8 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

8.1 Application Information

The LOG200 is a wide-dynamic-range current-to-voltage amplifier specifically designed to optimize current

measurements across 160 dB of dynamic range with unparalleled accuracy and speed. The LOG200 features

two logarithmic amplifiers, followed by a high-accuracy differential amplifier to convert current signals into a

single-ended voltage that represents the log-compressed ratio of the two currents. The current inputs are

designed to feature a high-speed response from one input, and a highly accurate reference signal on the other

input, allowing for a unique combination of fast transient response and high logarithmic conformity. The LOG200

ratio is internally set to 250 mV/decade of current-to-voltage conversion.

The LOG200 integrates an uncommitted high-speed amplifier to allow the output to be configured for a

differential- or filtered-response output. The device also features a precise reference current and reference

voltages designed to configure the device for optimal input current and common-mode voltages. The LOG200

operates with a single-ended 5-V supply or bipolar ±5-V supplies, with a total supply range from 4.5 V to 12.6 V.

VCM can be driven by either of the onboard voltage references (REF25 or REF165), or by an external source. I2

can be driven by an external source but is typically driven by the onboard current reference, IREF.

8.1.1 Logarithmic Transfer Function

The LOG200 uses a differential amplifier to compare the voltage outputs of two logarithmic amplifiers.

Logarithmic amplifiers rely on the feedback transistor relation of the base-emitter voltage (VBE) to the collector

current IC, according to the principle:

VBE= kTq ln ICIS

(1)

where

• k = the Boltzmann constant, 1.381 × 10-23 J/K

• T = absolute temperature in kelvins (K)

• q = the elementary charge, 1.602 × 10-19 C

• IS = the transistor reverse saturation current

For the basic logarithmic amplifier implementation shown in Figure 8-1, the following expression holds:

VOUT=−VBE=− kTq ln IINIS

(2)

+

–

V

OUT

V

CM

I

IN

+ V

BE –

Figure 8-1. Basic Logarithmic Amplifier

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LOG200

SBOSA28 – AUGUST 2023

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