MCP3304 Microchip Technology, MCP3304 Datasheet - Page 18

• Manufacturer Part Number: MCP3304
• Description: 13-Bit Differential Input, Low Power A/D Converter with SPI Serial Interface
• Manufacturer: Microchip Technology

MCP3302/04
FIGURE 6-3:
6.2.1
When the MCP3302/04 initiates, charge is stored on
the sample capacitor. When the sample period is com-
plete, 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 cap
while the conversion is taking place. For the MCP330X
devices, the recommended minimum clock speed dur-
ing the conversion cycle (T
meet this criteria may induce linearity errors into the
conversion outside the rated specifications. It should
be noted that during the entire conversion 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
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 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 2-9 for DNL and
INL errors versus V
between the high pass corner and the acquisition time.
The value of C will need to be quite large in order to
DS21697C-page 18
Legend
I
VA
C
LEAKAGE
SAMPLE
Biasing Solutions
R
C
R
CHx
SS
PIN
SS
R
VA
V
MAINTAINING MINIMUM CLOCK
SPEED
SS
T
S
CHx
=
=
=
=
=
=
=
=
=
signal source
source impedance
input channel pad
input pin capacitance
threshold voltage
leakage current at the pin
due to various junctions
sampling switch
sampling switch resistor
sample/hold capacitance
REF
Analog Input Model.
at V
CONV
DD
7 pF
C
PIN
= 5V. A trade-off exists
) is 105 kHz. Failure to
V
DD
V
V
T
T
= 0.6V
= 0.6V
I
±1 nA
LEAKAGE
bring down the high pass corner. The value of R will
need to be 1 kΩ, or less, since higher input impedances
require additional acquisition time. Using the RC values
in Figure 6-4, we have a 100 Hz corner frequency. See
Figure 2-12 for relation between input impedance and
acquisition time.
FIGURE 6-4:
circuit for bipolar operation.
Using an external operation amplifier on the input
allows for gain and also buffers the input signal from the
input to the ADC allowing for a higher source imped-
ance. This circuit is shown in Figure 6-5.
V
IN
10 µF
SS
Sampling
Switch
C
1 kΩ
1 µF
R
R
S
= 1 kΩ
Pseudo-differential biasing
V
© 2007 Microchip Technology Inc.
IN+
IN-
OUT
MCP1525
V
MCP330X
V
SS
C
= DAC capacitance
= 25 pF
IN
0.1 µF
SAMPLE
V
REF
V
DD
0.1 µF
= 5V