MCP3004/3008
EQUATION
Digital Output Code =
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.
1024 × VIN
--------------------------
VREF
VIN = analog input voltage
VREF = reference voltage
EQUATION
VREF
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.
------------
LSB Size =
1024
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.
VDD
Sampling
Switch
VT = 0.6V
RS = 1 kΩ
CHx
RSS
SS
CSAMPLE
= DAC capacitance
= 20 pF
CPIN
7 pF
ILEAKAGE
VA
VT = 0.6V
±1 nA
VSS
Legend
VA
Signal Source
Leakage Current At The Pin
Due To Various Junctions
=
I
=
LEAKAGE
SS
Source Impedance
Input Channel Pad
Input Pin Capacitance
Threshold Voltage
sampling switch
R
=
=
=
=
=
=
=
SS
CHx
sampling switch resistor
sample/hold capacitance
R
S
C
C
SAMPLE
PIN
V
T
FIGURE 4-1: Analog Input Model.
4
VDD = VREF = 5 V
SAMPLE = 200 ksps
f
3
2
1
0
VDD = VREF = 2.7 V
SAMPLE = 75 ksps
f
100
1000
10000
Input Resistance (Ohms)
FIGURE 4-2: Maximum Clock Frequency vs.
Input resistance (RS) to maintain less than a
0.1 LSB deviation in INL from nominal
conditions.
DS21295C-page 14
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
