AD7980BRMZRL7 Datasheet, PDF (16/28 Pages) Analog Devices – 16-Bit, 1 MSPS PulSAR ADC in MSOP/QFN

AD7980
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 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, a reference-decoupling capacitor value as small as
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
The AD7980 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 5.0 V. To reduce the
number of supplies needed, 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, as shown in Figure 29.
80
75
70
65
60
55
1
10
100
1000
FREQUENCY (kHz)
Figure 29. PSRR vs. Frequency
To ensure optimum performance, VDD should be roughly half
of REF, the voltage reference input. For example, if REF is 5.0 V,
VDD should be set to 2.5 V (±5%).
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 of a few Hz) and low battery-powered applications.
10.000
1.000
0.100
0.010
I_VDD
I_REF
I_VIO
0.001
10000
100000
SAMPLING RATE (SPS)
Figure 30. Operating Currents vs. Sampling Rate
1000000
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,
and digital hosts. This interface can use either a 3-wire or 4-wire
interface. 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 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
 In the CS mode if CNV or SDI is low when the ADC
conversion ends (see Figure 34 and Figure 38).
 In the chain mode if SCK is high during the CNV rising edge
(see Figure 42).
Rev. B | Page 16 of 28