The Microcontroller Idea Book - Jan Axelson's Lakeview Research

different ground symbols for the two ground paths. Also be sure to include decoupling
capacitors at pins 10 and 12.
Measuring Modes
To allow for different circuit requirements, the ADC0848 offers a choice of three softwareselectable
modes of operation: single-ended, differential, and pseudo-differential. Figure
9-5 illustrates.
In single-ended mode, each analog input is referenced to AGND. This is the simplest mode
and will work fine for many applications.
Listing 9-2 causes the ADC to convert each of the eight channels in turn and displays the
results on the host computer. For testing the circuits, you can connect a the wiper of a
potentiometer to each channel, with the other two leads connecting to AGND and VREF. Verify
that the readings for each channel vary from 0 to 255 as you vary the input voltage.
The other modes are useful for more critical measurements where you need to reject
background noise or offset voltages.
In differential mode, each channel is paired with an adjacent one, with the voltage on one
channel referenced to the voltage on the other.
For example, you could connect an output from a sensor to channel 2 and a ground or other
reference from that sensor to channel 1. With differential mode selected, channel 1 will read
the difference between channel 1’s and channel 2’s voltage. This mode cancels out errors
due to noise that is common to both channels in the pair, such as 60-cycle power-line
interference. However,because each channel uses two analog inputs, this mode limits you
to four channels.
The third mode of operation is pseudo-differential. In this mode, channels 1-7 are all
referenced to channel 8. This allows you to make 7 measurements, all with the same
reference. This mode is useful if you are have multiple sensors in the same location. Also,
if you connect channel 8 to a voltage greater than AGND, the converter’s range will shift to
match, with a 0 output occurring when an input equals channel 8’s voltage.
Reading the ADC
Using Sensors to Detect and Measure
To begin a conversion on the ADC0848, the 8052-BASIC writes to the converter indicating
the desired channel and mode. Bits 0-2 specify the channel (000=1, 001=2, 010=3, etc.),
and bits 3-4 specify the mode of operation (00=differential, 01=single-ended, 11=pseudo
differential). So, for example, in Figure 9-4’s circuit, to begin a single-ended conversion at
channel 5, you would write 0000 1101, or 0Dh, to the converter’s address.
The Microcontroller Idea Book 161