ADA4899-1_07 AD [Analog Devices], ADA4899-1_07 Datasheet - Page 15

• Manufacturer Part Number: ADA4899-1_07
• Description: Unity-Gain Stable, Ultralow Distortion, 1 nV/?Hz Voltage Noise, High Speed Op Amp
• Manufacturer: AD [Analog Devices]

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.
All resistors have a Johnson noise that is calculated by
where:
k is Boltzmann’s Constant (1.38 × 10
B is the bandwidth in Hz.
T is the absolute temperature in Kelvin.
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.
B
A
(4kBTR
V
V
4kTR1
4kTR3
N, R1
N, R3
RTI NOISE =
RTO NOISE = NG × RTI NOISE
R1
R3
)
Figure 48. Op Amp Noise Analysis Model
I
I
N–
N+
+
V
I
N+
N
2
2
V
R3
+ 4kTR3 + 4kTR1
N
V
4kTR2
2
N, R2
+ I
N–
2
R1 × R2
R1 + R2
–23
R2
J/K).
R1 + R2
B TO OUTPUT
2
R2
GAIN FROM
+ 4kTR2
A TO OUTPUT
GAIN FROM
2
NOISE GAIN =
V
OUT
NG = 1 +
R1 + R2
R1
= –
R2
R2
R1
R1
=
2
Rev. B | Page 15 of 20
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.
Table 5. ADA4899-1, Single-Ended Driver for AD7677
16-Bit, 1 MSPS, f
Parameter
Second Harmonic Distortion
Third Harmonic Distortion
THD
SFDR
SNR
The ADA4899-1 configured as a single-ended-to-differential
driver for the
associated performance.
ANALOG
Table 6. ADA4899-1, Single Ended-to-Differential Driver for
AD7677 16-Bit, 1 MSPS, f
Parameter
THD
SFDR
SNR
ANALOG
ANALOG
INPUT
INPUT
INPUT
+
–
+2.5V REF
Figure 50. Single-Ended-to-Differential ADC Driver
25Ω
25Ω
590Ω
590Ω
+2.5V
AD7677
REF
Figure 49. Single-Ended Input ADC Driver
C
= 50 kHz
+5V
–5V
590Ω
590Ω
+5V
+5V
–5V
–5V
ADA4899-1
is shown in Figure 50. Table 6 shows the
ADA4899-1
ADA4899-1
C
= 500 kHz
15Ω
15Ω
590Ω
590Ω
+5V
–5V
ADA4899-1
Measurement (dB)
−116.5
−111.9
−108.6
+101.4
+92.6
Measurement (dB)
−92.7
+91.8
+90.6
2.7nF
2.7nF
15Ω
15Ω
ADA4899-1
2.7nF
2.7nF
IN+
IN–
AD7677
IN+
IN–
AD7677