LM 2907

Hi,

I am sorry, maybe you need to change you forum to the "amplifiers", thank you!

Regards, Sunny.

Yes, the LM2907-8 pin is internally set at ground. The input must cross through ground by about 30mV in order to count. To save yourself from using the extra -3V rail, you can also try AC coupling through a capacitor.

Tim

Dear  Tim,

Thank you for your response. Below you will see my input signal to the LM 2907. The center dotted line is 0 Vdc. As you can see it is not a square wave. I don't know why it is like this but if you think this is the problem I will try buffering it.

I don't know what you mean by AC coupling through a cap. Coupling what? The input?

What should the output be, a DC voltage, a PWM signal?

I believe the input signal into the LM2907 might be causing the problem. Can you try inputting a clean square or sine wave from a f/n generator into the LM2907 to see if that solves the issue?

When I mentioned AC coupling, I meant from the output of your sensor to the input of the LM2907, in order to ensure zero crossing, but that was assuming a sine wave, not a square wave, so you can disregard that statement. AC coupling a square wave would give exactly the input you show, which we are trying to avoid.

Tim

Good suggestion Tim. I will try it. Thank you. I'll keep you posted.

Hello Tim. I have a square wave from +4V to -4V at the input and the output still does not respond. It's at about -100 mV and does not change with frequency. Note the sawtooth on the output signal. It is also about 100 mV p-p. I tried changing out the chip and it didn't matter. I am using the "minimum component tachometer" circuit shown on page 9 of the datasheet.

The graph on Figure 19 in the datasheet shows that increasing frequency at the input will increase Vout and reduce the ripple at pin 3. You look like you are running at about 100Hz from what I can make out of your figure. Can you try increasing the input frequency? This should increase your Vout.

Tim

Can you also send your schematic? I'd like to double check it.

Tim, The circuit is here. I have increased the frequency but still no response at the output. Perhaps I have not chosen the right chip. I need to measure motor rpm down to 0.1 Hz. This must be converted to V dc or PWM to be used as an error value for PID motor controllers.

If you copied the schematic exactly, it should work to produce a DC output of ~1V per 67Hz at the input.

Some debugging that you might do below:

- Check the output when you remove the input signal. It should be 0V DC.

- Scope the signals at pin 2 and 3. Note that pin 3 is virtually tied to the output through the internal op amp feedback loop.

Tim

The output is 0V with or without an input signal. The only difference is that I see the ripple with the input connected. The signal at pin 2 is 0V with a similar ripple. Pin 3 follows the input exactly.

Tony,

Taking a second look at your scope shot from earlier, it looks like CH2 is in AC coupling mode. Put it in DC coupling mode. Let me know if that solves it.

Tim

Tim,

You are correct. I am sorry to have taken your time over something so stupid. Thank you but now it appears I have chosen the wrong device or perhaps I need to change circuit values. The output approaches 0V at about 6 Hz. I need to be able to resolve to 0.1 Hz. Any recommendations?

Tony

At low frequencies, your C2 cap would have to be very large to reduce the ripple at the output. You would see very slow response times in this situation. I would not recommend LM2907 for this low of a frequency.

I would use a microcontroller.

Tim

Dear Tim,

Sorry I couldn't get back to you yesterday. I have not been able to get frequency counter software to work with an Arduino. I also have a TI C2000 Piccolo. Can you recommend a program to measure frequency?

I'm sorry to have taken your time with my stupid scope mistake. Thank you very much for your help.

Tony

Unfortunately, I would not be able to help you with that. For software/microcontroller questions, please post your question on the appropriate Microcontrollers forum.

Regards,

Tim

OK. Thanks again for your help Tim.