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MCP3002-I

2.7V Dual Channel 10-Bit A/D Converter with SPI Serial Interface

Microchip Technology 

4.0 DEVICE OPERATION

The MCP3002 A/D converter employs 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 second rising edge of

the serial clock after the start bit has been received.

Following this sample time, the input switch of the

converter opens and 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 200 ksps are possible on the MCP3002. See

Section 6.2 “Maintaining Minimum Clock Speed” for

information on minimum clock rates. Communication with

the device is done using a 3-wire SPI compatible interface.

4.1 Analog Inputs

The MCP3002 device offers the choice of using the

analog input channels configured as two single-ended

inputs that are referenced to VSS or a single pseudo-

differential input. The configuration setup is done as part

of the serial command before each conversion begins.

When used in the psuedo-differential mode, CH0 and

CH1 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 the reference voltage,

VDD. 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.

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.

In this diagram, it is shown 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.

Ideally, the impedance of the signal source should be

near zero. This is achievable with an operational

amplifier such as the MCP601 which has a closed loop

output impedance of tens of ohms. The adverse affects

of higher source impedances are shown in Figure 4-2.

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 {[VDD + (IN-)] - 1 LSB}, then the out-

put code will be 3FFh. If the voltage level at IN- is more

than 1 LSB below VSS, then 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 aboveVSS,

then the 3FFh code will not be seen unless the IN+

input level goes above VDD level. If the voltage at IN+

is equal to or greater than {[VDD + (IN-)] - 1 LSB}, then

the output code will be 3FFh.

© 2008 Microchip Technology Inc.

MCP3002

4.2 Digital Output Code

The digital output code produced by an A/D Converter

is a function of the input signal and the reference

voltage. For the MCP3002, VDD is used as the

reference voltage.

LSB Size = -V----R---E----F-

1024

As the VDD level is reduced, the LSB size is reduced

accordingly. The theoretical digital output code

produced by the A/D Converter is shown below.

Digital Output Code = 1----0---2---4----*---V----I--N--

VDD

Where:

VIN = analog input voltage

VDD = supply voltage

DS21294D-page 15

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