Data Sheet

AD8305

Rev. C | Page 17 of 24

LOG-RATIO APPLICATIONS

It is often desirable to determine the ratio of two currents, for

example, in absorbance measurements. These are commonly

used to assess the attenuation of a passive optical component,

such as an optical filter or variable optical attenuator. In these

situations, a reference detector is used to measure the incident

power entering the component. The exiting power is then

measured using a second detector and the ratio is calculated to

determine the attenuation factor. Because the AD8305 is

fundamentally a ratiometric device, having nearly identical

Figure 38 illustrates the AD8305 log-ratio capabilities in optical

absorbance measurements. Here a reference detector diode is

optical reference power level. A second detector measures the

calculates the logarithm of the ratio of these two currents, as

shown in Equation 11, and which is reformulated in power terms

in Equation 12. Both of these equations include the internal

via pin VRDZ. If the true (nonoffset) log ratio shown in Equation 4

is preferred, VRDZ should be grounded to remove the offset. As

already noted, the use of a negative supply at Pin VNEG allows

also allow the input pins INPT and IREF to be set to ground

potential. Therefore, the AD8305 may also be used to determine

the log ratio of two voltages.

Figure 38 also illustrates how a second order Sallen-Key low-pass

filter can be realized using two external capacitors and one

resistor. Here, the corner frequency is set to 1 kHz and the filter

Q is chosen to provide an optimally flat (overshoot-free) pulse

response. To scale this frequency either up or down, simply

scale the capacitors by the appropriate factor. Note that one of

the resistors needed to realize this filter is the output resistance

of 4.55 kΩ present at Pin VLOG. While this does not ratio exactly

to the external resistor, which may slightly alter the Q of the

filter, the effect on pulse response is be negligible for most

purposes. Note that the gain of the buffer (×2.5) is an integral

part of this illustrative filter design; in general, the filter may be

redesigned for other closed-loop gains.

The transfer characteristics can be expressed in terms of optical

power. If we assume that the two detectors have equal

responsivities, the relationship is

(11)

characteristic can be written as

(12)

Figure 39 illustrates the linear-in-dB relationship between the

absorbance and the output of the circuit in Figure 38.

ATTENUATION (dB)

2.5

50

0

5

10

15

20

25

30

35

40

45

2.0

1.5

1.0

0.5

0

Figure 39. Example of an Absorbance Transfer Function