# How can I convert 9 volts DC to 1 millivolt DC?

I need to be able to convert 9 volts to 1 millivolt. Is there a schematic/circuit that can do this? If so, could you please provide one or details on how to make one? Thank you!!!

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seandogue2 years ago

You can hack a mV source using a precision voltage reference and a quality buffer-opamp.

9V => precision-ref => V-divider => opamp

iceng2 years ago

You don't want a millivolt because Johnston noise becomes significant at room temperature.

2 years ago

"Maaaa- mie, Maaa-mie. "

That Johnston noise ?

2 years ago

Testing credolity

2 years ago

Sorry its Johnson thermal noise.

I'm very distracted by my wife's medical drama

http://www.daycounter.com/Calculators/Thermal-Nois...

https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyqu...

verence2 years ago

You want a regulated 1mV source?!?* What current? What for?

vak52 (author)  verence2 years ago
Hello!
Thank you for your reply. I need this regulated 1mV source for some brain stimulation experiments I am doing. I am planning on using a normal 9V battery, which is typically 500mAH. I'm not entirely sure how much current i want to be drawn out of it, maybe like 30 mA.

Hope this clears some things up.

Thanks
2 years ago

Unless you drill directly into the brain and place the electrodes there, 1mV will do nothing. Proof: Put your finger across the terminals of a 9V (that's 9000mV) battery - effect: none. And even directly inside the brain tissue, I'd guess that 1mV will do much. Chemical reactions need a certain voltage potential.

To drive 30mA from 1mV, there must not be more than 33mOhm resistance in the whole circuit of contacts, wires, electrodes and the brain tissue in question (ignoring the sources inner resistance by assuming that it is regulated).

And of course iceng is right about the thermal noise. Not to forget the electromagnetic noise your circuit will pick up.

Tell us, what you really want to do and maybe we can help you.

2 years ago

You want to encourage someone to pass current through their brain ?

Hmm. Actually, it might be fun to watch.

kmossman2 years ago

Maybe the most simple approach would be to use a voltage divider

https://learn.sparkfun.com/tutorials/voltage-divid...

If you want it regulated you can use a regulator like the 7805

http://www.engineersgarage.com/sites/default/files... [use Figure 8]

OR

a resistor and silicon diode [1N4001] in series.

Then use a voltage divider to get 1mV.

BTW it is Johnson–Nyquist noise or Johnson noise, and it is not that significant relative to 1mV. from Wiki, "For a 1 kΩ resistor at room temperature and a 10 kHz bandwidth, the RMS noise voltage is 400 nV.[5] A useful rule of thumb to remember is that 50 Ω at 1 Hz bandwidth correspond to 1 nV noise at room temperature."

2 years ago

BTW if you use a 7805 then the input voltage to the resistor divider is 5V

If you use a resistor and Si diode then the input voltage to the resistor divider is about 0.6V