AD823A

Data Sheet

Rev. B | Page 18 of 20

Table 8. RMS Noise Contributions of Photodiode Preamp

Contributor

Expression

(μV)1

RF

2

π







N

F

f

R

4kT

55.17

VNOISE



N

F

D

F

M

S

NOISE

f

C

2C

C

C

C

V













2

π

138.5

RSS Total

149.1

1 RMS noise with RF = 50 kΩ, CS = 5 pF, CF = 1.2 pF, CM = 1.3 pF, and CD = 0.6 pF.

ACTIVE FILTER

The AD823A is an ideal candidate for an active filter because of

its low input bias current and its low input capacitance. Low

input bias current reduces dc error in the signal path while low

input capacitance improves the accuracy of the active filter.

As a general rule of thumb, the bandwidth of the amplifier should

be at least 10 times bigger than the cutoff frequency of the filter

implemented. Therefore, the AD823A is capable of implementing

active filters of up to 1.7 MHz.

AD823A

RT

49.9Ω

R2

1.12kΩ

R1

1.12kΩ

C1

200pF

+VS

–VS

VOUT

VIN

C2

100pF

Figure 46. Two-Pole Sallen-Key Active Filter

Figure 46 shows an example of a second-order Butterworth

filter, which is implemented by the Sallen-Key topology. This

structure can be duplicated to produce higher-order filters.

3

–36

–33

–30

–27

–24

–21

–18

–15

–12

–9

–6

–3

0

100

1k

10k

100k

10M

1M

FREQUENCY (Hz)

Figure 47. Two-Pole Butterworth Active Filter Response

Figure 47 shows the two-pole Butterworth active filter’s response.

Note that it has a maximally flat pass band, a −3 dB bandwidth

of 1 MHz, and a 12 dB/octave roll-off in the stop band.

The cutoff frequency (fc) and the Q factor of the Butterworth

filter can be calculated by:

2

1

2

1

2

1

C

C

R

R

f

c





(7)



2

2

1

2

1

2

1

C

R

R

C

C

R

R

Q







(8)

Therefore, one can easily adjust the cutoff frequency by

appropriately factoring the resistor and capacitor values. For

example, a 100 kHz filter can be implemented by increasing the

values of R1 and R2 by 10 times. Note that the Q factor remains

the same in this case.