AD7731 Analog Devices, AD7731 Datasheet - Page 17

• Manufacturer Part Number: AD7731
• Description: Low Noise, High Throughput 24-Bit Sigma-Delta ADC
• Manufacturer: Analog Devices

Specifications of AD7731

• Resolution (bits): 24bit
• # Chan: 3
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Bip,Diff-Uni
• Ain Range: (2Vref/PGA Gain) p-p,(Vref/PGA Gain) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: DIP,SOIC,SOP

Bit
Location
MR12
MR11
MR10–MR9
MR8
MR7
MR6–MR4
REV. A
REV. 0
Bit
Mnemonic
B/U
DEN
D1–D0
WL
HIREF
RN2–RN0
Digital Output Bits. These bits determine the digital outputs on the AIN3/D1 and AIN4/D0
Input Range Bits. These bits determine the analog input range for the selected analog input.
Description
Bipolar/Unipolar Bit. A 0 in this bit selects bipolar operation and the output coding is
00...000 for negative full-scale input, 10...000 for zero input and 11...111 for positive full-
scale input. A 1 in this bit selects unipolar operation and the output coding is 00...000 for
zero input and 11...111 for positive full-scale input.
Digital Output Enable Bit. With this bit at 1, the AIN3/D1 and AIN4/D0 pins assume their
digital output functions and the output drivers connected to these pins are enabled. In this
mode, the user effectively has two port bits which can be programmed over the serial interface.
pins respectively when the DEN bit is a 1. For example, a 1 written to the D1 bit of the
Mode Register (with the DEN bit also a 1) will put a logic 1 on the AIN3/D1 pin. This logic
1 will remain on this pin until a 0 is written to the D1 bit (in which case, the AIN3/D1 pin
goes to a logic 0) or the digital output function is disabled by writing a 0 to the DEN bit.
Data Word Length Bit. This bit determines the word length of the Data Register. A 0 in this
bit selects 16-bit word length when reading from the data register (i.e., RDY returns high
after 16 serial clock cycles in the read operation). A 1 in this bit selects 24-bit word length for
the Data Register.
High Reference Bit. This bit should be set in accordance with the reference voltage which is
being used on the part. If the reference voltage is 2.5 V, the HIREF bit should be set to 0. If
the reference voltage is 5 V, the HIREF bit should be set to a 1. With the HIREF bit set
correctly for the appropriate applied reference voltage, the input ranges are 0 mV to +20 mV,
+40 mV, +80 mV, +160 mV, +320 mV, +640 mV and +1.28 V for unipolar operation and
It is possible for a user with a 2.5 V reference to set the HIREF bit to a 1. In this case, the
part is operating with a 2.5 V reference but assumes it has a 5 V reference. As a result, the
input ranges on the part become 0 mV to +10 mV through 0 mV to +640 mV for unipolar
operation and 10 mV through 640 mV for bipolar operation. However, the output noise
from the part (in nV) will remain unchanged so the resolution of the part (in LSBs) will re-
duce by 1.
The different input ranges are outlined in Table XII. The table is valid for a reference voltage
of 2.5 V with the HIREF bit at 0 or for a reference voltage of 5 V with the HIREF bit at a
logic 1.
RN2
0
0
0
0
1
1
1
1
20 mV, 40 mV, 80 mV, 160 mV, 320 mV, 640 mV and 1.28 V for bipolar operation.
RN1
0
0
1
1
0
0
1
1
RN0
0
1
0
1
0
1
0
1
B/U Bit = 0
–20 mV to +20 mV
–20 mV to +20 mV
–40 mV to +40 mV
–80 mV to +80 mV
–160 mV to +160 mV
–320 mV to +320 mV
–640 mV to +640 mV
–1.28 V to +1.28 V
–17–
Table XII. Input Range Selection
Input Range
B/U Bit = 1
0 mV to +20 mV
0 mV to +20 mV
0 mV to +40 mV
0 mV to +80 mV
0 mV to +160 mV
0 mV to +320 mV
0 mV to +640 mV
0 mV to +1.28 V
Power-On/Reset Default
AD7731