ADA4841-2_15 Datasheet, PDF (17/20 Pages) Analog Devices – Low Power, Low Noise and Distortion, Rail-to-Rail Output Amplifiers

Data Sheet
ADA4841-1/ADA4841-2
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 ADR364 reference voltage. If
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.
+5.2V
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.
C2
1320pF
+5V 10F
INPUT
R1
249
R2
249
C1
1320pF
0.1F
U1
0.1F
OUTPUT
10F
–5V
R3
840
R4
499
Figure 49. Two-Pole 500 kHz Reconstruction Filter Schematic
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
1
f_C  _2R1C1
The quality factor, or Q, is shown in the equation
Q ¹
3K
For minimum peaking, set Q equal to 0.707.
The gain, or K, of the amplifier is
K  ^R4 1
R3
Resistor values are kept low for minimal noise contribution,
offset voltage, and optimal frequency response.
100nF
ADR364
10F
100nF
0V TO 4.096V
ADA4841 ^100nF
33
–0.25V
2.7nF
REF
IN+
VDD
AD7685
IN–
GND
VIO
SDI
SCK
SDO
CNV
Figure 50. ADC Driver Schematic
Rev. F | Page 17 of 20