AD7946 Analog Devices, AD7946 Datasheet - Page 15

• Manufacturer Part Number: AD7946
• Manufacturer: Analog Devices

Specifications of AD7946

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 500kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

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.
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
1000
0.01
100
0.1
10
90
80
70
60
50
40
30
1
10
1
Figure 30. Operating Currents vs. Sampling Rate
100
Figure 29. PSRR vs. Frequency
10
SAMPLING RATE (SPS)
FREQUENCY (kHz)
1k
100
VDD = 5V
VDD = 5V
10k
VIO
1k
100k
10k
1M
Rev. A | Page 15 of 24
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:
•
•
•
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
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
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.
5V
NOTES
1. OPTIONAL REFERENCE BUFFER AND FILTER
The system power supply directly.
A reference voltage with enough current output capability,
such as the ADR43x.
A reference buffer, such as the
which can also filter the system power supply, as shown
in Figure 31.
1kΩ
1µF
47kΩ
Figure 31. Example of Application Circuit
ADA4841
ADR364
5V
AD8603
(NOTE 1)
addresses these nicely. The combina-
reference voltage. Note that, like any
10µF
AD8031
REF
10kΩ
or AD8603,
AD7946
VDD
AD7946
1µF
VIO