AD8605ARTZ-R2 datasheet(17/24 Pages) AD

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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

( )

TOTAL

where:

en is the input voltage noise density of the AD860x.

in is the input current noise density of the AD860x.

RS 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 RS = 10 kΩ, the total voltage noise density is

roughly 15 nV/√Hz.

For RS < 3.9 kΩ, en dominates and en,TOTAL ≈ en.

The current noise of the AD860x is so low that its total density

does not become a significant term unless RS is greater than 6 MΩ.

The total equivalent rms noise over a specific bandwidth is

expressed as

(

) BW

TOTAL

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.

FREQUENCY (Hz)

10M

100k

10k

100

100M

–20

–40

–60

–80

–100

–120

–140

–160

–180

Figure 47. Channel Separation vs. Frequency

TIME (10µs/DIV)

VS = ±2.5V

AV = 1

RL = 10kΩ

CL = 1000pF

Figure 48. AD8606 Capacitive Load Drive Without Snubber

V+

V–

AD8605

VIN

200mV

Figure 49. Snubber Network Configuration

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