AD7731BR-REEL7 Analog Devices Inc, AD7731BR-REEL7 Datasheet - Page 42

• Manufacturer Part Number: AD7731BR-REEL7
• Description: IC ADC 24BIT SIGMA-DELTA 24-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD7731BR-REEL7

• Rohs Status: RoHS non-compliant
• Number Of Bits: 24
• Sampling Rate (per Second): 6.4k
• Data Interface: DSP, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 125mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (0.300", 7.50mm Width)
• For Use With: EVAL-AD7731EBZ - BOARD EVALUATION FOR AD7731

AD7731
Bipolar Input Signals
As mentioned previously, some applications will require that the
AD7731 handle input signals that are negative with respect to
system ground. The number of applications requiring this are
limited but with the addition of some external components the
AD7731 is capable of handling such signals. Figure 25 outlines
one approach to the problem.
The example shown is a system that is driven from 5 V sup-
plies. In such a circuit, two issues must be addressed. The first
is how to get the AD7731 to handle input voltages below ground
and the second is how to generate a suitable reference voltage
for the AD7731. The circuit of Figure 25 attempts to address
these two issues simultaneously.
The AD7731’s analog and digital supplies can be split such that
AV
DGND can also be at separate potentials. The only stipulation
is that AV
Figure 25, the DV
to go down to –2.5 V with respect to system ground. This
means that all logic signals to the part must not exceed 3 V with
respect to system ground. The AV
respect to system ground.
The resistor string R1, R2 and R3 takes the 5 V supply voltage
and generates a differential voltage of nominally 5 V. Amplifiers
DD
+5V
–5V
and DV
ALL VOLTAGE VALUES ARE WITH
RESPECT TO SYSTEM GROUND.
DD
or DV
DD
5k
5k
R1
R3
DD
can be at separate potentials and AGND and
DD
R2
10k
is operated at +3 V which allows the AGND
OR 1/2 OP213
OR 1/2 OP213
must not exceed the AGND by 5.5 V. In
1/2 OP284
1/2 OP284
A1
A2
+5V
–5V
+5V
–5V
REF IN(+)
REF IN(–)
DD
AIN1(+)
AIN1(–)
AGND
AV
is operated at +2.5 V with
DD
Figure 25. Bipolar Input Signals on the AD7731
AGND
AV
DD
100nA
100nA
BUFFER
GROUND
SYSTEM
PGA
–42–
DGND
A1 and A2 buffer the resistor string voltages and provide the
AV
IN(–) voltages for the AD7731. The differential reference volt-
age for the part is +5 V. If the input voltage is from a transducer
excited by the 5 V, the AD7731 retains its ratiometric opera-
tion with this reference voltage varying in sympathy with the
analog input voltage.
The values of the resistors in the resistor string are chosen as-
suming the maximum input voltage range of 1.28 V is applied
to the AD7731. The minimum input voltage must be 1.2 V above
the AD7731’s AGND, while the maximum input voltage must be
0.95 V below the AD7731’s AV
ranges, the resistor ratios in the resistor string can be changed
to allow a larger DV
R2 = 10 k and R3 = 6.8 k , the AV
become +3.49 V and –1.56 V respectively. This allows the
AD7731 to be used with a +3.6 V DV
analog input ranges of 320 mV and below.
An alternate scheme is to generate the AV
ages from regulators or Zener diodes driven from the +5 V and
–5 V supplies respectively. The reference voltage for the part
can be generated from an AD780 whose GND pin is connected
to the AD7731’s AGND pin.
AND CONTROL LOGIC
SERIAL INTERFACE
MICROCONTROLLER
DD
POL
CALIBRATION
and AGND voltages as well as the REF IN(+) and REF
MODULATOR
SIGMA-DELTA A/D CONVERTER
SIGMA-
DELTA
RDY
DV
DD
REGISTER BANK
+3V
PROGRAMMABLE
DD
AD7731
GENERATION
voltage. For example, if R1 = 3 k ,
DIGITAL
FILTER
CLOCK
RESET
DD
. For smaller input voltage
DD
DD
MCLK IN
MCLK OUT
SCLK
CS
DIN
STANDBY
SYNC
DOUT
and AGND voltages
voltage while allowing
DD
and AGND volt-
REV. A
REV. 0