Log112 reference current regarding

Jayaram,

The reference output of the LOG112 is tested to 10mA output but the nominal short-circuit current is 16mA (listed in the specification table). The short circuit current is not particularly accurate. While some devices might perform reasonably well at 14mA output, others may perform very poorly. The output voltage could be inaccurate and very temperature sensitive if it nears the "soft" current limit region.

I have another concern with your plans. The logging characteristics of the LOG112 lose accuracy rapidly as the logging current exceeds 1mA. This can be seen in the graph  lower-right of page 5 of the data sheet. While this graph indicates the measurement is made on the I1 input, both logging transistors are identical. You should expect an equal and opposite error on I2. Though the reference current is constant and the resulting error might be calibrated, I would expect a calibrated point to be much more temperature sensitive at this high input current.

Regards, Bruce

Bruce,

Thanks for your reply...Our application is like this

1. We are generating reference current from the Vref and the same is fed to pin 14.

2.The current to pin1 is from our measurement device.

3. Please let me know the maximum value of current which can be generated and fed to pin14 without losing accuracy & without being temperature sensitive.

We shall design our circuit based on your reply.

Regards,

Jayaram C B

Jayaram,

As I indicated in my previous answer, accuracy degrades rapidly at greater than 2mA input. I suggest that you not exceed approximately 3mA on either input terminal. Please see the graph on page 5 in the lower-right corner to see how the errors increase as current is increased.

Regards, Bruce.

Bruce,

Thanks for your reply. From the datasheet of Log114, i could see a current range of upto 10mA for Log114 whereas the same is 3.5mA for Log112. Please let me know the limit of I1 and I2 in case i am using Log114 instead of Log112.

Regards,

Jayaram C B

Jayaram,

The LOG114 is capable of handling inputs up to 10mA but, again, the logging accuracy is poor beyond approximately 3mA. This can best be seen the four graphs on the lower portion of page 7.

What is your range of input current on the variable input? What is the source of your input current? Perhaps we can recommend an alternative approach to scale the input or output to better suit your needs.

Regards, Bruce.

Bruce,

We tried testing Log114 with a photodiode connected today and had tried the dual supply configuration(Fig 1 in page11 of the datasheet). The Rref value used by us was 2.5K ohm.Voltage at pin 16 was 2.544V. We measured the current Iref and it was 2 mA(instead of the expected 1mA). The voltage at pin 4(with respect to common ground was        -2.5V. Is this behavior an expected one? Pl let us know.

We had connected the die pad to -IN4.

Regards,

Jayaram C B

Jayaram,

No, certainly this behavior is not expected. Check the connection to pin 5. It should be 0V (ground).

Was the die pad accidentally connected to -IN4?  It's not clear that this would cause permanent damage.

Regards, Bruce.