AD8605/AD8606/AD8608

Rev. D | Page 14 of 20

FREQUENCY (Hz)

0.1

0.01

0.0001

20

20k

100

1k

0.001

10k

Figure 45. THD + N

TOTAL NOISE INCLUDING SOURCE RESISTORS

The low input current noise and input bias current of the

AD8605 make it the ideal amplifier for circuits with substantial

input source resistance such as photodiodes. Input offset voltage

increases by less than 0.5 nV per 1 kΩ of source resistance at

room temperature and increases to 10 nV at 85°C. The total

noise density of the circuit is

()

S

S

n

n

TOTAL

n

TR

k

R

i

e

e

4

2

2

,

+

+

=

where:

T is the ambient temperature in Kelvin (T = 273 + °C)

roughly 15 nV/√Hz.

The current noise of the AD8605 is so low that its total density

6 MΩ. The total equivalent rms noise over a specific bandwidth

is expressed as

(

e

E

TOTAL

n

n

,

=

where BW is the bandwidth in hertz.

Note that the analysis above is valid for frequencies greater than

100 Hz and assumes relatively flat noise, above 10 kHz. For

lower frequencies, flicker noise (1/f) must be considered.

CHANNEL SEPARATION

Channel separation, or inverse crosstalk, is a measure of the

signal feed from one amplifier (channel) to an other on the

same IC.

The AD8606 has a channel separation of greater than −160 dB

up to frequencies of 1 MHz, allowing the two amplifiers to

amplify ac signals independently in most applications.

FREQUENCY (Hz)

10M

1M

100k

10k

1k

100

100M

–20

0

–40

–60

–80

–100

–120

–140

–160

–180

Figure 46. Channel Separation vs. Frequency

CAPACITIVE LOAD DRIVE

The AD8605 can drive large capacitive loads without oscillation.

Figure 47 shows the output of the AD8606 in response to a

200 mV input signal. In this case, the amplifier was configured

in positive unity gain, worst case for stability, while driving a

1,000 pF load at its output. Driving larger capacitive loads in

unity gain may require the use of additional circuitry.

TIME (10

µs/DIV)

Figure 47. Capacitive Load Drive without Snubber

A snubber network, shown in Figure 48, helps reduce the signal

overshoot to a minimum and maintain stability. Although this

circuit does not recover the loss of bandwidth induced by large

capacitive loads, it greatly reduces the overshoot and ringing.

This method does not reduce the maximum output swing of

the amplifier.

Figure 49 shows a scope photograph of the output at the

snubber circuit. The overshoot is reduced from over 70% to

less than 5%, and the ringing is eliminated by the snubber.