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ADC
4.096 V
REF
REFCAP
REFIO
AGND
22
P
F
REFGND
C
REF
REFSEL
AVDD
REF5040
(See the device datasheet for
a detailed pin configuration.)
AVDD
OUT
DVDD
1
P
F
SBAS686 – JULY 2015
8.3.8.2 External Reference
For applications that require a better reference voltage or a common reference voltage for multiple devices, the
ADS8694 and ADS8698 offer a provision to use an external reference along with an internal buffer to drive the
ADC reference pin. In order to select the external reference mode, either tie the REFSEL pin high or connect this
pin to the DVDD supply. In this mode, an external 4.096-V reference must be applied at REFIO (pin 5), which
becomes an input pin. Any low-power, low-drift, or small-size external reference can be used in this mode
because the internal buffer is optimally designed to handle the dynamic loading on the REFCAP pin, which is
internally connected to the ADC reference input. The output of the external reference must be appropriately
filtered to minimize the resulting effect of the reference noise on system performance. A typical connection
diagram for this mode is shown in
Figure 62. Device Connections for Using an External 4.096-V Reference
The output of the internal reference buffer appears at the REFCAP pin. A minimum capacitance of 10 µF must
be placed between REFCAP (pin 7) and REFGND (pin 6). Place another capacitor of 1 µF as close to the
REFCAP pin as possible for decoupling high-frequency signals. Do not use the internal buffer to drive external ac
or dc loads because of the limited current output capability of this buffer.
28
Copyright © 2015, Texas Instruments Incorporated
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