pc87591l National Semiconductor Corporation, pc87591l Datasheet - Page 186

• Manufacturer Part Number: pc87591l
• Description: Lpc Mobile Embedded Controllers
• Manufacturer: National Semiconductor Corporation

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Embedded Controller Modules
For positive input voltages higher than V
The divider should be calculated so that its output is lower than the full-scale value (V
voltage.
For negative input voltages, a resistive level-shifter should be used in front of the analog input (see Figure 61b). The level-
shifter should be calculated so that its output is higher than 0V for the minimum input signal voltage.
Filtering the Noise on Voltage Input Signals
Noise may be coupled to the input signal for various reasons, including close proximity to digital circuits. The slow change
rate of the input signals, makes it possible to use an external low pass filter (LPF). This can be implemented by placing a
capacitor (C
least 22 times the maximum signal frequency required to be measured with a 10-bit accuracy (a smaller capacitor may be
used when a lower accuracy is acceptable). The formula below demonstrates the calculation of the components.
Calculating the Voltage Channel Delay
The Voltage measurement delay time (t
should be long enough to guarantee the voltage settling at the input of the A/D converter to within 1/2 LSB. This includes
the input interface circuits of the ADC and the externally added noise-rejection filters (if applicable). Figure 62 shows the
equivalent R-C circuit of a filtered input. R
preceding section, “Filtering the Noise on Voltage Input Signals”). C
cludes the input the multiplexer.
Since the R-C of the input circuit is short, typically, all that is required is a very short delay, which shortens the overall mea-
surement time. The delay length may be sized during development by increasing the delay time from the minimum value to
the point were the ADC readout is not affected by the delay value (i.e., use the minimal delay to which the ADC readout is
invariant).
.
Thermistor-Based Temperature Measurement
The ADC is capable of measuring temperature by means of NTC or PTC thermistor devices. These elements change their
resistance according to their temperature. The resistance change is converted to voltage by connecting the element in a
voltage divider between AV
by the parameters of the NTC/PTC thermistor in use.
F
) between the divider output and AGND, as shown in Figure 61. The cutoff frequency of this LPF should be at
V
IN
(12V)
f
(-3dB)
Signal
Input
V
IN
where: R
CC
C
F
= 1 / (2
Figure 61. Measurement of Positive and Negative Voltages
and AGND, as shown in Figure 63. The translation of voltage to temperature is determined
a. V
Figure 62. Filtered Voltage Input Equivalent Circuit
IN
R
eq
> V
eq
C
VD
F
= (R
FS
R
FS
R
AGND
eq
*
2
) is the period between input selection and the A/D conversion start. This delay
1
, a resistor divider should be used in front of the analog input (see Figure 61a).
represents the equivalent resistance of the input divider or level-shifter (see the
(Continued)
1 *
ADn
*
R
ADn
R
2
) / (R
eq *
1
C
+ R
F
186
)
2
)
Interface
Circuit
Input
V
F
IN
is the filter capacitor. The Input Interface Circuit in-
(-12V)
AGND
C
A/D Converter
F
AV
b. V
CC
FS
) for the maximum input signal
IN
< 0
R
R
2
1
ADn
Revision 1.07