Datasheet

AD7176-2 Data Sheet
Rev. A | Page 26 of 68
ADA4940
The ADA4940-1/ADA4940-2 is an alternate option to drive
the AD7176-2. It is a low noise, low distortion fully differential
amplifier with very low power consumption (1.25 mA of quiescent
current). The AD7176-2 REFOUT pin can be used to connect
to the ADA4940-1/ADA4940-2 to set the common-mode output
to 2.5 V. This option requires the use of external resistors to set
the gain of the amplifier.
AD7176-2 REFERENCE
The AD7176-2 offers the user the option of either supplying an
external reference to the REF+ and REF− pins of the device or
allowing the use of the internal 2.5 V, low noise, low drift reference.
Select the reference source to be used by the analog input by setting
the REF_SELx bits (Bits[5:4]) in the setup configuration registers
appropriately. The structure of the Setup Configuration 0 register
is shown in Table 16. The AD7176-2 defaults on power-up to
use the internal 2.5 V reference.
External Reference
The AD7176-2 has a fully differential reference input applied
through the REF+ and REF− pins. Standard low noise, low drift
voltage references, such as the ADR445, ADR444, and ADR441,
are recommended for use. The external reference should be
applied to the AD7176-2 reference pins as shown in Figure 32.
The output of any external reference should be decoupled to
AVSS. As shown in Figure 32, the ADR445 output is decoupled
with a 0.1 μF capacitor at its output for stability purposes. The
output is then connected to a 4.7 μF capacitor, which acts as a
reservoir for any dynamic charge required by the ADC, and
followed by a 0.1 μF decoupling capacitor at the REF+ input.
This capacitor is placed as close as possible to the REF+ and
REF− pins. The REF− pin is connected directly to the AVSS
potential. On power-up of the AD7176-2, the internal reference
is enabled by default and is output on the REFOUT pin. When
an external reference is used instead of the internal reference to
supply the AD7176-2, attention must be paid to the output of
the REFOUT pin. If the internal reference is not being used
elsewhere in the application, ensure that the REFOUT pin is not
hardwired to AVSS because this will draw a large current on
power-up. On power-up if the internal reference is not being
used, write to the ADC mode register, disabling the internal
reference. This is controlled by the REF_EN bit (Bit 15) in the
ADC mode register and is shown in Table 17.
Figure 32. External Reference ADR445 Connected to AD7176-2 Reference Pins
Table 16. Setup Configuration 0 Register
Reg Name Bits Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Reset RW
0x20 SETUPCON0 [15:8] RESERVED BI_UNIPOLAR0 RESERVED 0x1020 RW
[7:0] RESERVED REF_SEL0 RESERVED
Table 17. ADC Mode Register
Reg Name Bits Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 Reset RW
0x01 ADCMODE [15:8] REF_EN RESERVED SING_CYC RESERVED DELAY 0x8000 RW
[7:0] RESERVED MODE CLOCKSEL RESERVED
2
3
REF–
REF+
4.7µF0.1µF
1
1
11
1
0.1µF0.1µF
5.5V TO 18V
ADR445
2
5V VREF
AD7176-2
1
ALL DECOUPLING IS TO AVSS.
2
ANY OF THE ADR44x FAMILY REFERENCES MAY BE USED.
ADR444 OR ADR441 BOTH ENABLE REUSE OF THE 5V ANALOG SUPPLY
NEEDED FOR AVDD1 TO POWER THE REFERENCE VIN.
11037-159