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ADC08831 Просмотр технического описания (PDF) - National ->Texas Instruments

Номер в каталогеADC08831 National-Semiconductor
National ->Texas Instruments National-Semiconductor
Компоненты Описание8-Bit Serial I/O CMOS A/D Converters with Multiplexer and Sample/Hold Function
ADC08831 Datasheet PDF : 24 Pages
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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 VCC 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 VIH 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 VCC. 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, VREF, defines the voltage span of the analog input (the
difference between VIN(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. 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 VREF pin can be
tied to VCC (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 VCC
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 VREF/256).
a) Ratiometric
DS100108-28
b) Absolute with a Reduced Span
FIGURE 1. Reference Examples
DS100108-29
13
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