AD8605/AD8606/AD8608
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.
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 kꢀ of source resistance at
room temperature and increases to 10 nV at 85°C. The total
noise density of the circuit is
0
–20
–40
2
2
en
= en
+
(
inRS
)
+ 4kTRS
,TOTAL
where:
–60
en is the input voltage noise density of the AD8605.
in is the input current noise density of the AD8605.
RS is the source resistance at the noninverting terminal.
k is Boltzmann’s constant (1.38 × 10−23 J/K).
–80
–100
–120
–140
–160
–180
T is the ambient temperature in Kelvin (T = 273 + °C).
For example, with RS = 10 kꢀ, the total voltage noise density is
roughly 15 nV/√Hz.
For RS < 3.9 kꢀ, en dominates and en, TOTAL ≈ en.
100
1k
10k
100k
1M
10M
100M
FREQUENCY (Hz)
The current noise of the AD8605 is so low that its total density does
not become a significant term unless RS is greater than 6 Mꢀ.
Figure 46. Channel Separation vs. Frequency
The total equivalent rms noise over a specific bandwidth is
expressed as
V
= ±2.5V
= 1
= 10kΩ
= 1000pF
S
A
R
C
V
L
L
En =
en,TOTAL
BW
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.
TIME (10µs/DIV)
Figure 47. AD8606 Capacitive Load Drive Without Snubber
CAPACITIVE LOAD DRIVE
V+
The AD860x 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 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.
4
2
200mV
1
AD8605
V
3
IN
R
R
C
L
S
L
8
C
S
V–
Figure 48. Snubber Network Configuration
Rev. H | Page 16 of 24
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