ADA4841-1 Analog Devices, ADA4841-1 Datasheet - Page 17

• Manufacturer Part Number: ADA4841-1
• Description: Low Power, Low Noise and Distortion, Rail-to-Rail Output Amplifier
• Manufacturer: Analog Devices

Specifications of ADA4841-1

• Vcc-vee: 2.7V to 12V
• Isy Per Amplifier: 1.5mA
• Packages: SOIC,SOT
• -3db Bandwidth: 80MHz
• Slew Rate: 13V/µs
• Vos: 40µV
• Ib: 3µA
• # Opamps Per Pkg: 1
• Input Noise (nv/rthz): 2.1nV/rtHz

APPLICATIONS INFORMATION
TYPICAL PERFORMANCE VALUES
To reduce design time and eliminate uncertainty Table 6
provides a convenient reference for typical gains, component
values, and performance parameters.
16-BIT ADC DRIVER
The combination of low noise, low power, and high speed
make the ADA4841-1/ADA4841-2 the perfect driver solution
for low power, 16-bit ADCs, such as the AD7685. Figure 50
shows a typical 16-bit single-supply application.
There are different challenges to a single-supply, high resolution
design, and the ADA4841-1/ADA4841-2 address these nicely.
In a single-supply system, a main challenge is using the
amplifier in buffer mode with the lowest output noise and
preserving linearity compatible with the ADC.
Rail-to-rail input amplifiers are usually higher noise than the
ADA4841-1/ADA4841-2 and cannot be used in this mode
because of the nonlinear region around the crossover point of
their input stages. The ADA4841-1/ADA4841-2, which have no
crossover region but have a wide linear input range from 100 mV
below ground to 1 V below positive rail, solve this problem, as
shown in Figure 50. The amplifier, when configured as a
follower, has a linear signal range from 0.25 V above the minus
supply voltage (limited by the amplifier’s output stage) to 1 V
below the positive supply (limited by the amplifier input stage).
A 0 V to +4.096 V signal range can be accommodated with a
positive supply as low as +5.2 V and a negative power supply of
−0.25 V. The 5.2 V supply also allows the use of a small, low
dropout, low temperature drift
ground is used as the amplifier negative supply, then note that at
the low end of the input range close to ground, the ADA4841-1/
ADA4841-2 exhibit substantial nonlinearity, as any rail-to-rail
output amplifier. The ADA4841-1/ADA4841-2 drive a one-
pole, low-pass filter. This filter limits the already very low noise
contribution from the amplifier to the AD7685.
ADR364
0V TO 4.096V
reference voltage. If
ADA4841
–0.25V
+5.2V
Figure 50. ADC Driver Schematic
100nF
100nF
Rev. E | Page 17 of 20
2.7nF
33Ω
ADR364
RECONSTRUCTION FILTER
The ADA4841-1/ADA4841-2 can also be used as a reconstruction
filter at the output of DACs for suppression of the sampling
frequency. The filter shown in Figure 49 is a two-pole, 500 kHz
Sallen-Key LPF with a fixed gain of G = +1.6.
Setting the resistors and capacitors equal to each other greatly
simplifies the design equations for the Sallen-Key filter. The corner
frequency, or −3 dB frequency, can be described by the equation
The quality factor, or Q, is shown in the equation
For minimum peaking, set Q equal to 0.707.
The gain, or K, of the amplifier is
Resistor values are kept low for minimal noise contribution,
offset voltage, and optimal frequency response.
10μF
INPUT
IN+
IN–
Q
K
f
C
Figure 49. Two-Pole 500 kHz Reconstruction Filter Schematic
=
=
GND
=
REF
3
R4
R3
2
−
π
1
AD7685
R
249Ω
1
K
+
100nF
R1
1
C
1
1
VDD
249Ω
1320pF
R2
SDO
SCK
CNV
VIO
SDI
840Ω
C1
R3
ADA4841-1/ADA4841-2
U1
+5V
–5V
1320pF
C2
499Ω
R4
0.1μF
0.1μF
10μF
10μF
OUTPUT