Microchip Technology MCP3421DM-WS Data Sheet
MCP3421
DS22003E-page 12
© 2009 Microchip Technology Inc.
4.5
Input Voltage Range
The differential (V
IN
) and common mode voltage
(V
INCOM
) at the input pins without considering PGA
setting are defined by:
The input signal levels are amplified by the internal
programmable gain amplifier (PGA) at the front end of
the
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.
voltage range: (a) Differential input voltage range and
(b) Absolute maximum input voltage range.
4.5.1
DIFFERENTIAL INPUT VOLTAGE
RANGE
RANGE
The device performs conversions using its internal
reference voltage (V
reference voltage (V
REF
= 2.048V). Therefore, the
absolute value of the differential input voltage (V
IN
),
with PGA setting is included, needs to be less than the
internal reference voltage. The device will output
saturated output codes (all 0s or all 1s except sign bit)
if the absolute value of the input voltage (V
internal reference voltage. The device will output
saturated output codes (all 0s or all 1s except sign bit)
if the absolute value of the input voltage (V
IN
), with
PGA setting is included, is greater than the internal
reference voltage (V
reference voltage (V
REF
= 2.048V). The input full-scale
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
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
ABSOLUTE MAXIMUM INPUT
VOLTAGE RANGE
VOLTAGE RANGE
The input voltage at each input pin must be less than
the following absolute maximum input voltage limits:
the following absolute maximum input voltage limits:
• Input voltage < V
DD
+0.3V
• Input voltage > V
SS
-0.3V
Any input voltage outside this range can turn on the
input ESD protection diodes, and result in input
leakage current, causing conversion errors, or
permanently damage the device.
input ESD protection diodes, and result in input
leakage 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.
so that the input voltage does not exceed the absolute
maximum input voltage range.
4.6
Input Impedance
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
frequency that is generated by on-board clock. The
differential input impedance varies with the PGA
settings. The typical differential input impedance during
a normal mode operation is given by:
a 3.2 pF sampling capacitor. This capacitor is switched
(charged and discharged) at a rate of the sampling
frequency that is generated by on-board clock. The
differential input impedance varies with the PGA
settings. The typical differential input impedance during
a normal 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.
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.
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 and Anti-aliasing Filter
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 (
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 (
) may not give enough
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.
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
Self-Calibration
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.
gain for each conversion. This provides reliable
conversion results from conversion-to-conversion over
variations in temperature as well as power supply
fluctuations.
V
IN
V
IN
+
V
IN
-
–
=
V
INCOM
V
IN
+
V
IN
-
+
2
-------------------------------
=
Where:
V
IN
=
V
IN
+ - V
IN
-
V
REF
=
2.048V
V
REF
–
V
IN
PGA
•
(
)
V
REF
1LSB
–
(
)
≤
≤
Z
IN
(f) = 2.25 M
Ω
/PGA