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.nalanda.nitc.ac.in/indust...sil/an9609.pdf






so close!

thanks
bonanz[/url]

 

Is that a capacitor, @ the frequency input?

 

You can either use a MAX1681 to generate a -5 volts, or use the circuit of figure 2. If you have tried figure 2 and it doesn't work, you have done something wrong.

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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.intersil.com/data/fn/fn817.pdf