The set up of my experiment so far:

A. I use a simple LM317 linear voltage regulator to supply constant voltage of +5V to Vdd of my Laser driver circuit.

enter image description here

B. Also I use another LM317 linear voltage regulator to supply voltage Vlda ( Up-to 12 volts ). Just the same way I shown in the previous figure , the source is from a 12V lead acid battery with 7.2 AmpHour rating.

C: The circuit of the Laser Driver:

enter image description here

** Questions:**

A : I don't get a clear pulse waveform and it seems very noisy. This is not the case if I directly supply voltage from a battery without the LM317 in between.

enter image description here

B : What is the difference between AGND ( analog ground ) and just Ground ( please see the highlighted areas in Laser Driver circuit ). I have shorted them as of now.

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    \$\begingroup\$ One comment: I don't know where you got that schematic, but whoever drew it didn't have any idea what they were doing. The LM317 needs at least 1.5V differential between the input and output (up to ~2V at 500 mA load current). With a 6V input and 5V output, the regulator won't be regulating at all, it'll just behave like a big diode. In other words, the absolute minimum voltage you can supply the LM317 with is 6.5V, with a minimum of 7 giving you a little bit of headroom. \$\endgroup\$ Apr 4, 2013 at 3:42
  • 1
    \$\begingroup\$ Right, and if you're using an LM317 to supply Vlda (12V) from a 12V battery (probably ~13.2V fully charged), you'll also have no regulation. \$\endgroup\$
    – HikeOnPast
    Apr 4, 2013 at 4:10
  • \$\begingroup\$ i cant see the circuits as i have to login... this is not good for the future of this question.. \$\endgroup\$
    – Spoon
    Apr 4, 2013 at 9:34
  • \$\begingroup\$ Ok, I was on mobile .. I can see it now I'm a desktop.. \$\endgroup\$
    – Spoon
    Apr 4, 2013 at 9:40

1 Answer 1


ICs that differentiate analog and digital ground generally do to to separate 'noisy' power circuitry from 'quiet' analog or digital circuitry. This allows you to keep ground places separated in the layout and allows for a single-point connection between power and signal ground, which is optimal from a noise standpoint.

The LM317 is not a low-dropout type of linear regulator. The attached datasheet has a set of curves on page 7 called "dropout voltage" that illustrates this point:

enter image description here

You're trying to control an output that's within 1 volt of input. The LM317 is not an appropriate choice for this function - it is not regulating anything, as earlier comments have stated.

  • \$\begingroup\$ Thanks for all the answers. My basic idea was to provide a regulated clean DC power supply to the Vdd and Vlda as mentioned above to my laser driver. I realize that LM317T is no good as of now. Is there any other better way to do this ? Any other regulator with a low noise and low drop out voltage ... \$\endgroup\$ Apr 4, 2013 at 17:21
  • \$\begingroup\$ How much current will be drawn? \$\endgroup\$ Apr 4, 2013 at 20:15

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