ADA4899-1YCPZ-R2 datasheet(15/20 Pages) AD

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ADA4899-1

Rev. A | Page 15 of 20

NOISE

To analyze the noise performance of an amplifier circuit, first

identify the noise sources, then determine if the source has a

significant contribution to the overall noise performance of the

amplifier. To simplify the noise calculations, noise spectral

densities were used, rather than actual voltages to leave

bandwidth out of the expressions (noise spectral density, which

is generally expressed in nV/√Hz, is equivalent to the noise in a

1 Hz bandwidth).

The noise model shown in Figure 48 has six individual noise

sources: the Johnson noise of the three resistors, the op amp

voltage noise, and the current noise in each input of the

amplifier. Each noise source has its own contribution to the

noise at the output. Noise is generally specified referred to input

(RTI), but it is often simpler to calculate the noise referred to

the output (RTO) and then divide by the noise gain to obtain

the RTI noise.

GAIN FROM

"B" TO OUTPUT

= – R2

GAIN FROM

"A" TO OUTPUT

NOISE GAIN =

NG = 1 + R2

IN–

VN, R1

VN, R3

IN+

4kTR2

4kTR1

4kTR3

VN, R2

VN2 + 4kTR3 + 4kTR1

R1 + R2

IN+2R32 + IN–2

R1 × R2

+ 4kTR2

R1 + R2

RTI NOISE =

RTO NOISE = NG × RTI NOISE

VOUT

Figure 48. Op Amp Noise Analysis Model

All resistors have a Johnson noise that is calculated by

)

(4kBTR

where:

k is Boltzmann’s Constant (1.38 × 10–23 J/K).

T is the absolute temperature in Kelvin.

B is the bandwidth in Hz.

R is the resistance in ohms.

A simple relationship that is easy to remember is that a 50 Ω

resistor generates a Johnson noise of 1 nV√Hz at 25°C.

In applications where noise sensitivity is critical, care must be

taken not to introduce other significant noise sources to the

amplifier. Each resistor is a noise source. Attention to the

following areas is critical to maintain low noise performance:

design, layout, and component selection. A summary of noise

performance for the amplifier and associated resistors can be

seen in Table 4.

ADC DRIVER

The ultralow noise and distortion performance of the

ADA4899-1 makes it an excellent candidate for driving 16-bit

ADCs. The schematic for a single-ended input buffer using the

ADA4899-1 and the AD7677, a 1 MSPS, 16-bit ADC, is shown

in Figure 49. Table 5 shows the performance data of the

ADA4899-1 and the AD7677.

+5V

ANALOG

INPUT

ADA4899-1

–5V

2.7nF

25Ω

15Ω

+5V

ANALOG

INPUT

–

ADA4899-1

–5V

2.7nF

25Ω

15Ω

AD7677

IN+

IN–

+2.5V

REF

–5V

REF

Figure 49. Single-Ended Input ADC Driver

Table 5. ADA4899-1, Single-Ended Driver for AD7677

16-Bit, 1 MSPS, fc = 50 kHz

Parameter

Measurement (dB)

Second Harmonic Distortion

−116.5

Third Harmonic Distortion

−111.9

THD

−108.6

SFDR

+101.4

SNR

+92.6

The ADA4899-1 configured as a single-ended-to-differential

driver for the AD7677 is shown in Figure 50. Table 6 shows the

associated performance.

+5V

+2.5V REF

ANALOG

INPUT

ADA4899-1

–5V

590Ω

+5V

+2.5V

REF

+2.5V

ADA4899-1

–5V

2.7nF

590Ω

15Ω

590Ω

AD7677

IN+

IN–

REF

+5V

–5V

Figure 50. Single-Ended-to-Differential ADC Driver

Table 6. ADA4899-1, Single Ended-to-Differential Driver for

AD7677 16-Bit, 1 MSPS, fc = 500 kHz

Parameter

Measurement (dB)

THD

−92.7

SFDR

+91.8

SNR

+90.6

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