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AD549

Rev. H | Page 15 of 20

gain that multiplies the op amp input voltage noise contribu-

tion. A single-pole filter at the output of the amplifier limits the

op amp output voltage noise bandwidth to 26 Hz, comparable

to the signal bandwidth. This greatly improves the signal-to-

noise ratio of the preamplifier (in this case, by a factor of 3).

Input current, IB, contributes an output voltage error, VE1,

proportional to the feedback resistance

VE1 = IB × RF

The input voltage offset of the op amp causes an error current

through the photodiode shunt resistance, RS

10µ

1µ

100n

10n

100

10k

100k

FREQUENCY (Hz)

IF AND CS, NO FILTERS

IF AND CS, WITH FILTERS

CONTRIBUTION,

WITH FILTER

EN CONTRIBUTION,

NO FILTER

AD549

OPEN-LOOP GAIN

I = VOS/RS

The error current results in an error voltage (VE2) at the

amplifier output equal to

VE2 = (1 + RF/RS)VOS

Given typical values of photodiode shunt resistance (on the order

of 109 Ω), RF/RS can easily be greater than 1, especially if a large

feedback resistance is used. Also, RF/RS increases with tempera-

ture because photodiode shunt resistance typically drops by a

factor of 2 for every 10°C rise in temperature. An op amp with

low offset voltage and low drift must be used to maintain accuracy.

The AD549K offers a guaranteed maximum 0.25 mV offset

voltage and 5 mV/°C drift for very sensitive applications.

Figure 44. Spectral Density of the Photodiode Preamp Noise

Sources vs. Frequency

Photodiode Preamp Noise

Noise limits the signal resolution obtainable with the preamp.

The output voltage noise divided by the feedback resistance is

the minimum current signal that can be detected. This mini-

mum detectable current divided by the responsivity of the

photodiode represents the lowest light power that is detectable

by the preamp.

LOG RATIO AMPLIFIER

Logarithmic ratio circuits are useful for processing signals with

wide dynamic range. The 60 fA maximum input current of the

AD549L makes it possible to build a log ratio amplifier with

1% log conformance for input currents ranging from 10 pA to

1 mA, a dynamic range of 160 dB.

Noise sources associated with the photodiode, amplifier, and

feedback resistance are shown in Figure 43; Figure 44 is the

spectral density vs. frequency plot of the contribution of each of

the noise sources to the output voltage noise (circuit parameters

in Figure 42 are assumed). The rms contribution of each noise

source to the total output voltage noise is obtained by

integrating the square of its spectral density function over

frequency. The rms value of the output voltage noise is the

square root of the sum of all contributions. Minimizing the total

area under these curves optimizes the resolution of the

preamplifier for a given bandwidth.

The log ratio amplifier in Figure 45 provides an output voltage

proportional to the log base 10 of the ratio of Input Current I1

and Input Current I2. Resistor R1 and Resistor R2 are provided

for voltage inputs. Because NPN devices are used in the feedback

loop of the front-end amplifiers that provide the log transfer

function, the output is valid only for positive input voltages and

input currents. The input currents set the Collector Current IC1

and Collector Current IC2 of a matched pair of log transistors,

Q1 and Q2, to develop Voltage VA and Voltage VB

VA, VB = –(kT/q)ln IC/IES

where IES is the saturation current of the transistor.

The difference of VA and VB is taken by the subtractor section to

obtain

VC = (kT/q)ln(IC2/IC1)

VC is scaled up by the ratio of (R9 + R10)/R8, which is equal to

approximately 16 at room temperature, resulting in the output

voltage

Figure 43. Photodiode Preamp Noise Sources

VOUT = 1 V × log(IC2/IC1)

The photodiode preamp in Figure 41 can detect a signal current

of 26 fA rms at a bandwidth of 16 Hz, which, assuming a

photodiode responsivity of 0.5 A/W, translates to a 52 fW rms

minimum detectable power. The photodiode used has a high

source resistance and low junction capacitance. CF sets the

signal bandwidth with RF and also limits the peak in the noise

R8 is a resistor with a positive 3500 ppm/°C temperature coeffi-

cient to provide the necessary temperature compensation. The

parallel combination of R15 and R7 is provided to keep the gain

of the subtractor section for positive and negative inputs matched

over temperature.

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