• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

I can't get these 2 circuits to like eachother

Status
Not open for further replies.

bonanz

New Member
Okay i've done some playing around with this circuit and i cannot get it to work for my application. I have pin 26 grounded because it will not be measuring voltages. i am not using it as a typical dmm i'm using it to display a voltage in a circuit that will always be positive. I am using the LM2907 frequency to voltage converter below and have it configured so that the output voltage is linearly proportional to the input frequency (ie 200mv ~ 200hz) the max freqency i'd measure would be about 1200Hz which would correspond to 1.2v work perfectly on the 2v scale and i would just leave the decimal points unconnected. I have hooked up the DVM circuit, and did a test on a voltage divider to see if it was measuring correctly and it did measure the voltage, but i only had to connect terminal one to the voltage point and did not connect terminal 2 to anything and the voltage displayed correctly. (this is btw with the negative supply grounded). The LM2907 freq to voltage converter circuit i have completely working and outputing the corresponding voltage when i use a regular multimeter, but i would like to integrate this circuit with it to have a permanent display. anyone have any ideas what i'm doing worng? I tried the circuit suggested in the appnotes about the CMR problem but i couldnt get that to work either.
link to 7107 appnote:
http://www.electro-tech-online.com/custompdfs/2003/11/an9609.pdf






so close! :)

thanks
bonanz[/url]
 

bonanz

New Member
Hi,

yes that is a capacitor at the frequency input for the AC coupling since the signal i'm measuring does not go below zero.

I have tried hooking up the display with the -5 just using a lab powersupply to see if it works but it did the same as if i had it grounded. it will read a voltage divider correctly but not the output from the FTV converter. what could i have done incorrectly with the CMR input on Fig 2 of the appnote? all the values of resistros should be the same shouldnt they? i used that exact opamp. I gave V+ of the opamp 5v and grounded v-. am i supposed to ground the unused pins of a 3130? The same principle should work for figure 2 if i have components selected to have it range from 0-2v correct? I'm a little confused about what the extra pins shoudl or should not do on this opamp. Here is a link to the datasheet, are pins 1, 5, 8 supposed to be left floating?

also why would the 7170 read a voltage from input 1 without input 2 being connected to anything (without the opamp i mean) and then if i put input 2 to ground the display messes up. why would it work with a voltage divider and not the FTV converter? is it that CMR problem for sure?

Thanks for your responses/help,
bonanz

http://www.electro-tech-online.com/custompdfs/2003/11/fn817.pdf
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top