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

• Manufacturer Part Number: ADA4841-1YRZ
• Manufacturer: Analog Devices

APPLICATIONS
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 the perfect driver solution for low power,
16-bit ADCs, such as the AD7685. Figure 47 shows a typical
16-bit single-supply application. There are different challenges
to do a single-supply, high resolution design, and the
ADA4841-1 addresses these nicely. In a single-supply system,
one of the main challenges is to use the amplifier in buffer
mode to have the lowest output noise and still preserve linearity
compatible with the ADC. Rail-to-rail input amplifiers are
usually higher noise than the ADA4841-1 and cannot be used
in this mode because of the nonlinear region around the
crossover point of their input stages. The ADA4841-1, which
has no crossover region but has a wide linear input range from
100 mV below ground to 1 V below positive rail, solves this
problem, as shown in Figure 47. It can accept the 0 V to 4.096 V
input range with a supply as low as 5.2 V. This supply also
allows the use of a small, low dropout, low temperature drift
ADR364
range close to ground, the ADA4841-1 can exhibit some
nonlinearity, such as any rail-to-rail output amplifier. The
ADA4841-1 drives a one-pole, low-pass filter. This filter limits
the already very low noise contribution from the amplifier to
the AD7685.
0V TO 4.096V
Table 6. Recommended Values and Typical Performance
Gain
+1
+2
−1
+5
+10
+20
R
0
499
499
499
499
499
reference voltage. Note that at the low end of the input
F
(Ω)
ADA4841
R
N/A
499
499
124
54.9
26.1
G
Figure 47. ADC Driver Schematic
+5.2V
(Ω)
100nF
100nF
−3 dB BW (MHz)
77
34
38
11
5
2.3
2.7nF
33Ω
ADR364
10μF
IN+
IN–
GND
REF
Slew Rate (V/μs)
12.5
12.5
12.5
12
12
11.2
AD7685
100nF
VDD
SDO
SCK
CNV
VIO
SDI
Rev. A | Page 17 of 20
Peaking
(dB)
0.9
0.3
0.4
0
0
0
RECONSTRUCTION FILTER
The ADA4841-1 can also be used as a reconstruction filter at
the output of DACs for suppression of the sampling frequency.
The filter shown in Figure 48 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.
INPUT
Output Noise ADA4841-1
Only (nV/√Hz)
2
4
4
10
20
40
Q
K
f
C
Figure 48. Two-Pole 500 kHz Reconstruction Filter Schematic
=
=
=
49Ω
3
R
R
R
2
T
−
4
3
1
π
K
R
+
1
499Ω
1
1
C
R1
1
499Ω
R2
660pF
499Ω
C1
R3
U1
Total Output Noise Including
Resistors (nV/√Hz)
2
5.73
5.73
11.9
21.1
42.2
+5V
–5V
660pF
C2
280Ω
R4
0.1μF
0.1μF
10μF
10μF
ADA4841-1
OUTPUT