Electronics > Projects, Designs, and Technical Stuff

Is 30mV still acceptable as the lowest voltage for a linear power supply?

<< < (3/5) > >>

bloguetronica:
Hi,

I just wanted to let you know that I've assembled the new module, and now it works beautifully! When the module's output voltage is set to 0V, it now reads a residual voltage of 16uV instead of 30mV. This is much better than before. Pulling the output of the first half of the OPA2180 worked.

I should add that I've measured the output of the charge pump, and observed little noise. Probably due to the 22K resistor, the charge pump does not seem to have an appreciable contribution to noise at that point. The input of the charge pump is filtered, so its switching has little influence over the 5V rail (confirmed that too).

Anyway, I'll post more photos as soon as possible.

Kind regards, Samuel Lourenço

duak:
I recently found a problem with a circuit using an LM358 operated with its inputs and outputs operating near 0 V.  When connected to a PWM servo, it would generate a large rail-to-rail sawtooth output when it should be near zero.  With non zero input voltages, it worked fine.  Turned out that the general common noise noise (100 mVp-p, 16 kHz)  coupled to the inputs, outputs and power supply of the LM358 appeared to cause an internal stage to wind up and produce the output.  Improving the signal input and power supply filtering as well as adding series resistors on the outputs solved the problem.  The LM358 is an old part with some unusual characteristics that newer parts probably don't have. 

I'm only mentioning this because a lab type supply might be used in a noisy environment and be expected to provide a zero output.  This circuit uses a negative supply, so it's probably OK.  For a simple test, try injecting a square wave back into the supply through a capacitor while varying the output voltage to see what happens.

Cheers,

David Hess:
Another way this is sometimes done is to place a diode in series with the negative power input (for a positive regulator) to provide about -0.6 volts of bias.  The disadvantage of doing this is that it requires either the input or output to float.


--- Quote from: duak on April 10, 2019, 05:06:31 pm ---I recently found a problem with a circuit using an LM358 operated with its inputs and outputs operating near 0 V.  When connected to a PWM servo, it would generate a large rail-to-rail sawtooth output when it should be near zero.  With non zero input voltages, it worked fine.  Turned out that the general common noise noise (100 mVp-p, 16 kHz)  coupled to the inputs, outputs and power supply of the LM358 appeared to cause an internal stage to wind up and produce the output.  Improving the signal input and power supply filtering as well as adding series resistors on the outputs solved the problem.  The LM358 is an old part with some unusual characteristics that newer parts probably don't have.
--- End quote ---

The PNP emitter follower output of the 358/324 can only actively sink current down to about 0.6 volts.  Below that, a 50 microamp NPN current mirror operates down to the Vce(sat) of the NPN transistor.  What I suspect you saw was saturation of the 358/324 when the output could no longer sink enough current to maintain feedback; like most operational amplifiers, the 358/324 lacks protection against saturation which is something which distinguishes operational amplifiers from most comparators. (1)

Improved replacements for the 358/324 like the LT1013/LT1014 and of course rail-to-rail output parts can actively drive their outputs all the way to saturation.

(1) The comparator version of the 358/324 is the 393/339 and also lacks saturation protection which is unusual for a comparator but not so much for a slow one.  Faster comparators, even discrete ones, always include protection against saturation.

bloguetronica:

--- Quote from: duak on April 10, 2019, 05:06:31 pm ---I recently found a problem with a circuit using an LM358 operated with its inputs and outputs operating near 0 V.  When connected to a PWM servo, it would generate a large rail-to-rail sawtooth output when it should be near zero.  With non zero input voltages, it worked fine.  Turned out that the general common noise noise (100 mVp-p, 16 kHz)  coupled to the inputs, outputs and power supply of the LM358 appeared to cause an internal stage to wind up and produce the output.  Improving the signal input and power supply filtering as well as adding series resistors on the outputs solved the problem.  The LM358 is an old part with some unusual characteristics that newer parts probably don't have. 

I'm only mentioning this because a lab type supply might be used in a noisy environment and be expected to provide a zero output.  This circuit uses a negative supply, so it's probably OK.  For a simple test, try injecting a square wave back into the supply through a capacitor while varying the output voltage to see what happens.

Cheers,

--- End quote ---
Hi,

I should mention that this is not a two-quadrant power supply. It is single quadrant, and it is only able to source, and not sink, current. Therefore, if you inject a square wave signal while its output is at 0V, that square wave will appear.

Note that the negative "supply" that this module uses is just for biasing the output of the first half of the op-amp. It does not drive the output whatsoever.

Kind regards, Samuel Lourenço

bloguetronica:
Hi all,

Did some tests without load. Here are the measurements with the output voltage of the module set to 5, 10, 20, 50, 100, 200 and 500mV. This module is surprisingly precise. Or I was lucky with the components that I've got (resistors, DAC and op-amp). I see no appreciable offset at all!

Kind regards, Samuel Lourenço

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod