The Adc A Channel Analog to Digital Converter

The ADC0834 is an 8-bit analog to digital converter (A/D converter) with 4-channel synchronous serial output. The information box below will explain what each of these terms mean.

Analog to Digital Converter (A/D converter or ADC for short) A device that measures an analog voltage sample and returns a binary number that describes the sample.

8-bit: The number of binary digits the ADC0834 uses to describe the analog voltage it samples. 8-bit is also the resolution of the A/D converter. You can count from 0 to 255 (decimal) using an 8-bit binary number. This means that the ADC0834 can approximate the voltage it measures as one of 256 levels. A higher resolution converter, such as 12-bit, would break the same voltage range into 4096 levels because you can count from 0 to 4095 with 12 binary bits.

Synchronous serial means that data bits are sent one after another along a single data line, in a time-coordinated manner (in sync). The ADC0834 depends on a clock signal sent by the BASIC Stamp to time the sending of each serial output bit.

4-Channel means that four different analog voltages can be input to this A/D converter, one to each channel. Any channel can be converted, but only one channel can be converted at a time. The BASIC Stamp module sends signals to the ADC0834 to let the ADC0834 know which channel to convert.

Take a peek ahead at the schematic in Figure 1-6 on page 30. The BASIC Stamp module must be wired to the ADC0834, and then programmed to send binary control signals to it. The PBASIC program will make the ADC0834 do its job, and will allow the BASIC Stamp module to read and store the 8-serial-bits transmitted by the ADC0834. The notation for the ADC0834's inputs and outputs that we will be using are shown in Table 1-1 below.

Table 1-1: Measured voltages during charge cycle

CH0

Channel 0 Analog Input

CH1

Channel 1 Analog Input

CH2

Channel 2 Analog Input.

CH3

Channel 3 Analog Input

V+

Voltage supply

VREF

Reference Voltage

AGND

Analog Ground

DGND

Data Ground

DO

Data Out (the serial data output)

DI

Data In (the serial data input)

CS

Chip Select (active low)

Why are we using V+ instead of Vcc? If you're wondering why the ADC0834 A/D Converter chip's Vcc pin (pin 14) is not used, then please refer to page 18 of the manufacturer's data sheet for this device, which can be downloaded from National Semiconductor's web site www.national.com. Here you will see that an internal diode goes from V+ (pin 1) to Vcc along with a zener diode going to ground, which provides for a better degree of voltage regulation to the internal circuitry. By supplying voltage to the V+ pin in this way, the Vcc voltage level will always be guaranteed to be at the correct voltage. That said, this selection is based on your author's personal preference, and you are free to wire Vdd directly to Vcc (pin 14) if you so choose.

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