AD7985 datasheet(19/28 Pages) AD

19 / 28 page

Data Sheet

ADA4896-2/ADA4897-1/ADA4897-2

Rev.

| Page 19 of 28

NOISE CONSIDERATIONS

Figure 48 illustrates the primary noise contributors for the

typical gain configurations. The total rms output noise is

the root-mean-square of all the contributions.

iep

ien

+ vout_en –

ven

4kT × RS

vn _ RS =

4kT × RG

vn _ RG =

4kT × RF

vn _ RF =

Figure 48. Noise Sources in Typical Connection

The output noise spectral density can be calculated by

[]

ien

kTR

ven

iep

kTRs

kTR

vout

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

(6)

where:

k is Boltzmann’s constant.

T is the absolute temperature (degrees Kelvin).

iep and ien represent the amplifier input current noise spectral

density (pA/√Hz).

ven is the amplifier input voltage noise spectral density (nV/√Hz).

RS is the source resistance, as shown in Figure 48.

RF and RG are the feedback network resistances, as shown in

Figure 48.

Source resistance noise, amplifier voltage noise (ven), and the

voltage noise from the amplifier current noise (iep × RS) are all

subject to the noise gain term (1 + RF/RG). Note that with a

1 nV/√Hz input voltage noise and 2.8 pA/√Hz input current

noise, the noise contributions of the amplifier are relatively

small for source resistances from approximately 50 Ω to 700 Ω.

Figure 49 shows the total RTI noise due to the amplifier vs. the

source resistance. In addition, the value of the feedback resistors

used affects the noise. It is recommended that the value of the

feedback resistors be maintained between 250 Ω and 1 kΩ to

keep the total noise low.

500

SOURCE RESISTANCE (Ω)

0.5

500

50k

TOTAL

AMPLIFIER NOISE

AMPLIFIER AND

RESISTOR NOISE

SOURCE

RESISTANCE NOISE

Figure 49. RTI Noise vs. Source Resistance

CAPACITANCE DRIVE

Capacitance at the output of an amplifier creates a delay within the

feedback path that, if within the bandwidth of the loop, can create

excessive ringing and oscillation. The ADA4896-2/ADA4897-1/

ADA4897-2 show the most peaking at a gain of +2 (see Figure 9).

Placing a small snub resistor (RSNUB) in series with the amplifier

output and the capacitive load mitigates the problem. Figure 50

shows the effect of using a snub resistor (RSNUB) on reducing the

peaking for the worst-case frequency response (gain of +2).

Using RSNUB = 100 Ω eliminates the peaking entirely, with the

trade-off that the closed-loop gain is reduced by 0.8 dB due to

attenuation at the output. RSNUB can be adjusted from 0 Ω to

100 Ω to maintain an acceptable level of peaking and closed-

loop gain (see Figure 50).

–5

–4

–3

–2

–1

FREQUENCY (MHz)

0.1

100

RSNUB = 50Ω

RSNUB = 0Ω

RSNUB = 100Ω

ADA4896-2

1kΩ

249Ω

39pF

RSNUB

VIN

VOUT

VS = +5V

VOUT = 200mV p-p

G = +2

Figure 50. Using a Snub Resistor to Reduce Peaking

Due to Output Capacitive Load

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