MCP3301_07 Datasheet, PDF (17/32 Pages) Microchip Technology – 13-Bit Differential Input, Low Power A/D Converter with SPI™ Serial Interface

MCP3301
R_SS ^CHx
VA
V_DD
V_T = 0.6V
C_PIN
7 pF
V_T = 0.6V
I_LEAKAGE
±1 nA
Legend
VA = signal source
R_ss = 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_s = sampling switch resistor
C_sample = sample/hold capacitance
Sampling
Switch
SS R_S = 1 kΩ
C_SAMPLE
= DAC capacitance
= 25 pF
V_SS
FIGURE 6-3:
Analog Input Model.
6.2.1
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 the
MCP330X 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.
6.3 Biasing Solutions
For pseudo-differential bipolar operation, the biasing
circuit shown in Figure 6-4 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 2-9 for DNL and INL
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 kΩ or less, since higher input impedances
require additional acquisition time. Using the values in
Figure 6-4, we have a 100 Hz corner frequency. See
Figure 2-12 for the relationship between input imped-
ance and acquisition time.
V_DD = 5V
0.1 µF
10 µF C
V_IN
1 kΩ
IN+
Ρ _IN-
MCP3301
V_REF
1 µF
V_OUT
V_IN
MCP1525
0.1 µF
FIGURE 6-4:
Pseudo-differential biasing
circuit for bipolar operation.
Using an external operational amplifier on the input
allows for gain and buffers the input signal from the
input to the ADC, allowing for a higher source
impedance. This circuit is shown in Figure 6-5.
© 2007 Microchip Technology Inc.
DS21700C-page 17