AD79821 datasheet(15/24 Pages) AD

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AD7946

Rev. A | Page 15 of 24

POWER SUPPLY

The AD7946 is specified at 4.5 V to 5.5 V. It uses two power

supply pins: a core supply VDD and a digital input/output

interface supply VIO. VIO allows direct interface with any

logic between 1.8 V and VDD. To reduce the supplies needed,

the VIO and VDD can be tied together. The AD7946 is

independent of power supply sequencing between VIO and

VDD. Additionally, it is very insensitive to power supply

variations over a wide frequency range, as shown in Figure 29,

which represents PSRR over frequency.

FREQUENCY (kHz)

100

10k

VDD = 5V

Figure 29. PSRR vs. Frequency

The AD7946 powers down automatically at the end of each

conversion phase and, therefore, the power scales linearly with

the sampling rate, as shown in Figure 30. This makes the part

ideal for low sampling rate (even a few Hz) and low battery-

powered applications.

0.001

0.1

0.01

100

1000

100

10k

100k

SAMPLING RATE (SPS)

VIO

VDD = 5V

Figure 30. Operating Currents vs. Sampling Rate

SUPPLYING THE ADC FROM THE REFERENCE

For simplified applications, the AD7946, with its low operating

current, can be supplied directly using the reference circuit

shown in Figure 31. The reference line can be driven by one of

the following:

•

The system power supply directly.

•

A reference voltage with enough current output capability,

such as the ADR43x.

•

A reference buffer, such as the AD8031 or AD8603,

which can also filter the system power supply, as shown

in Figure 31.

AD8603

AD7946

VIO

REF

(NOTE 1)

VDD

10µF

1µF

10kΩ

1kΩ

47kΩ

1µF

NOTES

1. OPTIONAL REFERENCE BUFFER AND FILTER

Figure 31. Example of Application Circuit

SINGLE-SUPPLY APPLICATION

Figure 32 shows a typical 14-bit single-supply application. There

are different challenges to doing a single-supply, high resolution

design, and the ADA4841 addresses these nicely. The combina-

tion of low noise, low power, wide input range, rail-to-rail output,

and high speed make the ADA4841 a perfect driver solution for

low power, single-supply 14-bit ADCs, such as the AD7946. 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 usually have higher noise than the ADA4841

and cannot be used on their entire input range in buffer mode

because of the nonlinear region around the crossover point of

their input stage. The ADA4841, which has no crossover region

but has a wide linear input range from ground to 1 V below

positive rail, solves this issue, as shown in Figure 32, where it

can accept the 0 V to 4.096 V input range with a supply as low

as 5.2 V. This supply allows using a small, low dropout, low

temperature drift ADR364 reference voltage. Note that, like any

rail-to-rail output amplifier at the low end of its output range

close to ground, the ADA4841 can exhibit some nonlinearity on

a small region of approximately 25 mV from ground. The

ADA4841 drives a 1-pole, low-pass filter. This filter limits the

already very low noise contribution from the amplifier to the

AD7946.

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