Datasheet

Data Sheet AD8605/AD8606/AD8608
Rev. N | Page 17 of 24
TOTAL NOISE INCLUDING SOURCE RESISTORS
The low input current noise and input bias current of the
AD860x 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
(
)
SS
nn
TOTAL
n
TRk
R
ie
e 4
2
2
,
+
+
=
where:
e
n
is the input voltage noise density of the AD860x.
i
n
is the input current noise density of the AD860x.
R
S
is the source resistance at the noninverting terminal.
k is Boltzmann’s constant (1.38 × 10
−23
J/K).
T is the ambient temperature in Kelvin (T = 273 + °C).
For example, with R
S
= 10 kΩ, the total voltage noise density is
roughly 15 nV/√Hz.
For R
S
< 3.9 kΩ, e
n
dominates and e
n, TOTAL
≈ e
n
.
The current noise of the AD860x is so low that its total density
does not become a significant term unless R
S
is greater than 6 MΩ.
The total equivalent rms noise over a specific bandwidth is
expressed as
( )
BWeE
TOTALn
n
,
=
where BW is the bandwidth in hertz.
Note that the previous analysis 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 another 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.
CAPACITIVE LOAD DRIVE
The AD860x can drive large capacitive loads without oscillation.
Figure 48 shows the output of the AD8606 in response to a
200 mV input signal. In this case, the amplifier is configured
in positive unity gain, worst case for stability, while driving a
1000 pF load at its output. Driving larger capacitive loads in
unity gain can require the use of additional circuitry.
A snubber network, shown in Figure 49, 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.
CHANNEL SEPARATION (dB)
FREQUENCY (Hz)
10M1M100k10k1k100 100M
–20
0
–40
–60
–80
–100
–120
–140
–160
–180
02731-046
Figure 47. Channel Separation vs. Frequency
TIME (10µs/DIV)
VOLT
AGE (100mV/DIV)
02731-047
V
S
= ±2.5V
A
V
= 1
R
L
= 10kΩ
C
L
= 1000pF
Figure 48. AD8606 Capacitive Load Drive Without Snubber
R
S
C
S
R
L
C
L
V+
V–
4
2
3
8
1
AD8605
V
IN
200mV
02731-049
Figure 49. Snubber Network Configuration