I am looking to build an absolute value circuit using a pair of op-amps, hopefully using a single supply and would like to simulate my design using TINA-TI.

The ultimate aim is to measure the voltage difference between an input and a reference voltage that is not necessarily ground. The modified LT1001 circuit in the answer to this question would be okay, but it is dual supply.

TI provide a useful application bulletin with various designs for absolute value circuits. Figure 5 in the application bulletin offers a nice single supply variant. I have reproduced it here in case of a stale link later.

I have entered the design into TINA-TI - I think without errors, but the simulation is not working with the output swinging immediately to mid-supply.

TINA-TI schematic entry

Output waveform

Why aren't I seeing the waveform as shown in the application bulletin?

Also at the bottom of the clip (below Figure 5.1) it contains the sentence "Notice that the input range of the circuit is 4V below the power supply rail.". What does this mean - the diagram shows the input to be -4V to +4V and the output 0 to 4V for a 5V supply?

  • \$\begingroup\$ Well, for one thing, your input voltage is substantially less than that used in the example design, where the input was an 8Vₚ₋ₚ sine wave. By 4V below the supply rail it means you can safely have an input of -4V; 4V below the negative supply rail (which is ground in this case). \$\endgroup\$ – Hearth Feb 2 '19 at 16:32
  • \$\begingroup\$ I'm also inclined to ask, where did you get these specific values for the resistors from? I haven't bothered doing any math, but it's possible your problem lies there. \$\endgroup\$ – Hearth Feb 2 '19 at 16:35
  • \$\begingroup\$ Thank you, that makes sense now (0V being a power supply rail in this case). The values come from a table at the back of the application bulletin. There are various options, I used the "Lowest Cost" one. Originally, I had an OPA2337 in the circuit but changed it as per the note under Figure 5.1 (both gave the same [wrong] output waveform). \$\endgroup\$ – iwbnwif Feb 2 '19 at 16:39
  • \$\begingroup\$ I have updated the output image to show the 8Vp-p input. The output remains the same. \$\endgroup\$ – iwbnwif Feb 2 '19 at 16:41

Found your problem.

enter image description here

See where the arrow's pointing? That indicates that your positive supply rail should connect there.

We all make that mistake. Who knows why the symbol has that on the bottom; I guess it's probably been flipped from its original orientation.

| improve this answer | |
  • \$\begingroup\$ I know it's not good SE etiquette, but really thanks for this! I had been staring at it for 2 hours without seeing that one. \$\endgroup\$ – iwbnwif Feb 2 '19 at 16:48
  • 2
    \$\begingroup\$ It's a mistake I've made countless times, and yet I still never notice it quickly. \$\endgroup\$ – Hearth Feb 2 '19 at 16:49
  • \$\begingroup\$ "Who knows why the symbol has that on the bottom; I guess it's probably been flipped from its original orientation." No, that's a characteristic of TINA. All op amps are that way. As far as I can tell, it's done that way to make it easier to position power supply voltages. A default voltage source will have the + at the top, and it can be installed just below the op amp. Likewise, the - pin of a source can be installed just above an op amp. I personally don't agree with the choice, but they didn't ask me. \$\endgroup\$ – WhatRoughBeast Feb 2 '19 at 18:32
  • \$\begingroup\$ @WhatRoughBeast I would agree with you that it seems like the wrong decision, but I suppose I can see why they did it if that was their thought process. \$\endgroup\$ – Hearth Feb 2 '19 at 19:31

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