AD7980 Datasheet, PDF (13/22 Pages) Analog Devices – 16-bit, 1 MSPS PulSAR ADC in MSOP/QFN

Preliminary Technical Data
VOLTAGE REFERENCE INPUT
The AD7980 voltage reference input, REF, has a dynamic, input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
When REF is driven by a very low impedance source, for
example, a reference buffer using the AD8031 or the AD8605, a
10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate for
optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift ADR43x reference.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor (for example, 100 nF) between the
REF and GND pins.
POWER SUPPLY
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 AD7980 is
independent of power supply sequencing between VIO and
VDD. Additionally, it is very insensitive to power supply
variations over a wide frequency range.
The AD7980 powers down automatically at the end of each
conversion phase and, therefore, the power scales linearly with
the sampling rate. This makes the part ideal for low sampling
rate (even a few Hz) and low battery-powered applications.
AD7980
DIGITAL INTERFACE
Though the AD7980 has a reduced number of pins, it offers
flexibility in its serial interface modes.
The AD7980, when in CS mode, is compatible with SPI, QSPI,
digital hosts, and DSPs, e.g., Blackfin® ADSP-BF53x or ADSP-
219x. This interface can use either 3-wire or 4-wire. A 3-wire
interface using the CNV, SCK, and SDO signals minimizes
wiring connections useful, for instance, in isolated applications.
A 4-wire interface using the SDI, CNV, SCK, and SDO signals
allows CNV, which initiates the conversions, to be independent
of the readback timing (SDI). This is useful in low jitter
sampling or simultaneous sampling applications.
The AD7980, when in chain mode, 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 AD7980 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:
• In the CS mode, if CNV or SDI is low when the ADC
conversion ends (Figure 18 and Figure 22).
• In the chain mode, if SCK is high during the CNV rising edge
(Figure 26).
Rev Pr C | Page 13 of 22