7.3.2 Voltage and Current References

The LOG200 integrates two separate voltage references (2.5 V and 1.65 V) and a current reference (1 μA). The

voltage references are designed to be used as the input common-mode reference (2.5 V) and output reference

(1.65 V); however, the references can also be used for other functions requiring precise voltages within the

system, as long as the maximum current limitations are observed. These voltage references are established

relative to the voltage applied to the REFGND pin; therefore, establish the current return path to the REFGND

pin rather than to VS–. The current reference is designed to be used as the input to the I2 pin. If the current

reference is instead used for another function in the system, establish the corresponding current return path to

the VS– supply potential. If any of the references are unused, float the corresponding pins.

7.3.3 Adaptive Photodiode Bias

The LOG200 includes an IBIAS current output feature that can be used to bias a photodiode with a voltage that

is proportional to the photocurrent. The current from the IBIAS pin is nominally 1.1 times the input current of the

I1 pin. When an RBIAS resistance is placed in parallel with the photodiode, 1.0 times the input current is drawn

through the photodiode and the remaining 0.1 times the input current flows through RBIAS. This configuration

establishes a bias voltage across that resistance. As the anode end of the photodiode (connected to the I1 input)

is held at VCM, the cathode voltage effectively rises by 0.1 × RBIAS × I1, thus providing a current-dependent

reverse bias voltage for the photodiode.

This feature creates very small bias voltages for applications with low photodiode currents, reducing the dark

current of the photodiode. In applications with high photodiode currents (which often require larger photodiodes),

higher reverse-bias voltages are developed, thus reducing the effective capacitance of the photodiode and

increasing the effective device bandwidth. If this feature is not used, float the IBIAS pin.

7.3.4 Auxiliary Operational Amplifier

The LOG200 features an additional wide bandwidth amplifier to support functions such as single-ended to

differential conversion, or single-ended gain or filter blocks.

7.4 Device Functional Modes

The LOG200 has a maximum supply voltage of 12.6 V (±6.3 V) and a minimum supply voltage of 4.5 V

(±2.25 V). The device has two VCM pins (not internally connected to each other). Drive both VCM pins to the

same potential by one of the two onboard voltage references, or by an external source. Likewise, drive the

reference input of the difference amplifier by a reference or other low-impedance source. For proper operation,

do not float the VCM, VCM2, and REFA pins.

Typically, apply the test current to be measured through the I1 input. Apply a fixed reference current, whether

external or provided by the onboard IREF, through the I2 input. Two external currents can be measured through

I1 and I2, but only the logarithmic ratio of the two currents can be measured, rather than the absolute values of

either. The IBIAS feature is used to provide a reverse voltage bias for an input photodiode. If not used, float the

IBIAS pin or connect the pin to the positive supply voltage VS+.

The LOG200 also features an auxiliary amplifier that is used to create a differential output voltage or for any

other purpose in the system (provided the amplifier input common-mode limitations and other conditions are

met). If the auxiliary amplifier is not needed, apply a midsupply voltage or one of the onboard reference voltages

to the noninverting input to keep the auxiliary amplifier fixed within the input common-mode range. Short the

output and inverting input together, which causes the amplifier to act as a buffer in a known state, rather than

float the pins, which can lead to erratic behavior in noisy environments.

LOG200

SBOSA28 – AUGUST 2023

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