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Log112-Vlogout not coming as expected

Jayaram C B
Intellectual 370 points
Other Parts Discussed in Thread: LOG112

Hi,

We have an application wherein Log112 is used.Supply for the same is +15V(pin6) and -15V(pin 9). To test the healthiness of circuit, we do the following tests.

Test 1

1.Connect equal resistance of 3.09M between pins 8 & 1(I1) and pins 8 & 14(I2)

2.Measure output at pin5

3.Output shall be 0V since equal current is fed to pins 1 and 14.

In our case, we are getting a voltage of  4.5V instead of 0V.

 

Test 2

1.Connect resistance of 30.9M between pins 8 & 1(I1) and 3.09 M pins 8 & 14(I2)

2.Measure output at pin5

3.Output shall be 0.5V 

In our case, we are getting a voltage of  8.2V instead of 0.5V.

 

Any idea why this is happening? Thanks in advance for your time.

 

Regards,

Jayaram C B

  • 0
    TI__Guru* 93105 points

    Hello Jayaram,

    Something is definitely not working correctly in your LOG112 circuit. Using case 1 as an example, the I1/I2 ratio of 1:1 should produce an output voltage of 0 V, as you have stated. However, to achieve an output of 4.5 V indicates an I1/I2 ratio of 109:1. The ratio would have to be even more extreme to achieve the 8.2 V output as stated in the second case.

    The LOG112 requires careful attention to layout in terms of a PC board that has very low leakage currents, having proper supply bypassing and correct capacitive compensation (CC) applied. 50/60 Hz noise pick up can overwhelm the minute input currents causing the output to be corrupted. Shielded input lines and sometimes circuit shielding is sometimes required to keep noise from being coupled into the input circuits. 

    Make sure the PC board has been correctly cleaned and all solder flux and extraneous contaminates are eliminated. If you have two DVMs capable of measuring the low level I1 and I2 currents, you should be able to verify that the correct currents are flowing to the inputs with the selected resistors. Also, I suggest you observe the output with an O-scope to see make sure the output dc level is free of oscillation and excessive noise. 

    Regards, Thomas

    PA - Linear Applications Engineering