Precision High-Speed Logarithmic Amplifier with 2.5V Reference and Uncommitted Output Op Amps

The LOG114 is specifically designed

for measuring low-level and wide dynamic range currents in communications, lasers,

medical, and industrial systems. The device computes the logarithm or log-ratio of

an input current or voltage relative to a reference current or voltage (logarithmic

transimpedance amplifier).

High

precision is verified over a wide dynamic range of input signals on either bipolar

(±5V) or single (5V) supply. Special temperature drift compensation circuitry is

included on-chip. In log-ratio applications, the signal current can be from a high

impedance source such as a photodiode or resistor in series with a low impedance

voltage source. The reference current is provided by a resistor in series with a

precision internal voltage reference, photo diode, or active current source.

The output signal at

VLOGOUT has a scale factor of 0.375V/decade of input current, which

limits the output so that the output fits within a 5V or 10V range. The output can

be scaled and offset with one of the available additional amplifiers, so the output

matches a wide variety of ADC input ranges. Stable DC performance allows accurate

measurement of low-level signals over a wide temperature range. The LOG114 is

specified over a −5°C to 75°C temperature range and can operate from −40°C to

85°C.