MCP3004_07 datasheet(14/34 Pages) MICROCHIP

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MCP3004/3008

DS21295C-page 14

4.2

Reference Input

For each device in the family, the reference input

(VREF) determines the analog input voltage range. As

the reference input is reduced, the LSB size is reduced

accordingly.

EQUATION

The theoretical digital output code produced by the A/D

converter is a function of the analog input signal and the

reference input, as shown below.

EQUATION

When using an external voltage reference device, the

system designer should always refer to the manufac-

turer’s recommendations for circuit layout. Any instabil-

ity in the operation of the reference device will have a

direct effect on the operation of the A/D converter.

FIGURE 4-1:

Analog Input Model.

FIGURE 4-2:

Maximum Clock Frequency vs.

Input resistance (RS) to maintain less than a

0.1 LSB deviation in INL from nominal

conditions.

LSB Size

REF

1024

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

Digital Output Code

1024

REF

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

VIN = analog input voltage

VREF = reference voltage

CHx

7pF

T = 0.6V

LEAKAGE

Sampling

Switch

S = 1 kΩ

SAMPLE

= DAC capacitance

= 20 pF

±1 nA

Legend

= Signal Source

ILEAKAGE = Leakage Current At The Pin

Due To Various Junctions

RSS = Source Impedance

= sampling switch

CHx

= Input Channel Pad

RS = sampling switch resistor

CPIN = Input Pin Capacitance

CSAMPLE = sample/hold capacitance

VT = Threshold Voltage

100

1000

10000

Input Resistance (Ohms)

VDD = VREF = 5 V

fSAMPLE = 200 ksps

VDD = VREF = 2.7 V

fSAMPLE = 75 ksps

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