AD7151BRMZ Analog Devices Inc, AD7151BRMZ Datasheet - Page 23

• Manufacturer Part Number: AD7151BRMZ
• Description: IC CAP CONV 1CH ULT LP 10MSOP
• Manufacturer: Analog Devices Inc
• Type: Capacitance-to-Digital Converterr

Specifications of AD7151BRMZ

• Input Type: Voltage
• Output Type: Digital
• Interface: 2-Wire Serial
• Current - Supply: 120µA
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Resolution (bits): 12bit
• Sampling Rate: 100SPS
• Input Channel Type: Single Ended
• Data Interface: 2-Wire, I2C, Serial
• Supply Voltage Range - Analog: 2.7V To 3.6V
• Supply Current: 70µA
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

HARDWARE DESIGN CONSIDERATIONS
OVERVIEW
The AD7151 is an interface to capacitive sensors.
On the input side, the sensor (C
between the AD7151 EXC and CIN pins. The way it is
connected and the electrical parameters of the sensor
connection, such as parasitic resistance or capacitance, can
affect the system performance. Therefore, any circuit with
additional components in the capacitive front end, such as
overvoltage protection, has to be carefully designed considering
the AD7151 specified limits and information provided in this
section.
On the output side, the AD7151 can work as a standalone
device, using the power-up default register settings and flagging
the result on digital outputs. Alternatively, the AD7151 can be
interfaced to a microcontroller via the 2-wire serial interface,
offering flexibility by overwriting the AD7151 register values
from the host with a user-specific setup.
PARASITIC CAPACITANCE TO GROUND
The CDC architecture used in the AD7151 measures the
capacitance C
pin. In theory, any capacitance C
the CDC result (see Figure 42).
The practical implementation of the circuitry in the chip
implies certain limits, and the result is gradually affected by
capacitance to ground (see Table 1 for information about the
allowed capacitance to GND for CIN and information about
excitation).
See Figure 4, Figure 5, Figure 6, and Figure 7.
C
C
GND1
GND2
X
Figure 42. Parasitic Capacitance to Ground
connected between the EXC pin and the CIN
C
X
EXC
CIN
X
GND
) can be connected directly
to ground should not affect
CDC
DATA
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PARASITIC RESISTANCE TO GROUND
The AD7151 CDC result is affected by a leakage current from
C
The equivalent resistance between C
maximized (see Figure 43).
See Figure 8, Figure 9, Figure 10, and Figure 11.
PARASITIC PARALLEL RESISTANCE
The AD7151 CDC measures the charge transfer between the
EXC and CIN pins. Any resistance connected in parallel to the
measured capacitance C
resistance of the sensor, also transfers charge. Therefore, the
parallel resistor is seen as an additional capacitance in the
output data. The equivalent parallel capacitance (or error
caused by the parallel resistance) can be approximately
calculated as
where R
frequency.
See Figure 13.
X
to ground; therefore, C
R
R
C
GND1
GND2
P
C
P
X
=
is the parallel resistance and f
R
R
P
P
EXC
Figure 43. Parasitic Resistance to Ground
×
C
CIN
Figure 44. Parasitic Parallel Resistance
X
f
1
EXC
CIN
EXC
×
X
4
(see Figure 44), such as the parasitic
X
should be isolated from the ground.
X
CDC
and ground should be
EXC
CDC
is the excitation
DATA
AD7151
DATA