MCP3204-BISL Microchip Technology, MCP3204-BISL Datasheet - Page 12

• Manufacturer Part Number: MCP3204-BISL
• Description: 2.7V 4-Channel/8-Channel 12-Bit A/D Converters with SPI Serial Interface
• Manufacturer: Microchip Technology

MCP3204/3208
3.0
3.1
Analog inputs for channels 0 - 7 respectively for the
multiplexed inputs. Each pair of channels can be pro-
grammed to be used as two independent channels in
single ended-mode or as a single pseudo-differential
input where one channel is IN+ and one channel is IN-.
See Section 4.1 and Section 5.0 for information on pro-
gramming the channel configuration.
3.2
The CS/SHDN pin is used to initiate communication
with the device when pulled low and will end a conver-
sion and put the device in low power standby when
pulled high. The CS/SHDN pin must be pulled high
between conversions.
3.3
The SPI clock pin is used to initiate a conversion and to
clock out each bit of the conversion as it takes place.
See Section 6.2 for constraints on clock speed.
3.4
The SPI port serial data input pin is used to load chan-
nel configuration data into the device.
3.5
The SPI serial data output pin is used to shift out the
results of the A/D conversion. Data will always change
on the falling edge of each clock as the conversion
takes place.
3.6
Analog ground connection to internal analog circuitry.
3.7
Digital ground connection to internal digital circuitry.
4.0
The MCP3204/3208 A/D Converters employ a conven-
tional SAR architecture. With this architecture, a sam-
ple is acquired on an internal sample/hold capacitor for
1.5 clock cycles starting on the fourth rising edge of the
serial clock after the start bit has been received. Follow-
ing this sample time, the device uses the collected
charge on the internal sample and hold capacitor to
produce a serial 12-bit digital output code. Conversion
rates of 100ksps are possible on the MCP3204/3208.
See Section 6.2 for information on minimum clock
rates. Communication with the device is done using a
4-wire SPI-compatible interface.
DS21298B-page 12
PIN DESCRIPTIONS
CH0 - CH7
CS/SHDN(Chip Select/Shutdown)
CLK (Serial Clock)
D
D
AGND
DGND
DEVICE OPERATION
IN
OUT
(Serial Data Input)
(Serial Data output)
Preliminary
4.1
The MCP3204/3208 devices offer the choice of using
the analog input channels configured as single-ended
inputs or pseudo-differential pairs. The MCP3204 can
be configured to provide two pseudo-differential input
pairs or four single-ended inputs. the MCP3208 can be
configured to provide four pseudo-differential input
pairs or eight single-ended inputs. Configuration is
done as part of the serial command before each con-
version begins. When used in the pseudo-differential
mode, each channel pair (i.e., CH0 and CH1, CH2 and
CH3 etc.) are programmed as the IN+ and IN- inputs as
part of the command string transmitted to the device.
The IN+ input can range from IN- to (V
IN- input is limited to ±100mV from the V
input can be used to cancel small signal com-
mon-mode noise which is present on both the IN+ and
IN- inputs.
When operating in the pseudo-differential mode, if the
voltage level of IN+ is equal to or less than IN-, the
resultant code will be 000h. If the voltage at IN+ is equal
to or greater than {[V
put code will be FFFh. If the voltage level at IN- is more
than 1 LSB below V
input will have to go below V
code. Conversely, if IN- is more than 1 LSB above V
then the FFFh code will not be seen unless the IN+
input level goes above V
For the A/D Converter to meet specification, the charge
holding capacitor, (C
to acquire a 12-bit accurate voltage level during the 1.5
clock cycle sampling period. The analog input model is
shown in Figure 4-1.
In this diagram it is shown that the source impedance
(R
ance, directly affecting the time that is required to
charge the capacitor, C
source impedances increase the offset, gain, and inte-
gral linearity errors of the conversion. See Figure 4-2.
4.2
For each device in the family, the reference input (V
determines the analog input voltage range. As the ref-
erence input is reduced, the LSB size is reduced
accordingly. The theoretical digital output code pro-
duced by the A/D Converter is a function of the analog
input signal and the reference input as shown below.
where:
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.
S
) adds to the internal sampling switch (R
V
V
IN
REF
= analog input voltage
Analog Inputs
Reference Input
= reference voltage
Digital Output Code = 4096 * V
SS
SAMPLE
REF
, then the voltage level at the IN+
+ (IN-)] - 1 LSB}, then the out-
REF
SAMPLE
1999 Microchip Technology Inc.
) must be given enough time
level.
SS
. Consequently, larger
to see the 000h output
V
REF
REF
SS
IN
rail. The IN-
+ IN-). The
SS
) imped-
REF
SS
)
,