MCP3422A1-E/MS Microchip Technology, MCP3422A1-E/MS Datasheet - Page 14

• Manufacturer Part Number: MCP3422A1-E/MS
• Description: 18-Bit, Multi-Channel Delta-Sigma ADC w/ I2C Interface and On-Board Reference ; 8L MSOP 3x3mm
• Manufacturer: Microchip Technology

MCP3422/3/4
4.5
The differential (V
(V
setting are defined by:
The input signal levels are amplified by the internal
programmable gain amplifier (PGA) at the front end of
the ΔΣ modulator.
The user needs to consider two conditions for the input
voltage range: (a) Differential input voltage range and
(b) Absolute maximum input voltage range.
4.5.1
The device performs conversions using its internal
reference voltage (V
absolute value of the differential input voltage (V
with PGA setting is included, needs to be less than the
internal reference voltage. The device will output satu-
rated output codes (all 0s or all 1s except sign bit) if the
absolute value of the input voltage (V
setting is included, is greater than the internal
reference voltage (V
voltage range is given by:
EQUATION 4-1:
If the input voltage level is greater than the above limit,
the user can use a voltage divider and bring down the
input level within the full-scale range. See
for more details of the input voltage divider circuit.
4.5.2
The input voltage at each input pin must be less than
the following absolute maximum input voltage limits:
• Input voltage < V
• Input voltage > V
Any input voltage outside this range can turn on the
input ESD protection diodes, and result in input leak-
age current, causing conversion errors, or permanently
damage the device.
Care must be taken in setting the input voltage ranges
so that the input voltage does not exceed the absolute
maximum input voltage range.
DS22088B-page 14
Where:
Where:
INCOM
V
REF
V
IN
n
) at the input pins without considering PGA
–
Input Voltage Range
V
REF
Differential Input Voltage Range
V
Absolute Maximum Input Voltage Range:
=
INCOM
=
=
V
≤
IN
(
V
nth input channel (n=1, 2, 3, or 4)
=
DD
SS
IN
IN
CHn+ - CHn-
2.048V
REF
=
) and common mode voltage
-0.3V
(
•
+0.3V
REF
CHn+
PGA
(
---------------------------------------------- -
= 2.048V). The input full-scale
CHn+
= 2.048V). Therefore, the
)
)
≤
–
)
(
(
2
V
+
CHn-
REF
(
CHn-
–
)
1LSB
IN
)
), with PGA
Figure 6-7
)
IN
),
4.6
The device uses a switched-capacitor input stage using
a 3.2 pF sampling capacitor. This capacitor is switched
(charged and discharged) at a rate of the sampling fre-
quency that is generated by on-board clock. The differ-
ential input impedance varies with the PGA settings.
The typical differential input impedance during a nor-
mal mode operation is given by:
Since the sampling capacitor is only switching to the
input pins during a conversion process, the above input
impedance is only valid during conversion periods. In a
low power standby mode, the above impedance is not
presented at the input pins. Therefore, only a leakage
current due to ESD diode is presented at the input pins.
The conversion accuracy can be affected by the input
signal source impedance when any external circuit is
connected to the input pins. The source impedance
adds to the internal impedance and directly affects the
time required to charge the internal sampling capacitor.
Therefore, a large input source impedance connected
to the input pins can degrade the system performance,
such as offset, gain, and Integral Non-Linearity (INL)
errors. Ideally, the input source impedance should be
zero. This can be achievable by using an operational
amplifier with a closed-loop output impedance of tens
of ohms.
4.7
Aliasing occurs when the input signal contains
time-varying signal components with frequency greater
than half the sample rate. In the aliasing conditions, the
device can output unexpected output codes. For
applications that are operating in electrical noise
environments, the time-varying signal noise or high
frequency interference components can be easily
added to the input signals and cause aliasing. Although
the device has an internal first order sinc filter, the filter
response
attenuation to all aliasing signal components. To avoid
the aliasing, an external anti-aliasing filter, which can
be accomplished with a simple RC low-pass filter, is
typically used at the input pins. The low-pass filter cuts
off the high frequency noise components and provides
a band-limited input signal to the input pins.
4.8
The device performs a self-calibration of offset and
gain for each conversion. This provides reliable
conversion results from conversion-to-conversion over
variations in temperature as well as power supply
fluctuations.
Aliasing and Anti-aliasing Filter
Input Impedance
Self-Calibration
(Figure
Z
IN
(f) = 2.25 M
2-11)
© 2008 Microchip Technology Inc.
may
Ω
/PGA
not
give
enough