7 Detailed Description

7.1 Overview

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 for optical

communications, medical diagnostics, and industrial process control measurements. 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 device integrates an

uncommitted high-speed amplifier to allow the output to be configured for differential or filtered responses, with a

fast settling time to drive successive approximation analog-to-digital converters (SAR ADCs). The LOG200 also

features a separate reference current and reference voltage designed to configure the device for optimal input

current and common-mode voltages.

7.2 Functional Block Diagram

I2

I1

Logarithmic

Amplifier

(A)

VCM

Logarithmic

Amplifier

VCM

(B)

Difference

Amplifier

Auxiliary

Amplifier

REFA

OUTA

OUTB

+IN

–IN

Voltage

References

REF25

REF165

Current

Reference

IREF

Adaptive

Photodiode Bias

IBIAS

A.

Either IREF or an external source drive I2.

B.

Either REF25, REF165, or an external source drive VCM. Comply with the input common-mode voltage constraints so that the input

logarithmic amplifiers have sufficient headroom.

7.3 Feature Description

7.3.1 High Speed, Logarithmic Current-to-Voltage Conversion

The LOG200 converts current into voltage using an advanced, high-speed amplifier architecture. By dynamically

controlling the amplifier open-loop gain, the LOG200 achieves transient response from low-to-high current and

high-to-low current measurements significantly faster than previous-generation logarithmic amplifiers.

The LOG200 features two current inputs, I1 and I2. The I1 input is optimized for speed, facilitating the excellent

transient response of the device to changes in the current to be measured. The I2 input is optimized for precision

and accuracy, intended for use with a current reference such as the onboard 1-μA reference. If an external

current in excess of 100 μA is used for I2, implementation of a snubber network can improve device stability.

The effective capacitance at a current input pin establishes the effective bandwidth of the corresponding

feedback loop, and thus the effective device bandwidth. Photodiode capacitance and system parasitics both

play a role and must be considered for stability and transient performance analyses.

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LOG200

SBOSA28 – AUGUST 2023

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