AD7148 Analog Devices, AD7148 Datasheet - Page 16

• Manufacturer Part Number: AD7148
• Manufacturer: Analog Devices

Specifications of AD7148

• Resolution (bits): 16bit
• # Chan: 8
• Sample Rate: 250kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: SE-Uni
• Ain Range: ± 8 pF (Delta C)
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD7148
CAPACITANCE SENSOR INPUT CONFIGURATION
Each input connection from the external capacitance sensors to
the AD7148 converter can be uniquely configured by using the
registers in Bank 2 (see Table 39 through Table 42). These registers
are used to configure input pin connection setups, sensor offsets,
sensor sensitivities, and sensor limits for each stage. Each sensor
can be individually optimized. For example, a button sensor
connected to STAGE0 can have different sensitivity and offset
values from those of a button with a different function that is
connected to a different stage.
CINx INPUT MULTIPLEXER SETUP
The CINx_CONNECTION_SETUP register bits provide options
for connecting the sensor input pins to the CDC (see Table 39
and Table 40).
The AD7148 has an on-chip multiplexer to route the input
signals from each pin to the input of the converter. Each input
pin can be tied to either the negative or the positive input of the
CDC, or it can be left floating. Each input can also be internally
connected to the BIAS signal to help prevent cross coupling. If
an input is not used, always connect it to BIAS.
Connecting a CINx input pin to the positive CDC input results
in an increase in CDC output code when the corresponding sensor
is activated. Connecting a CINx input pin to the negative CDC
input results in a decrease in CDC output code when the corre-
sponding sensor is activated.
The AD7148 performs a sequence of eight conversions. The
multiplexer can have different connection settings for each of
the eight conversions by using the CINx_CONNECTION_
SETUP bits. For example, CIN0 can be connected to the
negative CDC input or left floating. The same holds true for all
eight conversion stages.
Two bits in each sequence stage register control the mux setting for
the input pin, as shown in Figure 25.
CIN0
CIN1
CIN2
CIN3
CIN4
CIN5
CIN6
CIN7
CINx_CONNECTION
_SETUP BITS
00
01
10
11
Figure 25. Input Mux Configuration Options
Rev. A | Page 16 of 56
CINx FLOATING
CINx CONNECTED TO
NEGATIVE CDC INPUT
CINx CONNECTED TO
POSITIVE CDC INPUT
CINx CONNECTED TO
BIAS
CIN SETTING
SINGLE-ENDED CONNECTIONS TO THE CDC
A single-ended connection to the CDC is defined as having one
CINx input connected to either the positive or the negative CDC
input. A differential connection to the CDC is defined as having
one CINx input connected to the positive CDC input and a second
CINx input connected to the negative input of the CDC.
When a single-ended connection to the CDC is made in any
stage, the SE_CONNECTION_SETUP bits in the STAGEx_
CONNECTION_SETUP registers should be applied. These bits
ensure that, during a single-ended connection to the CDC, the
input paths to both terminals are matched. This matching of
input paths, in turn, improves the power supply rejection of the
converter measurement.
Table 11. Application of SE_CONNECTION_SETUP Bits
Bit Values
00
01
10
11
If more than one CINx input is connected to either the positive
or negative input of the converter for the same conversion, set
SE_CONNECTION_SETUP = 11. For example, if CIN0 and
CIN3 are connected to the positive input of the CDC,
SE_CONNECTION_SETUP = 11.
Description
Do not use.
Single-ended connection. For this stage, there is one
CINx connected to the positive CDC input.
Single-ended connection. For this stage, there is one
CINx connected to the negative CDC input.
Differential connection. For this stage, there is one
CINx connected to the negative CDC input and one
CINx connected to the positive CDC input.
+
–
CDC