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I've built a DC signal mixer that doesn't quite work as expected. The supply voltage is 9V, virtual ground at 4.5V and the mixer uses a TL074 quad op amp. While the input voltage is 0-9V, the output voltage is 1.3-9V. How can I remove that 1.3V offset? I tried biasing the input and the output with no success so far.

enter image description here

Any ideas why? I need my signal to 'sit'at 0V at minimum.

https://www.circuitlab.com/circuit/wvu5n3/dc-mixer/

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    \$\begingroup\$ It is not clear what exactly your power voltages are and how they are applied. You show two different ground symbols, which creates more confusion. You say the opamps are powered from a positive supply only, but yet the positive inputs are grounded. You say TL074 but the schematic says TL081. Analyzing this is pointless because we can't tell what to believe. Stop the hand waving and show what your circuit really is. Also, you need to read the datasheet. Take a look at the large headroom required by these opamps. \$\endgroup\$ – Olin Lathrop Sep 15 '13 at 12:11
  • \$\begingroup\$ If you don't have a -9V supply, this won't work (I think) \$\endgroup\$ – pjc50 Sep 15 '13 at 12:22
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If your supply is 9V and you have created a midrail at 4.5V that is a good start however, to expect any op-amp to produce an output voltage that drops down to the 0V rail is naive. OK that's being a little harsh for newbies in electronics.

If you look at page 7 of this document, about halfway down is the spec of the peak voltage swing and typically this is +/-13.5V. Note the power supply voltage is +/-15V and what it's telling you is that the op-amp cannot swing to within 1.5V of the power rails.

You can get better op-amps - these are usually referred to as "rail to rail" devices because they can swing their outputs (and usually their inputs) to very close to both supply rails. Make sure you pick one that can operate from your 9V supply - a lot of them are intended for 5V applications.

The LT1677 is a good example - it can swing its outputs to about 200mV of either supply but there are probably better ones that you will find that achieve maybe 50mV.

If you really need 0.0mV swing on the output then you should consider haveing a negative supply.

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    \$\begingroup\$ The LM324 is close enough, 0.1V with 9V supply. Luckily I have a bunch of them. Thanks for the help gents! \$\endgroup\$ – alkopop79 Sep 15 '13 at 13:47
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The issue is that TL07x is not a rail-to-rail opamp... From the documentation from Texas instruments:

TL 07x series output voltage vs supply voltage

As you can see, the single 9V supply is +-4.5V here, translates roughly to +-3V output, which just about describes the situation you're in.

Andy aka beat me on this... I blame IrfaneView :)

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  • \$\begingroup\$ I likes Irfan view (I also use it at work) \$\endgroup\$ – Andy aka Sep 15 '13 at 12:41
  • \$\begingroup\$ @Andyaka I like it too, but I have a lot of clutter icons on my desktop, and I couldn't find it. So of course Irfanview is to blame, not me, who can't keep an order on the desktop... :) \$\endgroup\$ – ppeterka Sep 15 '13 at 12:42
  • \$\begingroup\$ I use paint for cropping quick screen shots. I still prefer that for this type of work but irfan is cool. \$\endgroup\$ – Andy aka Sep 15 '13 at 13:00

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