ADC
4.096
V
REF
REFCAP
REFIO
AGND
22
P
F
REFGND
10
P
F
REFSEL
AVDD
1
P
F
SBAS582C – JULY 2014 – REVISED APRIL 2015
8.3.7 Multiplexer (MUX)
The ADS8684 and ADS8688 feature an integrated 4- and 8-channel analog multiplexer, respectively. For each
analog input channel, the voltage difference between the positive analog input AIN_
n
P and the negative ground
input AIN_
n
GND is conditioned by the analog front-end circuitry before being fed into the multiplexer. The output
of the multiplexer is directly sampled by the ADC. The multiplexer in the device can scan these analog inputs in
either manual or auto-scan mode, as explained in
section. In manual mode
(MAN_Ch_n), the channel is selected for every sample via a register write; in auto-scan mode (AUTO_RST), the
channel number is incremented automatically on every CS falling edge after the present channel is sampled. The
analog inputs can be selected for an auto scan with register settings (refer to
section). The devices automatically scan only the selected analog inputs in ascending order.
The maximum overall throughput for the ADS8684 and ADS8688 is specified at 500 kSPS across all channels.
The per channel throughput is dependent on the number of channels selected in the multiplexer scanning
sequence. For example, the throughput per channel is equal to 250 kSPS if only two channels are selected, but
is equal to 125 kSPS per channel if four channels are selected (as in the ADS8684), and so forth.
Refer to
for command register settings to switch between the auto-scan mode and manual mode for
individual analog channels.
8.3.8 Reference
The ADS8684 and ADS8688 can operate with either an internal voltage reference or an external voltage
reference using the internal buffer. The internal or external reference selection is determined by an external
REFSEL pin. The devices have a built-in buffer amplifier to drive the actual reference input of the internal ADC
core for maximizing performance.
8.3.8.1 Internal Reference
The devices have an internal 4.096-V (nominal value) reference. In order to select the internal reference, the
REFSEL pin must be tied low or connected to AGND. When the internal reference is used, REFIO (pin 5)
becomes an output pin with the internal reference value. TI recommends placing a 10-µF (minimum) decoupling
capacitor between the REFIO pin and REFGND (pin 6), as shown in
. The capacitor must be placed as
close to the REFIO pin as possible. The output impedance of the internal band-gap circuit creates a low-pass
filter with this capacitor to band-limit the noise of the reference. The use of a smaller capacitor value allows
higher reference noise in the system, thus degrading SNR and SINAD performance. Do not use the REFIO pin to
drive external ac or dc loads because REFIO has limited current output capability. The REFIO pin can be used
as a source if followed by a suitable op amp buffer (such as the
Figure 57. Device Connections for Using an Internal 4.096-V Reference
26
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