A single positive supply of 5V is all that is required for operation over specified 1pA to 3mA input range. Dual-supply operation extends the specified input current range to 10mA.
The ADL5304 accepts two current inputs to the logarithmic argument. The numerator input, INUM, flows in the collector of an NPN transistor, connected in feedback path around low offset JFET amplifier. The denominator current, IDEN, is treated in the same way, which allows for log ratio operation. The input summing nodes (INUM and IDEN) operate at constant default voltage of 1.5V. The VSM1 to VSM4 pins flank the INUM and IDEN inputs to provide guard voltage to minimize leakage currents.
Adaptive photodiode biasing is provided for optical measurements. A monitor current 1.1 times INUM is output at the IMON pin, and an external resistor, RMNTR, at 10 times the photodiode series resistance (RS) applies a voltage across the photodiode that 1st order keeps the internal PD junction at 0 V to minimize dark current.
The VLOG output is buffered and can be rescaled through internal gain setting resistors. The internal ILOG varies from -400uA to +400uA as INUM changes over 10 decades from 1pA to 10 mA. This corresponds to 0.5 to 2.5V at the VLOG pin in the default configuration
Accurate 1.5V (Pin 1P5V) and 2.0 V (Pin 2VLT) reference outputs allow precise repositioning of the intercept using external resistors.
The ADL5304 is available in a 32-lead, 5 x 5mm LFCSP and specified for operation from -40°C to +85°C.
