AD7942 Analog Devices, AD7942 Datasheet - Page 16

• Manufacturer Part Number: AD7942
• Manufacturer: Analog Devices

Specifications of AD7942

• Resolution (bits): 14bit
• # Chan: 1
• Sample Rate: 250kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: CSP,SOP

AD7942
Power Supply
The AD7942 is specified over a wide operating range from
2.3 V to 5.5 V. It has, unlike other low voltage converters, a
noise low enough to design a low supply (2.5 V) 14-bit resolu-
tion system with respectable performance. 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 AD7942 is indepen-
dent of power supply sequencing between VIO and VDD.
Additionally, it is insensitive to power supply variations over
a wide frequency range, as shown in Figure 27.
The AD7942 powers down automatically at the end of each
conversion phase and, therefore, the power scales linearly with
the sampling rate, as shown in Figure 28. This makes the part
ideal for low sampling rates (even rates of a few hertz) and low
battery-powered applications.
Supplying the ADC from the Reference
For simplified applications, the AD7942, with its low operating
current, can be supplied directly using the reference circuit, as
shown in Figure 29. The reference line can be driven by either
•
The system power supply directly,
0.001
1000
0.1
10
90
85
80
75
70
65
60
55
0
10
10
Figure 28. Operating Current vs. Sampling Rate
100
Figure 27. PSRR vs. Frequency
VDD = 5V
SAMPLING RATE (SPS)
100
1000
FREQUENCY (kHz)
VDD = 5V
10000
1000
VIO
100000
VDD = 2.5V
1000000
10000
Rev. B | Page 16 of 24
•
•
DIGITAL INTERFACE
Although the AD7942 has a reduced number of pins, it offers
flexibility in its serial interface modes.
When in CS mode, the AD7942 is compatible with SPI, QSPI,
digital hosts, and DSPs (for example, Blackfin® ADSP-BF53x or
ADSP-219x). A 3-wire interface using the CNV, SCK, and SDO
signals minimizes wiring connections, which is useful, for
instance, in isolated applications. A 4-wire interface using the
SDI, CNV, SCK, and SDO signals allows CNV, which initiates
conversions, to be independent of the readback timing (SDI).
This is useful in low jitter sampling or simultaneous sampling
applications.
When in chain mode, the AD7942 provides a daisy-chain
feature using the SDI input for cascading multiple ADCs on
a single data line similar to a shift register.
The mode in which the part operates depends on the SDI level
when the CNV rising edge occurs. The CS mode is selected if
SDI is high and the chain mode is selected if SDI is low. The
SDI hold time is such that when SDI and CNV are connected
together, the chain mode is always selected.
In either mode, the AD7942 offers the flexibility to optionally
force a start bit in front of the data bits. This start bit can be
used as a busy signal indicator to interrupt the digital host and
trigger the data reading. Otherwise, without a busy indicator,
the user must time out the maximum conversion time prior
to readback.
The busy indicator feature is enabled as follows:
•
•
NOTE 1: OPTIONAL REFERENCE BUFFER AND FILTER.
5V
A reference voltage with enough current output capability,
such as the ADR43x, or
A reference buffer, such as the AD8031, that can also filter
the system power supply (see Figure 29).
In the CS mode, if CNV or SDI is low when the ADC
conversion ends (see
In the chain mode, if SCK is high during the CNV rising
edge (see Figure 41).
10kΩ
1µF
Figure 29. Example of Application Circuit
(NOTE 1)
5V
AD8031
Figure 33
10µF
5V
and
REF
Figure 37
10Ω
AD7942
VDD
).
1µF
VIO