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I'm currently working on a project where I need to implement a voltage divider followed by a buffer amplifier to scale down a voltage signal by a factor of ~10. However, I'm encountering some issues with the implementation and could use some guidance.

Here's what I've done so far:

I set up a voltage divider using a 10k-ohm resistor and a 1k-ohm resistor. Following the voltage divider, I've connected a buffer amplifier (op-amp configured as a voltage follower) to ensure minimal loading effects and maintain signal integrity.

I have two voltage regulator ICs (L7812CV & L7912ACV) that deliver the bipolar voltage supply that the op-amp (OPA227P) requires. I have connected the output of the resistive voltage divider to the non-inverting input of the op-amp. The output of the op-amp is also fed back to the inverting input. However, despite these steps, I'm not getting the expected output at the buffer amplifier's output.

For some reason, the output has this huge 50 Hz noise, much like you would get from a power supply. The voltage regulators are being fed from a 12 V DC power supply, which itself does not have this noise. The noise is only in the output of the op-amp, and only when it is powered. It exists even when there is no input signal being fed to the voltage divider and shows up as soon as I connect the power supply.

Could anyone provide some insight into what might be going wrong? Are there any common pitfalls or considerations I should be aware of when implementing a voltage divider with a buffer amplifier?

I am always unsure how to connect the grounds in such circuits - I have the grounds of the input and output connected while the supply voltage to the op-amp is referenced to the ground of the voltage regulator ICs. Is this correct?

Any help or suggestions would be greatly appreciated. Thank you!

enter image description here

Update

I implemented the suggestions, most of which were overlapping so I must thank all those of you who answered. It was all very helpful advice! I certainly learnt a lot, including how NOT to draw a schematic!

I added a potentiometer to allow for adjustable scaling and now I have a version which works exactly as intended! enter image description here

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    \$\begingroup\$ Please show a schematic diagram so we can see all your intended connections. \$\endgroup\$ Commented May 3 at 15:39
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    \$\begingroup\$ Decoupling capacitor very nearest power supply pins is also needed ... \$\endgroup\$
    – Antonio51
    Commented May 3 at 16:02
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    \$\begingroup\$ FYI a 10k and a 1k resistor do not make a 10:1 voltage divider. They make an 11:1 voltage divider. For a 10:1 voltage divider use a 9k and a 1k. Or, to use more common resistors, use an 18k and a 2k. \$\endgroup\$ Commented May 3 at 17:19

3 Answers 3

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The quick answer is that all the grounds should be connected together for this circuit to work.

But you need to know the relationship of the 'ground' connection each of the 4 items you are connecting together (two supplies, input and output) to each other, including indirectly through external earth connections, to be sure this won't cause undesirable things to happen.

For example, if your power supply had a low-impedance connection to earth that is 18VAC and your oscilloscope BNC shell is also earthed, connecting the two together could damage the probe, oscilloscope or power supply.


Now that you have added the schematic there is a glaring issue. You should have bypass capacitors on the inputs of both regulators and the output of the 7812. You must have an output capacitor on the 7912 since, unlike the 7812, it is not stable without it. Use an aluminum electrolytic of something like 47uF, not a ceramic capacitor†. You can use 47uF caps of appropriate voltage rating(s) for all four, preferably with additional 100nF ceramic capacitors near the op-amp supply pins (though the op-amp you have is not a particularly fast one, so that's more of a nice-to-have).

It's also a good idea to have a series resistor between the op-amp output/inverting input node and the BNC connector. Otherwise relatively small capacitive loads (more than 100pF or so) from whatever it is connected to and the cable can start to cause overshoot and even oscillation, typically in the low nF range. Something like 100Ω is fine.

† Sensitivity to output capacitance and and ESR is a common characteristic with most linear LDO regulators and most linear negative voltage regulators. Some older non-LDO regulators such as LM317 and the LM78xx, LM78Mxx, LM78Lxx series do not suffer from this issue (but have a higher dropout voltage).

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This is how your circuit should look:

schematic

simulate this circuit – Schematic created using CircuitLab

  1. The circuit is easy to read. Yours is a bit of a mess. Where possible, draw higher potential nodes at the top, lower potentials lower down. Signals should propagate left-to-right where possible. This makes it easier for others to understand at a glance what's going on. In particular, notice how the ground node extends right across the schematic, and clearly shows how the various elements all share this node, all operating with respect to this reference point of 0V potential.

  2. Supplies are ±15V, not ±12. This is because the regulators require inputs at least 2.5V beyond their outputs.

  3. There are capacitors either side of the regulators, installed physically close to the ICs themselves. These are probably not optional, read and comply with the datasheets. Recommended capacitor values may differ from what I've shown here.

  4. There are supply decoupling capacitors C5 and C6 installed close to the op-amp. These compensate for large current transients that occur when the op-amp output is required to change suddenly.

  5. The op-amp output is not connected to ground, as it is in your circuit. That's just a misunderstanding/mistake on your part, I'm sure. The output of this system is the potential difference between OUT and ground. The input to this system is the potential difference between IN and that same ground.

  6. All grounds are common, connected together. The 0V ground node is shared by the supplies, regulators, input divider, and op-amp output. This fixes the noise problem you are experiencing, because you have not defined or implemented a proper, common, ground.

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Your power supply common/ground must be connected to the voltage divider ground, and also be used as the ground reference for the input and output.

The schematic shows the op-amp output connected to the bottom of the voltage divider, and Vin connected to both ends of the voltage divider - this can't be right.

If your power supplies are 12 volts, the 7812 and 7912 regulators won't be regulating, as they require at least 2 volts "headroom" to operate - the actual supply voltages at the op-amp will be around +/- 10 volts.

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