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AD7327 Datasheet(PDF) 19 Page - Analog Devices

Part No. AD7327
Description  500 kSPS, 8-Channel, Software-Selectable, True Bipolar Input, 12-Bit Plus Sign ADC
Download  36 Pages
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7327 Datasheet(HTML) 19 Page - Analog Devices

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AD7327
Rev. 0 | Page 19 of 36
TYPICAL CONNECTION DIAGRAM
Figure 32 shows a typical connection diagram for the AD7327.
In this configuration, the AGND pin is connected to the analog
ground plane of the system, and the DGND pin is connected to
the digital ground plane of the system. The analog inputs on the
AD7327 can be configured to operate in single-ended, true
differential, or pseudo differential mode. The AD7327 can operate
with either an internal or external reference. In Figure 32, the
AD7327 is configured to operate with the internal 2.5 V reference.
A 680 nF decoupling capacitor is required when operating with
the internal reference.
The VCC pin can be connected to either a 3 V supply voltage or a
5 V supply voltage. The VDD and VSS are the dual supplies for the
high voltage analog input structures. The voltage on these pins
must be equal to or greater than the highest analog input range
selected on the analog input channels (see Table 6). The VDRIVE
pin is connected to the supply voltage of the microprocessor.
The voltage applied to the VDRIVE input controls the voltage of
the serial interface. VDRIVE can be set to 3 V or 5 V.
AD7327
VCC
VDD1
SERIAL
INTERFACE
µC/µP
VIN0
VIN1
VIN2
VIN3
VIN4
VIN5
VIN6
VIN7
REFIN/OUT
CS
DOUT
VDRIVE
SCLK
DIN
DGND
10µF
0.1µF
+
10µF
0.1µF
+
10µF
0.1µF
ANALOG INPUTS
±10V, ±5V, ±2.5V
0V TO +10V
+15V
–15V
680nF
VSS1
VCC +2.7V TO 5.25V
1MINIMUM VDD AND VSS SUPPLY VOLTAGES
DEPEND ON THE HIGHEST ANALOG INPUT
RANGE SELECTED.
AGND
10µF
0.1µF
+
+3V SUPPLY
Figure 32. Typical Connection Diagram
ANALOG INPUT
Single-Ended Inputs
The AD7327 has a total of eight analog inputs when operating
the AD7327 in single-ended mode. Each analog input can be
independently programmed to one of the four analog input
ranges. In applications where the signal source is high
impedance, it is recommended to buffer the signal before
applying it to the ADC analog inputs. Figure 33 shows the
configuration of the AD7327 in single-ended mode.
AD73271
VIN+
V+
V–
VDD
VSS
VCC
5V
AGND
1ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 33. Single-Ended Mode Typical Connection Diagram
True Differential Mode
The AD7327 can have a total of four true differential analog
input pairs. Differential signals have some benefits over single-
ended signals, including better noise immunity based on the
device’s common-mode rejection and improvements in
distortion performance. Figure 34 defines the configuration of
the true differential analog inputs of the AD7327.
AD73271
VIN+
VIN
1ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 34. True Differential Inputs
The amplitude of the differential signal is the difference
between the signals applied to the VIN+ and VIN− pins in
each differential pair (VIN+ − VIN−). VIN+ and VIN− should
be simultaneously driven by two signals each of amplitude
±4 × VREF (depending on the input range selected) that are 180°
out of phase. Assuming the ±4 × VREF mode, the amplitude of
the differential signal is −20 V to +20 V p-p (2 × 4 × VREF),
regardless of the common mode.
The common mode is the average of the two signals
(VIN+ + VIN−)/2
and is therefore the voltage on which the two input signals are
centered.
This voltage is set up externally, and its range varies with
reference voltage. As the reference voltage increases, the
common-mode range decreases. When driving the differential
inputs with an amplifier, the actual common-mode range is
determined by the amplifier’s output swing. If the differential
inputs are not driven from an amplifier, the common-mode
range is determined by the supply voltage on the VDD and the
VSS supply pins.
When a conversion takes place, the common mode is rejected,
resulting in a noise-free signal of amplitude −2 × (4 × VREF) to
+2 × (4 × VREF) corresponding to Digital Codes −4096 to +4095.


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