MCP3421 datasheet(12/42 Pages) MICROCHIP

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MCP3421

DS22003E-page 12

4.5

Input Voltage Range

The differential (VIN) and common mode voltage

(VINCOM) 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

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

DIFFERENTIAL INPUT VOLTAGE

RANGE

The device performs conversions using its internal

reference voltage (VREF = 2.048V). Therefore, the

absolute value of the differential input voltage (VIN),

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 (VIN), with

PGA setting is included, is greater than the internal

reference voltage (VREF = 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 Figure 6-7

for more details of the input voltage divider circuit.

4.5.2

ABSOLUTE MAXIMUM INPUT

VOLTAGE RANGE

The input voltage at each input pin must be less than

the following absolute maximum input voltage limits:

• Input voltage < VDD+0.3V

• Input voltage > VSS-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,

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.

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:

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 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

(Figure 2-11)

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.

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.

IN+VIN-

–

INCOM

IN+VIN-

-------------------------------

Where:

VIN

=VIN+ - VIN-

VREF

2.048V

REF

–

PGA

•

()

REF

1LSB

–

()

≤≤

ZIN(f) = 2.25 MΩ/PGA

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