Rainbow Electronics DS1677 - Manual (Page 9)

DS1677
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ANALOG–TO–DIGITAL CONVERTER
The DS1677 provides a 3–channel 8–bit analog–to–digital converter. The A/D reference voltage (2.55
volt typical) is derived from an on–chip band–gap circuit. Three multiplexed analog inputs are provided
through the AIN0, AIN1, and AIN2 pins. The A/D converter is monotonic (no missing codes) and uses a
successive approximation technique to convert the analog signal into a digital code.
An A/D conversion is the process of assigning a digital code to an analog input voltage. This code
represents the input value as a fraction of the full scale voltage (FSV) range. Thus the FSV range is then
divided by the A/D converter into 256 codes (8 bits). The FSV range is bounded by an upper limit equal
to the reference voltage and the lower limit which is ground. The DS1677 has a FSV of 2.55 volt
(typical) which provides a resolution of 10 mV. An input voltage equal to the reference voltage converts
to FFh while an input voltage equal to ground converts to 00h. The relative linearity of the A/D converter
is
±0.5 LSB.
The A/D converter selects from one of three different analog inputs (AIN0 – AIN2). The input that is
selected is determined by the Analog Input Select (AIS) bits in the Control Register. Table 2 lists the
specific analog input that is selected by these 2 bits. Note also that the converter can be turned off by
these bits to reduce power. When the A/D is turned on by setting AIS0 and AIS1 to any value other than
0,0 the analog input voltage is converted and written to the ADC Register within 488
µs. An internal
analog filter at the input reduces high frequency noise. Subsequent updates occur approximately every
10 ms. If AIS0 and/or AIS1 are changed, updates will occur at the next 10 ms conversion time.
The Conversion Update In Progress (CU) bit in the Status Register indicates when the ADC Register can
be read. When this bit is a one, an update to the ADC Register will occur within 488
µs maximum.
However, when this bit is zero an update will not occur for at least 244 µs. The CU bit should be polled
before reading the ADC Register to insure that the contents are stable during a read cycle. Once a read
cycle to the ADC Register has been started, the DS1677 will not update that register until the read cycle
has been completed. It should also be mentioned that taking CS low will abort the read cycle and will
allow the ADC Register to be updated.
Figure 5 illustrates the timing of the CU bit relative to an instruction to begin conversion and the
completion of that conversion.