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MCP3008T Datasheet(PDF) 17 Page - Microchip Technology

Part No. MCP3008T
Description  2.7V 4-Channel/8-Channel 10-Bit A/D Converters with SPI Serial Interface
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Manufacturer  MICROCHIP [Microchip Technology]
Direct Link  http://www.microchip.com
Logo MICROCHIP - Microchip Technology

MCP3008T Datasheet(HTML) 17 Page - Microchip Technology

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© 2008 Microchip Technology Inc.
DS21295D-page 17
MCP3004/3008
4.0
DEVICE OPERATION
The MCP3004/3008 A/D converters employ a
conventional SAR architecture. With this architecture,
a sample is acquired on an internal sample/hold
capacitor for 1.5 clock cycles starting on the first rising
edge of the serial clock once CS has been pulled low.
Following this sample time, the device uses the
collected charge on the internal sample and hold
capacitor to produce a serial 10-bit digital output code.
Conversion rates of 100 ksps are possible on the
MCP3004/3008. See Section 6.2 “Maintaining Mini-
mum Clock Speed”
, “Maintaining Minimum Clock
Speed”, for information on minimum clock rates.
Communication with the device is accomplished using
a 4-wire SPI-compatible interface.
4.1
Analog Inputs
The MCP3004/3008 devices offer the choice of using
the analog input channels configured as single-ended
inputs or pseudo-differential pairs. The MCP3004 can
be configured to provide two pseudo-differential input
pairs or four single-ended inputs. The MCP3008 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
conversion 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 transmit-
ted to the device. The IN+ input can range from IN- to
(VREF + IN-). The IN- input is limited to ±100 mV from
the VSS rail. The IN- input can be used to cancel small
signal common-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 {[VREF + (IN-)] - 1 LSB}, then
the output code will be 3FFh. If the voltage level at IN-
is more than 1 LSB below VSS, the voltage level at the
IN+ input will have to go below VSS to see the 000h
output code. Conversely, if IN- is more than 1 LSB
above VSS, the 3FFh code will not be seen unless the
IN+ input level goes above VREF level.
For the A/D converter to meet specification, the charge
holding capacitor (CSAMPLE) must be given enough
time to acquire a 10-bit accurate voltage level during
the 1.5 clock cycle sampling period. The analog input
model is shown in Figure 4-1.
This diagram illustrates that the source impedance (RS)
adds to the internal sampling switch (RSS) impedance,
directly affecting the time that is required to charge the
capacitor (CSAMPLE). Consequently, larger source
impedances increase the offset, gain and integral
linearity errors of the conversion (see Figure 4-2).
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.
EQUATION 4-1:
LSB SIZE CALCULATION
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.
EQUATION 4-2:
DIGITAL OUTPUT CODE
CALCULATION
When using an external voltage reference device, the
system
designer
should
always
refer
to
the
manufacturer’s recommendations for circuit layout.
Any instability in the operation of the reference device
will have a direct effect on the operation of the A/D
converter.
LSB Size
V
REF
1024
-------------
=
Digital Output Code
1024
V
IN
×
V
REF
---------------------------
=
Where:
VIN
=
analog input voltage
VREF
=
analog input voltage


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