ADC08831 datasheet(13/23 Pages) NSC

13 / 23 page

Functional Description (Continued)

3.0 Reducing Power Consumption

The ADC08831 operate up to a 2MHz clock frequency, or

about 181 ksps. At 5V supply, it consumes about 1.7 mA or

8.5 mW when CS is logic low. The ADC08831 has a low

power mode to minimize total power consumption.

When the chip select is asserted with a logic high, some ana-

log circuitry and digital logic are pulled to a static, low power

condition. Also, DOUT, the output driver is taken into

TRI-STATE mode.

To optimize static power consumption, special attention is

needed to the digital input logic signals: CLK, CS, DI. Each

digital input has a large CMOS buffer between V

CC and

GND. A traditional TTL level high (2.4V) will be sufficient for

each input to read a logical “1”. However, there could be a

large V

IH to VCC voltage difference at each input. Such a

voltage difference would cause static power dissipation,

even when chip select pin is high and the part is in low power

mode.

Therefore, to minimize static power dissipation, it is recom-

mended that all digital input logic levels should equal the

converter’s supply. Various CMOS logic is particularly well

suited for this application.

The reference pin on the ADC08831 is not affected by the

power-down mode. To reduce static reference current during

non-conversion time, there are a couple options. First, a low

voltage external reference (ie, 2.5V could be used). A shunt

reference, such as the LM385-2.5, could be powered by a

logic gate that is the inverse of the signal on CS . When CS

is high, the reference is off. As a second option, an external,

low on-resistance switch could be used.

The ADC08832 is similar to the ADC08831, except its refer-

ence is derived from V

CC. The ADC08832 does enter a

low-power mode when CS is logic high, as the analog and

digital logic enter static current modes. However power dis-

sipation from the reference ladder occurs, regardless of the

signal on CS

4.0 REFERENCE CONSIDERATIONS

The voltage applied to the reference input on these convert-

ers, V

REF, defines the voltage span of the analog input (the

difference between V

IN(MAX) and VIN(MIN) over which the 256

possible output codes apply. The devices can be used either

in 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

which can be as low as 2.8k

Ω. This pin is the top of a resistor

divider string and capacitor array used for the successive ap-

proximation conversion.

In a ratiometric system the analog input voltage is propor-

tional to the voltage used for the A/D reference. This voltage

is typically the system power supply, so the V

REF pin can be

tied to V

CC (done internally on the ADC08832). This tech-

nique relaxes the stability requirements of the system refer-

ence 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 be-

tween very specific voltage limits, the reference pin can be

biased with a time and temperature stable voltage source.

The LM385, LM336 and LM4040 reference diodes are good

low current devices to use with these converters.

The maximum value of the reference is limited to the V

supply voltage. The minimum value, however, can be quite

small (see Typical Performance Characteristics) to allow di-

rect 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 LSB equals V

REF/256).

DS100108-28

a) Ratiometric

DS100108-29

b) Absolute with a Reduced Span

FIGURE 1. Reference Examples

ADC08831 Datasheet(PDF) 13 Page - National Semiconductor (TI)

ADC08831 Datasheet(HTML) 13 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Datasheet Download

Link URL