Providing the precision signal measurement demanded in high-volume optical networking, industrial and medical laboratory applications, Texas Instruments (TI) Incorporated introduced a low-power logarithmic amplifier which computes the logarithm or log ratio of an input current relative to a reference or second input current. (See amplifier.ti.com/hpl7736u ).

The unique LOG102 features low supply voltages, wide input dynamic range, excellent AC stability and two uncommitted on-chip op amps to provide unparalleled design flexibility and accuracy for laser diode monitoring, optical signal compression and absorbance measurement. Optical networking applications require measurement of very wide dynamic range due to signal variation through fiber-optic cables and laboratory applications require precise logarithmic measurements. The LOG102 features a complete solution for compressing many decades of signal strength as well as accurately measuring the ratio of two photodiode currents. "In response to high demand in the optical network market, TI has developed this new generation of a highly successful and unique analog circuit," said John Brown, strategic marketing engineer for TI's high-performance linear products.

The LOG102 measures input currents from 1nA to 1mA with 0.5% accuracy (output voltage changes 1V for each decade change of input current). This permits six decades of signal range to be measured with a 12-bit A/D converter resulting in an effective 20-bit dynamic range. With supply voltages ranging from +/-4.5V to +/-18V, and typical quiescent current of 1.25mA, the LOG102 is useful in many low-power applications. The two uncommitted op amps can be used for a variety of functions such as gain scaling, inverting, filtering, offsetting, and level comparison to detect loss of signal. This device has been designed for high stability over temperature, which is difficult to achieve in discrete designs. The LOG102 is specified from 0C to +70C with operation from -40C to +125C.

Texas Instruments Incorporated, Semiconductor Group, SC-02029 Literature Response Center, P.O. Box 954 Santa Clarita, CA 91380. Tel: 1-800-477-8924, ext. 4500

Here's a product that can't miss, if for no other reason than its genealogy. The parent of this product, the LOG100, is about 20 years old and still going strong. It was a product before its time, but found a perfect fit with the advent of optical networking. Due to the reemergence of interest in the LOG100, TI decided to update it. Thus, the LOG102 was born. Essentially, it is a smaller product that is less costly.

The LOG10x family is a perfect fit for optical networking because it accepts the input of a photo diode directly. What you're measuring is the logarithm of the ratio of a reference current to a photo diode current. It measures it over a wide dynamic range. A graph in the data sheet shows a plot with 6 decades of current range from nano amp to milli amp range. The output you get is a linear line in voltage against the logarithmic line of input current. That means you can measure the output voltage with a 12-bit converter and get an indication of the optical power level over a 6-decade range. The other interesting item is that you can put a photo diode on both inputs, which means the reference is another photo diode. Also, it's a direct transimpedance input - it operates the photo diode in a short circuit mode, which is the way the photo diodes are the most linear and most accurate. With two photo diodes you get the ratio of the optical power on the two photo diodes. It provides true measurements in dB - so it's like converting directly to dB.

TI thinks there will be wide use of the log amp, some will be optical and others will be wherever you have need for wide dynamic range. Audio measurements are one such area because the log amp gives a direct conversion to dB. For example, if you convert a signal to a unipolar average value DC voltage or current, you can now measure that over a 60-80 dB range. And that's what a logarithm does for you, it converts it to a human response unit. The logarithm and you get a readout that is linear in dB.

There are other situations for which the log amp could be used, such as measuring leakage currents. Imagine you have a summing junction with leakage current coming into that node. The leakage current could be measured over a very wide range. Additionally, there are some measurements that are not directly optical but indirectly optical. One of those would be measuring the absorbance of material. In a sample you put two light paths, one direct and the other through the sample. You measure how much of the light gets absorbed in a particular wavelength through the material. Again, you are dealing with a wide dynamic range and you are measuring the ratio of the transmission between two optical paths. Two photo diodes measure that directly at the two inputs of the LOG102.

You may wonder how TI managed to shrink the LOG102 packaging and maintain or improve the specs. The basic circuit in the LOG100 uses the logarithmic relationship of base-emitter voltage on the bipolar transistor versus the collector current. The trick in this device is in compensating that basic conversion over temperature. The bipolar transistor has a fundamental V-thermal that is proportional to Kelvin temperature. Taking that out of the equation requires temperature compensation. Essentially TI does it with the interconnection metal on the IC. A very large area of the chip is used to construct a resistor out of aluminum. Making a resistor out of aluminum is easy but getting the magnitude of the compensation correct required expertise in laser trimming devices, in their over-temperature behavior at the wafer probe level.

If you are a LOG100 aficionado then you know that the functionality is very similar to the LOG102 but here's where your boss will be interested - the 102 lowers the cost, and shrinks the size while making some improvements in accuracy. For comparison the LOG100 is available in a 14-Pin Ceramic DIP for about $50, and the LOG102 is packaged in an SO-14 for $18.60 (in 1,000 piece quantities).

Data sheet: http://www-s.ti.com/sc/ds/log102.pdf