 1.3V away from the rails. See Figure 18 in the data sheet. \$\endgroup\$ – TimWescott Dec 30 '18 at 20:05
  • \$\begingroup\$ The data sheet shows a phase margin vs. output current plot that looks distinctly weird. Are you by chance driving a capacitive load with this? I'm wondering if maybe it's oscillating at high output voltages. Look at it with an O-scope & report back? \$\endgroup\$ – TimWescott Dec 30 '18 at 20:14
  • \$\begingroup\$ These do not support RRI nor are they Push-Pull Out so 2 reasons why you cannot use these. \$\endgroup\$ – Sunnyskyguy EE75 Dec 30 '18 at 20:19
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schematic

simulate this circuit – Schematic created using CircuitLab

These are not Rail to rail input and R5 pullup to high Z internal CC source, Iol=50mA

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  • \$\begingroup\$ Thanks Tony and everybody else who answered this post. In the future I need to read the datasheet a little closer. Also I will provide a better schematic for any future questions. \$\endgroup\$ – Neil ForceTronics Jan 3 at 4:28
  • \$\begingroup\$ Did anyone understand my solution? @TimWescott ? \$\endgroup\$ – Sunnyskyguy EE75 Jan 3 at 5:08

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