AD7682 View Datasheet(PDF) - Analog Devices
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AD7682 Datasheet PDF : 28 Pages
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If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially on DNL.
Regardless, there is no need for an additional lower value
ceramic decoupling capacitor (for example, 100 nF) between the
REF and GND pins.
For applications that use multiple AD7682/AD7689s or other
PulSAR devices, it is more effective to use the internal reference
buffer to buffer the external reference voltage, thus reducing
SAR conversion crosstalk.
The voltage reference temperature coefficient (TC) directly impacts
full scale; therefore, in applications where full-scale accuracy
matters, care must be taken with the TC. For instance, a
±15 ppm/°C TC of the reference changes full scale by ±1 LSB/°C.
POWER SUPPLY
The AD7682/AD7689 use two power supply pins: an analog
and digital 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 pins can be tied together. The
AD7682/AD7689 are independent of power supply sequencing
between VIO and VDD. The only restriction is that CNV must
be low when powering up the AD7682/AD7689. Additionally, it
is very insensitive to power supply variations over a wide
frequency range, as shown in Figure 31.
75
70
65
60
55
50
45
40
35
30
1
10
100
1k
10k
FREQUENCY (kHz)
Figure 31. PSRR vs. Frequency
The AD7682/AD7689 power down automatically at the end of
each conversion phase; therefore, the operating currents and
power scale linearly with the sampling rate. This makes the part
ideal for low sampling rates (even of a few hertz) and low
battery-powered applications.
AD7682/AD7689
10000
1000 VDD = 5V, INTERNAL REF
100
VDD = 5V, EXTERNAL REF
10
VDD = 2.5V, EXTERNAL REF
1
VIO
0.1
0.010
0.001
10
100
1k
10k
100k
1M
SAMPLING RATE (sps)
Figure 32. Operating Currents vs. Sampling Rate
SUPPLYING THE ADC FROM THE REFERENCE
For simplified applications, the AD7682/AD7689, with their
low operating current, can be supplied directly using the
reference circuit as shown in Figure 33. The reference line can
be driven by
• The system power supply directly
• A reference voltage with enough current output capability,
such as the ADR43x/ADR44x
• A reference buffer, such as the AD8605, which can also
filter the system power supply, as shown in Figure 33
5V 10kΩ
1µF
5V
5V
AD8605 10µF
10Ω
1µF
0.1µF
0.1µF
1
REF
VDD
VIO
AD7689
1OPTIONAL REFERENCE BUFFER AND FILTER.
Figure 33. Example of an Application Circuit
Rev. 0 | Page 21 of 28
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