Data Sheet

ADA4895-1/ADA4895-2

Rev. B | Page 21 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 bandwidth of the two amplifiers; 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 ADA4895-1/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 ADA4895-1/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 ADA4895-1/ADA4895-2 noise and gain

bandwidth can provide a significant increase in performance

within certain transimpedance ranges.

Figure 55 shows an I/V converter with an electrical model of a

photomultiplier.

–

+

Figure 55. 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.

is the feedback resistance.

is the total capacitance at the amplifier summing junction

(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 56.

FREQUENCY (Hz)

RF NOISE

NOISE DUE TO AMPLIFIER

ven

1

1

GBP

Figure 56. Photomultiplier Voltage Noise Contributions

Table 12. RMS Noise Contributions of Photomultiplier

Preamplifier

Contributor

Expression

57

1

4

.

f

R

kT

2

F







Amplifier ven





57

.

1













3

F

D

F

M

S

f

C

C

C

C

C

ven

Amplifier ien

57

1.

f

R

ien

2

F





