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 RS less than 3 kΩ. For RS greater
than 3 kΩ, A/D clock speed needs to be reduced. For ex-
ample, with RS = 6 kΩ, 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.
15.0 A/D Converter (Continued)
Source impedances greater than 3 kΩ 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.
20006362
*The analog switch is closed only during the sample time.
FIGURE 29. A/D Pin Model (Single Ended Mode)
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.
16.0 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.
16.2.1 Procedure for Reading the Temperature Sensor
Voltage
The following steps should be followed for measuring the
temperature sensor voltage:
The equation for VOUT vs. temperature is:
VOUT = [(−8.0 mV/˚C) X T] + 1.65V
where T is the temperature in ˚C.
1. 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.
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
2. Wait 350 µs for the temperature sensor to stabilize. This
is only required after ENTS bit is changed from 0 to 1.
3. 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.
VOUT
.
16.2 OPERATION
4. Wait for the Programmable Gain Amplifier to stabilize
with the voltage for the newly selected channel.
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, VCC 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.
5. Set the ADBSY bit in the ENAD register. The other bits in
the ENAD register should be the same as in step 3.
6. Wait for the ADBSY bit to go to 0 and then read the
output of the A/D Converter result registers, ADRSTH
and ADRSTL.
7. 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.
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
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