LM1815: Output rage of the IC is +V to -5V square wave for sine wave input

Dear Varney,

I will check and confirm you that.

Meantime, i have two more query.

1. In Page no. 9 of LM1815 datasheet, it is given that pin no.10 is not ensured to drive TTL Loads.  Is it wrong If I connect that pin to MCU for measuring input frequency?

2. I need only +ve pulse at the output of IC i.e pin no. 10, input to the IC is sine wave. is possible to get output? if yes. can you please suggest me what need to be done to get +ve pulse output ?

Mr. Kumar,

As you pointed out pin 10 is not designed to drive a TTL device. Yes you can connect pin 10 to your controller, if the output voltage of pin 10 is at the same voltage as the controller. However It will be up to you to determine the compatibility of pin 10 and your controller. 

You might want to use a buffer between pin 10 and the controller or use a CMOS to TTL converter. 

Hello Varney,

Thanks for answering my first question.

Also, Can i get an answer for 2nd one?

Mr. Kumar,

  I am still trying to understand your circuit. As I think I understand you are outputting a 2.7Vp-p from U16 to the R123 10k resistor for VIN. 

Based on the threshold detect, R123 should be (VIN peak)1.35V / (VIN Current) 0.003 = 450 Ohm. It's possible that you never reach the threshold with a 10k resistor.

Minimum Rext = [(Vin peak)/3mA]

If I do not understand your input signals properly please provide more information

Hi Varney,

I will explain my circuit. Flow_sen_CH1 is the input signal with the range (60 to 650) mVpp & (8 to 114) Hz.

Then by using U16 IC. We are amplifying the signal with approximately gain of 5. 

Output pin i.e. pin no.10 of LM1815 is connecting to MCU input capture pin for calculating the frequency of the input signal i.e. Flow_sen_CH1.

I hope this helps you to understand our circuit

Mr. Kumar,

  That helps a bit, thanks. That also confirms that with a 2.7Vp-p you will see a positive peak of 1.35V, R123 is set to 10k. You will never see the threshold crossing at that level. Change R123 to 450 Ohms. 

You can connect a scope to VIN pin and look at the levels, my guess is that you will not see a 25mV threshold crossing with R123 set to 10k.

Hi Varney,

Ok i will check and let you know.

But i did not understand the concept of threshold crossing 100%. Also can i get output of LM1815 only +ve pulse by doing that?

Mr. Kumar,

In my first response I copied a section explaining the threshold. If the threshold does not see an arming threshold crossing it will not provide an output. I expect that you will get an output when the part sees a threshold crossing.

But now also i am getting output. i.e square wave varying from -V to +V. But my question is can i get only +ve pulse?

Mr. Kumar,

  I am not sure that I understand what you are looking for. However connect a scope to pin 12 since it is your reference out maybe this is what you are looking for. It is an open collector so make sure you pull it up to VCC. 

Hi Varney,

waiting for your reply

Santhosh - 

You indicated in your other post that this circuit was working for you now that you changed the setting on your o'scope - can we close this post now as well?