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AD7988-1_1208 Datasheet(PDF) 17 Page - Analog Devices
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AD7988-1_1208 Datasheet(HTML) 17 Page - Analog Devices
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Rev. C | Page 17 of 24
VOLTAGE REFERENCE INPUT
The AD7988-x 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 example, 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,
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
The AD7988-x 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 AD7988-x 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 33.
Figure 33. 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%). If REF = 2.5V, and VDD =
2.5 V, performance is degraded as can be seen in Table 2.
The AD7988-x powers down automatically at the end of each
Although the AD7988-x has a reduced number of pins, it offers
flexibility in its serial interface modes.
The AD7988-x, 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 and is 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
The AD7988-x, 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. CS mode is selected if SDI is
high, and 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.
The user must time out the maximum conversion time prior to
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