ADC0831-N, ADC0832-N, ADC0834-N, ADC0838-N

SNAS531B – AUGUST 1999 – REVISED MARCH 2013

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7. After 8 clock periods the conversion is completed. The SAR status line returns low to indicate this ½ clock

cycle later.

8. If the programmer prefers, the data can be provided in an LSB first format [this makes use of the shift enable

(SE) control line]. All 8 bits of the result are stored in an output shift register. On devices which do not include the

SE control line, the data, LSB first, is automatically shifted out the DO line, after the MSB first data stream. The

DO line then goes low and stays low until CS is returned high. On the ADC0838-N the SE line is brought out and

if held high, the value of the LSB remains valid on the DO line. When SE is forced low, the data is then clocked

out LSB first. The ADC0831-N is an exception in that its data is only output in MSB first format.

9. All internal registers are cleared when the CS line is high. If another conversion is desired, CS must make a

high to low transition followed by address information.

The DI and DO lines can be tied together and controlled through a bidirectional processor I/O bit with one wire.

This is possible because the DI input is only “looked-at” during the MUX addressing interval while the DO line is

still in a high impedance state.

Reference Considerations

The voltage applied to the reference input to these converters defines the voltage span of the analog input (the

used in either ratiometric applications or in systems requiring absolute accuracy. The reference pin must be

connected to a voltage source capable of driving the reference input resistance of typically 3.5 k

Ω. This pin is the

top of a resistor divider string used for the successive approximation conversion.

In a ratiometric system, the analog input voltage is proportional to the voltage used for the A/D reference. This

N). This technique relaxes the stability requirements of the system reference as the analog input and A/D

reference move together maintaining the same output code for a given input condition.

For absolute accuracy, where the analog input varies between very specific voltage limits, the reference pin can

be biased with a time and temperature stable voltage source. The LM385 and LM336 reference diodes are good

low current devices to use with these converters.

quite small (see Typical Performance Characteristics) to allow direct conversions of transducer outputs providing

less than a 5V output span. Particular care must be taken with regard to noise pickup, circuit layout and system

error voltage sources when operating with a reduced span due to the increased sensitivity of the converter (1

a) Ratiometric

b) Absolute with a reduced Span

Figure 23. Reference Examples

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