Datasheet

LP2950
VIN
GND
VDD
CS
SCLK
SDATA
LMT87
VDD
GND
OUT
ADCS747X
V
SUPPLY
1 µF
1 µF
0.1 µF
C
BP
DSP
Microcontroller
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22
ADCS7476 , ADCS7477 , ADCS7478
SNAS192G –APRIL 2003–REVISED MAY 2016 www.ti.com
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8.2 Typical Application
The ADCS747x are monolithic CMOS 12-, 10-, and 8-bit ADCs that use the supply voltage as a reference,
enabling the devices to operate with a full-scale input range of 0 to V
DD
. An example low-power application with
the LMT87, which is a wide range ±0.3°C accurate temperature sensor, is shown in Figure 32.
Figure 32. Typical Application Circuit
8.2.1 Design Requirements
A successful ADCS747x and LMT87 design is constrained by the following factors:
• V
IN
range must be 0 V to V
DD
where V
DD
can range from 2.7 V to 5.25 V.
8.2.2 Detailed Design Procedure
Designing for an accurate measurement requires careful attention to the timing requirements for the ADCS747x
parts.
Because the ADC747x parts use the supply voltage as a reference, ensuring that the supply voltage is settled to
its final level before exiting the shutdown mode and beginning a conversion is important. After the supply voltage
has settled, the CS is brought to a low level (ideally 0 V) to start a conversion.
Ensuring that any noise on the power supply is less than ½ LSB in amplitude is also important. The supply
voltage must be regarded as a precise voltage reference.
After the CS has been brought low, the user must wait for one complete conversion cycle (approximately 1 μs)
for meaningful data. The dummy conversion cycle is the start-up time of the ADCS747x. The ADCS747x digital
output can then be correlated to the LMT87 output level to get an accurate temperature reading. At V
DD
= 3.3 V,
1 LSB of ADCS7476 is 0.805 mV.