EVAL-AD7689EDZ Analog Devices Inc, EVAL-AD7689EDZ Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7689EDZ
• Description: BOARD EVAL AD7689
• Manufacturer: Analog Devices Inc
• Series: PulSAR®r

Specifications of EVAL-AD7689EDZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 250k
• Data Interface: Serial
• Inputs Per Adc: 8 Single Ended
• Input Range: ±VREF
• Power (typ) @ Conditions: 12.5mW @ 250kSPS, 5 V
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Utilized Ic / Part: AD7689
• Silicon Manufacturer: Analog Devices
• Application Sub Type: ADC
• Kit Application Type: Data Converter
• Silicon Core Number: AD7689
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7682/AD7689
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
V
complement format when using the bipolar input configuration.
Refer to the AN-581 Application Note, Biasing and Decoupling
Op Amps in Single Supply Applications, at
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.
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 V
differential inputs, signals common to both inputs are rejected,
as shown in Figure 29.
REF
= 4.096 V). Note that the conversion results are in twos
OR COM
OR INx–
GND
INx+
Figure 28. Equivalent Analog Input Circuit
C
PIN
VDD
D1
D2
REF
± 0.1 V). By using these
www.analog.com
R
IN
C
IN
for
Rev. B | Page 18 of 32
During the acquisition phase, the impedance of the analog inputs
can be modeled as a parallel combination of the capacitor, C
and the network formed by the series connection of R
C
is a lumped component composed of serial resistors and the on
resistance of the switches. C
the ADC sampling capacitor.
Selectable Low-Pass Filter
During the conversion phase, where the switches are opened,
the input impedance is limited to C
AD7689 are acquiring, R
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 R
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, t
violated, resulting in increased THD.
PIN
is primarily the pin capacitance. R
70
65
60
55
50
45
40
35
30
1
Figure 29. Analog Input CMRR vs. Frequency
10
IN
FREQUENCY (kHz)
and C
IN
is typically 27 pF and is mainly
100
IN
make a one-pole, low-pass
PIN
. While the AD7682/
IN
is typically 2.2 kΩ and
1k
IN
to 19 kΩ.
IN
10k
and C
ACQ
, is
PIN
IN
,
.