ADA4895-2

Data Sheet

Rev. 0 | Page 22 of 24

WIDEBAND PHOTOMULTIPLIER PREAMPLIFIER

A decompensated amplifier can provide significantly greater

speed in transimpedance applications than a unity-gain stable

amplifier. The speed increases by the square root of the ratio

of the two amplifiers’ bandwidth; that is, a 1 GHz GBP amplifier

is 10 times faster than a 10 MHz amplifier in the same trans-

impedance application if all other parameters are kept constant.

Additionally, the input voltage noise normally dominates the

total output rms noise because it is multiplied by the capacitive

noise gain network.





F

D

F

M

S

C

C

C

C

C







In the case of the ADA4895-2, the input noise is low, but the

capacitive noise gain network must be kept greater than 10 for

stability reasons.

One disadvantage of using the ADA4895-2 in transimpedance

applications is that the input current and input current noise

can create large offsets and output voltage noise when coupled

with an excessively high feedback resistance. Despite these two

issues, the ADA4895-2 noise and gain bandwidth can provide a

significant increase in performance within certain transimpedance

ranges.

Figure 47 shows an I/V converter with an electrical model of a

photomultiplier.

–

+

Figure 47. Wideband Photomultiplier Preamplifier

The basic transfer function is

F

F

F

PHOTO

OUT

R

sC

R

I

V







1

The stable bandwidth attainable with this preamplifier is a function

and the amplifier input capacitance.

mission of the amplifier that can result in peaking and instability.

for the pole effect and reduces the signal bandwidth. It can be

shown that the signal bandwidth resulting in a 45° phase margin



S

F

45

C

R

GBP

f







π

2

where:

GBP is the gain bandwidth product.

(amplifier + photomultiplier + board parasitics).

GBP

R

C

C

F

S

F







π

2

The frequency response in this case shows approximately 2 dB

bandwidth by half results in a flat frequency response with

approximately 5% transient overshoot.

The output noise over frequency for the preamplifier is shown

in Figure 48.

FREQUENCY (Hz)

RF NOISE

NOISE DUE TO AMPLIFIER

ven

1

1

GBP

Figure 48. Photomultiplier Voltage Noise Contributions

Table 10. RMS Noise Contributions of Photomultiplier

Preamplifier

Contributor

Expression

57

1

f

R

kT

4

2

F

.







Amplifier ven





57

.

1

f

C

C

C

C

C

ven

3

F

D

F

M

S













Amplifier ien

57

1

f

R

ien

2

F

.





