AD795

REV. A

–13–

will typically drop by a factor of two for every 10

°C rise in

temperature. In the AD795, both the offset voltage and drift are

low, this helps minimize these errors.

Minimizing Noise Contributions

The noise level limits the resolution obtainable from any pre-

amplifier. 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 photodiode sensitivity is the minimum detectable

light power.

Sources of noise in a typical preamp are shown in Figure 45.

The total noise contribution is defined as:

VOUT = (in

2

Rf

1

+ s (Cf ) Rf









2

Rf

Rd

1

+ s (Cd ) Rd

1

+ s (Cf ) Rf

























2

PHOTODIODE

OUTPUT

10

Ω

9

50pF

i

S

i

S

Rd

Cd

10pF

Cf

Rf

i

f

i

n

en

Figure 45. Noise Contributions of Various Sources

Figure 46, 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.

Minimizing the total area under these curves will optimize the

preamplifier’s overall noise performance.

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

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

photodiode preamplifier shown in Figure 45—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.

10pF

AD795

PHOTODIODE

GUARD

OUTPUT

10

Ω

9

OPTIONAL 26Hz

FILTER

FILTERED

OUTPUT

6

8

3

2

Figure 43. The AD795 Used as a Photodiode Preamplifier

Preamplifier Applications

The low input current and offset voltage levels of the AD795

together with its low voltage noise make this amplifier an

excellent choice for preamplifiers used in sensitive photodiode

applications. In a typical preamp circuit, shown in Figure 43,

the output of the amplifier is equal to:

where:

= photodiode signal current (Amps)

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

contribute an output voltage error which will be proportional to

will 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

PHOTODIODE

OUTPUT

10pF

10

Ω

9

I

D

OS

V

Rd

50pF

Cd

Cf

Rf

Figure 44. A Photodiode Model Showing DC Error

Sources