# Voltage Level Shift?

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#### VICI

##### New Member
Hi, I'm an undergrad (junior-to-be) EE student with a research project this summer and I was wondering if anyone could help me out.
I am trying to convert the frequency signal of my anemometer to a
voltage signal, but my frequency to voltage chip (LM2907) requires a +/-
15mV swing on either side of the inverting input. My input voltage is
currently 0-5v. I think I need a voltage level shift circuit(?) to shift my input voltage slightly negative. Can anyone help me out with that? Or recommend good webpages (or another name I could look up in my textbooks?)
Thanks Very Much!

#### audioguru

##### Well-Known Member
You have the 8-pins IC which has hysteresis and its input DC reference voltage is 0V. You could capacitor-couple the signal to the existing IC.

The 14-pins one can have its input DC reference voltage what ever you want.
Here are the notes from the datasheet:

#### Attachments

• LM2907-2917 notes.PNG
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#### VICI

##### New Member
Yes, except the 8-pin ICs were free, and the university beaurocracy didn't have to approve it first. I only have 2 weeks or so to finish what I have.
I'm not sure I understand capacitor coupling the anemometer signal. I wish I had a scanner to show my circuit layout for the chip. But I really just want to know about this "level-shift" ordeal. It's something with transistors to literally shift the voltage (0 = -1, 1 = 0, etc) but I can't find it in my books.

#### audioguru

##### Well-Known Member
Like I said, use a coupling capacitor for your level shifting.
A coupling capacitor can couple an AC signal from one DC voltage to any other DC voltage.

#### Attachments

• Level shift.PNG
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#### Russlk

##### New Member
The anemometer frequency could be quite low, so use large C and R (100uF, 100K).

#### almotions

##### New Member
can anybody give me a schmatic to shift an AC voltage of 5 Vp-p to fit into 0-5V range with 2.5V as the offset?

#### Nigel Goodwin

##### Super Moderator
A coupling capacitor and two resistors (forming a potential divider across the supply).

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