DS1677

9 of 18

ANALOG-TO-DIGITAL CONVERTER

The DS1677 provides a 3-channel 8-bit analog-to-digital converter. The A/D reference voltage (2.55V

typical) is derived from an on-chip band-gap circuit. Three multiplexed analog inputs are provided

through the AIN0, AIN1, and AIN2 pins. The ADC 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 ADC 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 2.55V (typical) that

provides a resolution of 10mV. 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 ADC is

0.5 LSB.

The ADC 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 ADC 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

s. An internal analog filter at the input

reduces high frequency noise. Subsequent updates occur approximately every 10ms. If AIS0 and/or AIS1

are changed, updates will occur at the next 10ms 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

s maximum.

However, when this bit is zero an update will not occur for at least 244

s. 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.