LOG114: Measure positive and negative current both using same log114

Hi Ravi,

have you seen the section "NEGATIVE INPUT CURRENTS" on page 17 of datasheet?

Kai

Dear Kai,

I have seen this section. But in my application the current can also be positive in some cases. So instead of using the above circuit i need a circuit where I can measure current of both polarities i.e., negative and positive. 

Hi Ravi,

then you must develop a circuit, which changes the polarity.

Kai

Dear Kai,

Do you mean a current rectifier?

Hi Ravi,

it depends on your application. Can you tell more about your project?

Kai

Hi Ravi,

The TI LOG amplifiers including LOG114 are unidirectional input current devices. That is dictated by the unidirectional current flow through he logging transistors used in the feedback of the input op amp transimpedance stages. See Figure 1 in the datasheet for details about the input stages.

Maybe if we learn more about your application we can offer some ideas.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi thomas and kai,

Our application is to use the log 114 as an ammeter. It should be able to show the absolute value of current irrespective of the direction of current i.e, we should be able to measure both negative and positive current(+7mA to -7mA). As pointed out by Kai we need to change the polarity of current and we should make it positive only(make it unidirectional as pointed out by thomas). Please give me suggestions for the same because the current is very small.

Hi Ravi,

what is your current source?

Kai

Hi kai,

My current source is a known voltage source in series with an unknown resistance which is to be measured.

Hi Ravi,

and your voltage source is generating an AC signal?

Kai

Dear Kai,

No it does not generate AC but the user can apply voltage from any direction.

Ravi

Hi Ravi,

can you show a scheme? What measuring voltage are we talking about? What range of resistance is to be measured? Why is the maximum current limited to +7mA of -7mA? Is this a short current limting? Provided by a resistor?

Kai

Hi Kai,

I should be able to measure the current in both the given schemes.

Hi Ravi,

I am not sure where you found that particular LOG114 model in TINA? When I check TINA for the LOG114 "Spice Macro" this much more detailed symbol is provided:

I am not sure what the A and B inputs correspond to, I1 and I2? And then what is IREF?

You are certainly aware that the LOG114 cannot be powered with a 700 V supply so that is something must have been overlooked in your simple schematic. Also, be aware that if the intended input voltage is 700 V that is a very high voltage level. The PC board environment and the potential for severe over voltage of low voltage ICs is a real possibility. Working with high voltage required much precaution compared to working with low voltage.

I realize that you want to maximize the current measurement resolution, but the LOG114 doesn't really appear to be the solution. Have you considered a conventional op amp transimpedance amplifier (TIA) operated from dual polarity supplies? It won't provide the resolution of a LOG amplifier, but it will readily accommodate the bipolar input current. The high voltage concerns would apply to it as well.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi thomas,

I am not powering the log 114 with 700v power supply. Here is the schematic as per above model. In this schematic the resistance R2 has to be calculated which maybe upto 1.5 teraohms(current in picoamperes) so we cannot measure it using conventional op amp transimpedance amplifier. The current through the log114 is controlled by the source itself so it will not exceed 10mA. 

Hi Ravi,

The first scheme 1 you show is connected as the LOG114 is intended, but scheme 2 is not. I recall if you attempt to operate the LOG114 in this manner the logging transistor has no current flow through it, the transimpedance gain becomes enormous and VLOGOUT rails. And note that even when the input current is flowing in the correct direction the minimum specified input current is 100 pA so it is not usable in picoampere applications having current levels lower than the 100 pA minimum.

I have looked for ways to accommodate a bipolar LOG114 input current, but every idea has resulted in one or more issues that make them unacceptable.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Thomas,

Is there any possibility to use circuit such as precision rectifier to change the polarity of current in scheme 2?

Regards, Ravi

Hi Ravi,

I had considered  applying an absolute value circuit before the LOG114 input, but it looks like much of the resolution will be lost. The absolute value circuits are voltage input circuits. It would require the wide range +/- input current be converted to a voltage range for the absolute value circuit to act upon it.

The problem is the absolute value circuit, and even the LOG114 operating in voltage input mode (See Voltage Input on Pg. 18 of the LOG114 datasheet) can't match the 7 to 8 decades of dynamic range the LOG114 provides in current input mode. The datasheet states, "Voltage inputs may be handled directly by using a low-impedance voltage source with series resistors, but the dynamic input range is limited to approximately three decades of input voltage." Therefore, the high resolution needed to detect the very low currents would not be attainable.

Here is a link to a Burr-Brown Applications Bulletin (AB121) that describes precision absolute value circuits.

http://www.ti.com/lit/an/sboa068/sboa068.pdf

If you have any ideas after looking it over please share them.

Regards, Thomas

Precision Amplifiers Applications Engineering

Hi Ravi,

I would do it this way:

ravi_lmc6081_3.TSC

Kai