Datasheet
2001-2011 Microchip Technology Inc. DS21700E-page 15
MCP3301
FIGURE 5-3: Analog Input Model.
5.2.1 MAINTAINING MINIMUM CLOCK
SPEED
When the MCP3301 initiates, charge is stored on the
sample capacitor. When the sample period is complete,
the device converts one bit for each clock that is
received. It is important for the user to note that a slow
clock rate will allow charge to bleed off the sample
capacitor while the conversion is taking place. For
MCP3301 devices, the recommended minimum clock
speed during the conversion cycle (t
CONV
) is 85 kHz.
Failure to meet this criteria may introduce linearity
errors into the conversion outside the rated specifica-
tions. It should be noted that, during the entire conver-
sion cycle, the A/D converter does not have
requirements for clock speed or duty cycle, as long as
all timing specifications are met.
5.3 Biasing Solutions
For pseudo-differential bipolar operation, the biasing
circuit shown in Figure 5-4 shows a single-ended input
AC coupled to the converter. This configuration will give
a digital output range of -4096 to +4095. With the 2.5V
reference, the LSB size is equal to 610 µV.
Although the ADC is not production tested with a 2.5V
reference as shown, linearity will not change more than
0.1 LSB. See Figure 2-2 and Figure 2-6 for INL and
DNL errors versus V
REF
at V
DD
= 5V. A trade-off exists
between the high-pass corner and the acquisition time.
The value of C will need to be quite large in order to
bring down the high-pass corner. The value of R needs
to be 1 k or less, since higher input impedances
require additional acquisition time.
Using the values in Figure 5-4, we have a 100 Hz cor-
ner frequency. See Figure 5-2 for the relationship
between input impedance and acquisition time.
FIGURE 5-4: Pseudo-differential biasing
circuit for bipolar operation.
C
PIN
VA
R
S
CHx
7pF
V
T
= 0.6V
V
T
= 0.6V
I
LEAKAGE
Sampling
Switch
SS
R
SS
= 1 k
C
SAMPLE
= DAC capacitance
V
SS
V
DD
= 25 pF
±1 nA
Legend
VA
=
signal source
R
S
=
source impedance
CHx
=
input channel pad
C
PIN
=
input pin capacitance
V
T
=
threshold voltage
I
LEAKAGE
=
leakage current at the pin due to various junctions
SS
=
sampling switch
R
SS
=
sampling switch resistor
C
SAMPLE
=
sample/hold capacitance
V
DD
= 5V
0.1 µF
IN+
IN-
V
REF
MCP3301
1µF
MCP1525
V
IN
V
OUT
0.1 µF
1k
10 µF
V
IN
R
C