MUX
PGA
1 M
:
OVP
1 M
:
OVP
2nd-Order
LPF
ADC
Driver
V
B
ADC
AIN_nP
AIN_nGND
CS
SCLK
SDI
SDO
DAISY
SBAS686 – JULY 2015
8.3 Feature Description
8.3.1 Analog Inputs
The ADS8694 and ADS8698 have either four or eight analog input channels, respectively, such that the positive
inputs AIN_
n
P
(n = 0 to 3 or 7)
are the single-ended analog inputs and the negative inputs AIN_
n
GND are tied to
GND.
shows the simplified circuit schematic for each analog input channel, including the input
overvoltage protection circuit, PGA, low-pass filter (LPF), high-speed ADC driver, and analog multiplexer.
NOTE: n = 0 to 3 for the ADS8694 and n = 0 to 7 for the ADS8698.
Figure 51. Front-End Circuit Schematic for Each Analog Input Channel
The devices can support multiple unipolar or bipolar, single-ended input voltage ranges based on the
configuration of the program registers. As explained in the
section, the input voltage
range for each analog channel can be configured to bipolar ±2.5 × V
REF
, ±1.25 × V
REF
, and ±0.625 × V
REF
or
unipolar 0 to 2.5 × V
REF
and 0 to 1.25 × V
REF
. With the internal or external reference voltage set to 4.096 V, the
input ranges of the device can be configured to bipolar ranges of ±10.24 V, ±5.12 V, and ±2.56 V or unipolar
ranges of 0 V to 10.24 V and 0 V to 5.12 V. Any of these input ranges can be assigned to any analog input
channel of the device. For instance, the ±2.5 × V
REF
range can be assigned to AIN_1P, the ±1.25 × V
REF
range
can be assigned to AIN_2P, the 0 V to 2.5 × V
REF
range can be assigned to AIN_3P, and so forth.
The devices sample the voltage difference (AIN_
n
P – AIN_
n
GND) between the selected analog input channel
and the AIN_
n
GND pin. The devices allow a ±0.1-V range on the AIN_
n
GND pin for all analog input channels.
This feature is useful in modular systems where the sensor or signal-conditioning block is further away from the
ADC on the board and when a difference in the ground potential of the sensor or signal conditioner from the ADC
ground is possible. In such cases, running separate wires from the AIN_
n
GND pin of the device to the sensor or
signal-conditioning ground is recommended.
If the analog input pins (AIN_
n
P) to the devices are left floating, the output of the ADC corresponds to an internal
biasing voltage. The output from the ADC must be considered as invalid if the devices are operated with floating
input pins. This condition does not cause any damage to the devices, which are fully functional when a valid
input voltage is applied to the pins.
8.3.2 Analog Input Impedance
Each analog input channel in the device presents a constant resistive impedance of 1 M
Ω
. The input impedance
is independent of either the ADC sampling frequency, the input signal frequency, or range. The primary
advantage of such high-impedance inputs is the ease of driving the ADC inputs without requiring driving
amplifiers with low output impedance. Bipolar, high-voltage power supplies are not required in the system
because this ADC does not require any high-voltage front-end drivers. In most applications, the signal sources or
sensor outputs can be directly connected to the ADC input, thus significantly simplifying the design of the signal
chain.
In order to maintain the dc accuracy of the system, matching the external source impedance on the AIN_
n
P input
pin with an equivalent resistance on the AIN_
n
GND pin is recommended. This matching helps to cancel any
additional offset error contributed by the external resistance.
Copyright © 2015, Texas Instruments Incorporated
21
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