COP8-REF-AM datasheet(55/83 Pages) NSC

55 / 83 page

15.0 A/D Converter (Continued)

Source impedances greater than 3 k

Ω on the analog input

lines will adversely affect the internal RC charging time

during input sampling. As shown in Figure 29, the analog

switch to the Sample & Hold capacitor is closed only during

the 3 A/D cycle sample time. Large source impedances on

the analog inputs may result in the Sample & Hold capacitor

not being charged to the correct voltage levels, causing

scale errors.

If large source resistance is necessary, the recommended

solution is to slow down the A/D clock speed in proportion to

the source resistance. The A/D Converter may be operated

at the maximum speed for R

S less than 3 k

Ω. For R

S greater

than 3 k

Ω, A/D clock speed needs to be reduced. For ex-

ample, with R

S =6k

Ω, the A/D Converter may be operated

at half the maximum speed. A/D Converter clock speed may

be slowed down by either increasing the A/D prescaler

divide-by or decreasing the CKI clock frequency. The A/D

minimum clock speed is 64 kHz.

16.0 Temperature Sensor

16.1 GENERAL DESCRIPTION

The Temperature Sensor on this device operates over a

−40˚C to +125˚C temperature range and produces an output

voltage proportional to the device temperature. The transfer

function is approximately linear. Refer to the A/D Converter

section to see how the Temperature Sensor is integrated

with the Programmable Gain Amplifier and A/D Converter.

The equation for V

OUT vs. temperature is:

OUT = [(−8.0 mV/˚C) X T] + 1.65V

where T is the temperature in ˚C.

The user can achieve greater temperature sensor accuracy

by performing a two temperature calibration to compensate

for device-to-device variations in slope and base value for

OUT.

16.2 OPERATION

The Temperature Sensor is used in conjunction with the

on-chip Programmable Gain Amplifier and A/D converter to

read the Temperature Sensor output voltage. The Program-

mable Gain Amplifier must be used and the gain can be set

at either 1 or 2, depending on the operating voltage of the

device. To use a gain of 2, V

CC should be greater than 4.5V.

The output voltage given in the above equation is for a gain

of 1. The Temperature Sensor is connected to channel 7 on

the A/D Converter multiplexor. See the A/D Converter sec-

tion for more details on using the ENAD and ADGAIN regis-

ters.

The Temperature Sensor is enabled by setting the ENTS bits

in the ADGAIN register to a 1. The circuit will draw power

when it’s enabled. When disabled, the current drawn is

extremely low. When using the HALT mode of the device, the

Temperature Sensor will draw current unless it is disabled by

software. Therefore, for minimal current in HALT mode, the

Temperature Sensor should be disabled prior to entering

HALT. A Reset will disable the Temperature Sensor.

16.2.1 Procedure for Reading the Temperature Sensor

Voltage

The following steps should be followed for measuring the

temperature sensor voltage:

Enable the Temperature Sensor by setting the ENTS bit

in the ADGAIN register to 1. The Programmable Gain

Amplifier gain should also be selected to be either 1 or 2.

Wait 350 µs for the temperature sensor to stabilize. This

is only required after ENTS bit is changed from 0 to 1.

Load the ENAD register with the channel number for the

temperature sensor, and the desired prescale value. The

ADMOD, MUX, and ADBSY bits should be 0.

Wait for the Programmable Gain Amplifier to stabilize

with the voltage for the newly selected channel.

Set the ADBSY bit in the ENAD register. The other bits in

the ENAD register should be the same as in step 3.

Wait for the ADBSY bit to go to 0 and then read the

output of the A/D Converter result registers, ADRSTH

and ADRSTL.

Subsequent readings of the temperature sensor can be

done by repeating steps 5 and 6, as long as the channel

number in ENAD has not changed from that of the

temperature sensor. If the channel number has been

changed to measure other channels, in between two

successive temperature sensor readings, then steps

1–6 should be followed.

20006362

*The analog switch is closed only during the sample time.

FIGURE 29. A/D Pin Model (Single Ended Mode)

COP8-REF-AM 数据表(PDF) 55 Page - National Semiconductor (TI)

COP8-REF-AM 数据表(HTML) 55 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 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

数据表下载