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I am looking for an operational amplifier which will fit my requirements.

My circuit consists of two cascaded non-inverting amplifiers.I have a classical non-inverting setup.The minus pin of both stages is at ground potential. My positive input to the first stage is a voltage that can be in uV region, therefore the op-amp needs to be able to operate as expected at this level.

I have a single supply voltage of +3.3v, no split rail, my input is always a positive voltage.

I need the amp to have the following characteristics:

Low noise, Ultra low offset, Low power, True Rail/Rail IO, Should work in a single supply of 3.3V

Any suggestions for an op-amp for my requirements? I have tried a MAX4239 but it seems not to work as expected when the positive input is only a few uV, the output load is a comparator input and an ADC input. VGND in schematic is GND

enter image description here

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  • \$\begingroup\$ My standard warning: Rail-to-Rail is a marketing term, not a technical one. You will not likely find a true R2R in & out op-amp that works as you hope. You must study the datasheets very closely. \$\endgroup\$ Oct 27, 2020 at 20:05
  • \$\begingroup\$ Ok, I can afford to lose some range on the bottom, if I made a virtual GND 100mV above ground, would this be enough away from the rail? \$\endgroup\$ Oct 27, 2020 at 20:09
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    \$\begingroup\$ First, gain an appreciation of what is being asked. Start here: electronicdesign.com/technologies/analog/article/21806660/… Then read more Bob Pease. \$\endgroup\$ Oct 27, 2020 at 20:20
  • \$\begingroup\$ "The minus pin of both stages is at ground potential" - totally ambiguous - don't use baby terms use proper terms. Presumably you mean the negative power supply pins? Also, show a schematic. \$\endgroup\$
    – Andy aka
    Oct 27, 2020 at 21:01
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    \$\begingroup\$ My circuit consists of two cascaded non-inverting amplifiers.I have a classical non-inverting setup. My text-to-circuit converter is broken so show the schematic. Asking for components is a "shopping question" which is Off topic. \$\endgroup\$ Oct 27, 2020 at 21:05

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Figuring out why the MAX4239 doesn't work for you is found in the datasheet. There's a line in the table called Output Voltage Swing. Since you're dealing with values close to your negative rail, you're concerned with VOL:

enter image description here

I think you're going to have trouble finding anything that swings less than 1mV. This is because rail-to-rail output stages are built using BJT common-emitter or FET common-source circuits, which have a non-zero \$V_{CE(sat)}\$ and \$R_{DS(on)}\$ respectively. As mentioned in the comments of your question, this is why you cannot achieve "true rail-to-rail output".

So sounds like you're either going to need a bias or a negative rail to stay out of the saturation range. One way to bias (out of several) is a technique highlighted in Chapter 6 of Analog Devices' free book called "Op Amp Applications" under Figure 6-117 which I've pasted below.

enter image description here

As for negative rails, they can be troublesome to generate cleanly if you don't already have one available. One example of a solution is the LM7705 from Texas Instruments which is designed specifically for problems like yours. However, this still might be too noisy for µV level signals so perhaps a bias generated from a tightly controlled voltage reference might be a better fit for you.

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  • \$\begingroup\$ Thank you, I have a better understanding now, would it be acceptable if I biased the VGND net at around 50mV? I would of course ensure the input would only be equal to or above 50mV \$\endgroup\$ Oct 28, 2020 at 8:45
  • \$\begingroup\$ @EoinOConnell I don't think that's going to work how you intend it to. A virtual ground like that only works if you're able to AC couple the signal into/out-of each stage. I highly recommend using a circuit simulator to check things like this; either LTspice or the built-in ngspice engine within KiCad. Also, if you are able to ensure the input stays above 50mV, then you shouldn't need to change anything in the first place. Since your amplifier gains are 10, even 5mV would work because the output of the first amp will always be above 50mV which is above VOL. \$\endgroup\$
    – Ste Kulov
    Oct 28, 2020 at 9:03

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