AD645

–7–

REV. B

AD645

2

3

6

8

FILTERED

OUTPUT

OPTIONAL 26Hz

FILTER

PHOTODIODE

GUARD

OUTPUT

10pF

10

Ω

9

Figure 30. The AD645 Used as a Sensitive Preamplifier

Preamplifier Applications

The low input current and offset voltage levels of the AD645 to-

gether with its low voltage noise make this amplifier an excellent

choice for preamplifiers used in sensitive photodiode applica-

tions. In a typical preamp circuit, shown in Figure 30, the out-

put of the amplifier is equal to:

where:

Rp = photodiode sensitivity (Amp/Watt)

Rf = the value of the feedback resistor, in ohms.

P = light power incident to photodiode surface, in watts.

An equivalent model for a photodiode and its dc error sources is

tribute an output voltage error which will be proportional to the

cause a “dark” current error due to the photodiode’s finite

equal to:

A shunt resistance on the order of 10

small photodiode. Resistance Rd is a junction resistance which

will typically drop by a factor of two for every 10

°C rise in tem-

perature. In the AD645, both the offset voltage and drift are

low, this helps minimize these errors.

PHOTODIODE

OUTPUT

10pF

10

Ω

9

OS

V

Rd

50pF

Cd

Cf

Rf

Figure 31. A Photodiode Model Showing DC Error

Sources

Minimizing Noise Contributions

The noise level limits the resolution obtainable from any pream-

plifier. The total output voltage noise divided by the feedback

resistance of the op amp defines the minimum detectable signal

current. The minimum detectable current divided by the photo-

diode sensitivity is the minimum detectable light power.

Sources of noise in a typical preamp are shown in Figure 32.

The total noise contribution is defined as:

V OUT =

in2 + i f 2 + is 2









Rf

1

+ s (Cf ) Rf









2

+ en

2







+

Rf

Rd

1

+ s (Cd ) Rd

1

+ s (Cf ) Rf

















2

Figure 33, a spectral density versus frequency plot of each

source’s noise contribution, shows that the bandwidth of the

amplifier’s input voltage noise contribution is much greater than

its signal bandwidth. In addition, capacitance at the summing

junction results in a “peaking” of noise gain in this configura-

tion. This effect can be substantial when large photodiodes with

large shunt capacitances are used. Capacitor Cf sets the signal

bandwidth and also limits the peak in the noise gain. Each

source’s rms or root-sum-square contribution to noise is ob-

tained by integrating the sum of the squares of all the noise

sources and then by obtaining the square root of this sum. Mini-

mizing the total area under these curves will optimize the

preamplifier’s overall noise performance.

PHOTODIODE

OUTPUT

10

Ω

9

50pF

Rd

Cd

10pF

Cf

Rf

i f

in

en

Figure 32. Noise Contributions of Various Sources

FREQUENCY – Hz

100

1k

10k

100k

10

1

10nV

100nV

1

µV

10

µV

SIGNAL BANDWIDTH

NO FILTER

WITH FILTER

e n

is &if

in

en

Figure 33. Voltage Noise Spectral Density of the Circuit of

Figure 32 With and Without an Output Filter

An output filter with a passband close to that of the signal can

greatly improve the preamplifier’s signal to noise ratio. The pho-

todiode preamplifier shown in Figure 32—without a bandpass

filter—has a total output noise of 50

µV rms. Using a 26 Hz

single pole output filter, the total output noise drops to 23

µV

rms, a factor of 2 improvement with no loss in signal bandwidth.

Using a “T” Network

A “T” network, shown in Figure 34, can be used to boost the ef-

fective transimpedance of an I to V converter, for a given feed-

back resistor value. Unfortunately, amplifier noise and offset

voltage contributions are also amplified by the “T” network

gain. A low noise, low offset voltage amplifier, such as the

AD645, is needed for this type of application.