ADA4841-1/ADA4841-2

Rev. E | Page 17 of 20

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.

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.

U1

C2

1320pF

R3

840

Ω

INPUT

+5V

10

μF

0.1

μF

–5V

10

μF

0.1

μF

OUTPUT

C1

1320pF

R2

249

Ω

R1

249

Ω

R4

499

Ω

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

1

2

1

C

R

π

=

The quality factor, or Q, is shown in the equation

K

Q

−

=

3

1

For minimum peaking, set Q equal to 0.707.

The gain, or K, of the amplifier is

1

+

=

R3

R4

K

Resistor values are kept low for minimal noise contribution,

offset voltage, and optimal frequency response.

AD7685

REF

GND

VDD

IN–

IN+

VIO

SDI

SCK

SDO

CNV

100nF

10

μF

ADR364

33

Ω

2.7nF

–0.25V

0V TO 4.096V

ADA4841

100nF

+5.2V

100nF

Figure 50. ADC Driver Schematic