EVAL-AD7689EDZ Datasheet, PDF (19/32 Pages) Analog Devices – 16-Bit, 4-Channel/8-Channel, 250 kSPS PulSAR ADC

Data Sheet
Unipolar or Bipolar
Figure 26 shows an example of the recommended connection
diagram for the AD7682/AD7689 when multiple supplies are
available.
Bipolar Single Supply
Figure 27 shows an example of a system with a bipolar input
using single supplies with the internal reference (optional
different VIO supply). This circuit is also useful when the
amplifier/signal conditioning circuit is remotely located with
some common mode present. Note that for any input config-
uration, the INx inputs are unipolar and are always referenced
to GND (no negative voltages even in bipolar range).
For this circuit, a rail-to-rail input/output amplifier can be used;
however, the offset voltage vs. input common-mode range should
be noted and taken into consideration (1 LSB = 62.5 μV with
VREF = 4.096 V). Note that the conversion results are in twos
complement format when using the bipolar input configuration.
Refer to the AN-581 Application Note, Biasing and Decoupling
Op Amps in Single Supply Applications, for additional details
about using single-supply amplifiers.
ANALOG INPUTS
Input Structure
Figure 28 shows an equivalent circuit of the input structure of
the AD7682/AD7689. The two diodes, D1 and D2, provide ESD
protection for the analog inputs, IN[7:0] and COM. Care must
be taken to ensure that the analog input signal does not exceed
the supply rails by more than 0.3 V because this causes the
diodes to become forward biased and to start conducting
current.
These diodes can handle a forward-biased current of 130 mA
maximum. For instance, these conditions may eventually occur
when the input buffer supplies are different from VDD. In such
a case, for example, an input buffer with a short circuit, the
current limitation can be used to protect the part.
VDD
INx+
OR INx–
OR COM
GND
D1
C_PIN
D2
R_IN
C_IN
Figure 28. Equivalent Analog Input Circuit
This analog input structure allows the sampling of the true
differential signal between INx+ and COM or INx+ and INx−.
(COM or INx− = GND ± 0.1 V or VREF ± 0.1 V). By using these
differential inputs, signals common to both inputs are rejected,
as shown in Figure 29.
AD7682/AD7689
70
65
60
55
50
45
40
35
30
1
10
100
1k
10k
FREQUENCY (kHz)
Figure 29. Analog Input CMRR vs. Frequency
During the acquisition phase, the impedance of the analog inputs
can be modeled as a parallel combination of the capacitor, CPIN,
and the network formed by the series connection of RIN and CIN.
CPIN is primarily the pin capacitance. RIN is typically 2.2 kΩ and
is a lumped component composed of serial resistors and the on
resistance of the switches. CIN is typically 27 pF and is mainly
the ADC sampling capacitor.
Selectable Low-Pass Filter
During the conversion phase, where the switches are opened,
the input impedance is limited to CPIN. While the AD7682/
AD7689 are acquiring, RIN and CIN make a one-pole, low-pass
filter that reduces undesirable aliasing effects and limits the
noise from the driving circuitry. The low-pass filter can be pro-
grammed for the full bandwidth or ¼ of the bandwidth with
CFG[6], as shown in Table 10. This setting changes RIN to 19 kΩ.
Note that the converter throughput must also be reduced by ¼
when using the filter. If the maximum throughput is used with the
bandwidth (BW) set to ¼, the converter acquisition time, tACQ, is
violated, resulting in increased THD.
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