ADL5304
Data Sheet
Rev. 0 | Page 24 of 32
Log Ratio Operation
Because the ADL5304 has two equal inputs, INUM and IDEN, log ratio operation is possible. The only difference between INUM
and IDEN is that the IMON current derives from the INUM signal and
allows adaptive photodiode bias at this input only. Assuming
that the ratio INUM/IDEN can, in general, be either greater or less
than unity, VLOG can be of either polarity, requiring a negative
supply in some cases. The value of VLOG depends on the
minimum ratio and the slope chosen for the application.
For example, if the ratio can vary from 1:1000 to 1000:1 and a slope
of 20 mV/dB is required, the peak swing is ±1.2 V around VOFS.
Option 5 in Table 4 provides this with an intercept IZ of 17.8 pA (VOFS = 1.5 V) with VLOG = ±1.2 V around VOFS = 1.5 V, which
results in 0.3 V ≤ VLOG ≤ 2.7 V.