AD795 datasheet(16/20 Pages) AD

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AD795

Data Sheet

Rev. D | Page 16 of 20

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 45,

the output of the amplifier is equal to:

VOUT = ID (Rf) = Rp (P) Rf

where:

ID is the photodiode signal current, in amps (A).

Rp is the photodiode sensitivity, in amps/watt (A/W).

Rf is the value of the feedback resistor, in ohms (Ω).

P is the light power incident to photodiode surface, in watts (W).

An equivalent model for a photodiode and its dc error sources

is shown in Figure 46. The amplifier’s input current, IB, contri-

butes an output voltage error, which is proportional to the value

of the feedback resistor. The offset voltage error, VOS, causes a

dark current error due to the photodiode’s finite shunt resistance,

Rd. The resulting output voltage error, VE, is equal to:

VE = (1 + Rf/Rd) VOS + Rf IB

A shunt resistance on the order of 109 Ω is typical for a small

photodiode. Resistance Rd is a junction resistance, which

typically drops by a factor of two for every 10°C rise in

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

low, which helps minimize these errors.

50pF

10pF

VOS

1GΩ

PHOTODIODE

OUTPUT

Figure 46. A Photodiode Model Showing DC Error Sources

MINIMIZING NOISE CONTRIBUTIONS

The noise level limits the resolution obtainable from any

preamplifier. 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 47.

The total noise contribution is defined as:

















































OUT

50pF

10pF

1GΩ

PHOTODIODE

OUTPUT

Figure 47. Noise Contributions of Various Sources

Figure 48, a spectral density vs. 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 configuration. This

effect can be substantial when large photodiodes with large shunt

capacitances are used. Capacitor Cf sets the signal bandwidth

and limits the peak in the noise gain. Each source’s rms or root-

sum-square contribution to noise is obtained 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 optimizes 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 47, 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.

10µV

1µV

100nV

10nV

100

10k

100k

FREQUENCY (Hz)

SIGNAL BANDWIDTH

WITH FILTER

NO FILTER

IQ AND IF

Figure 48. Voltage Noise Spectral Density of the Circuit of Figure 47 With and

Without an Output Filter

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