COP8AME9
15.0 A/D Converter
(Continued)
TABLE 22. A/D Converter Channel Selection when the Multiplexor Output is Disabled
Mode Select
ADMOD = 0
Single Ended
Mode
ADCH0
1
0
1
0
1
0
1
Channel No.
Temp sensor, if
enabled
10
11
12
13
14
15
Mode Select
ADMOD = 1
Differential
Mode
Channel Pairs
(+, −)
Not used
10, 11 (Note 12)
11, 10 (Note 12)
12, 13 (Note 12)
13, 12 (Note 12)
14, 15 (Note 12)
15, 14 (Note 12)
Mux Output
Disabled
MUX
0
0
0
0
0
0
0
Select Bits
ADCH3
0
1
1
1
1
1
1
ADCH2
1
0
0
1
1
1
1
ADCH1
1
1
1
0
0
1
1
Note 12:
Only if the programmable gain amplifier is bypassed.
15.1.1.2 Multiplexor Output Select
The MUX bit field allows the output of the A/D multiplexor
(with the exception of the temperature sensor channel) and
the input to the A/D to be connected directly to external pins.
This allows for an external, common filter/signal conditioning
circuit to be applied to all channels. The output of the exter-
nal conditioning circuit can then be connected directly to the
input of the Sample and Hold input on the A/D Converter.
See
for the single ended mode diagram. The
Multiplexor output is connected to ADCH4 and the A/D input
is connected to ADCH5. For differential mode, the differential
multiplexor outputs are available and should be converted to
a single ended voltage for connection to the A/D Converter
Input. The programmable gain amplifier must be bypassed
when using the multiplexor output feature in differential
mode. See
The channel assignments for this mode are shown in
When using the Mux Output feature, the delay though the
internal multiplexor to the pin, plus the delay of the external
filter circuit, plus the internal delay to the Sample and Hold
will exceed the three clock cycles that’s allowed in the con-
version. This requires that whenever the MUX bit = 1, that
the channel selected by ADCH3:0 bits, be enabled, even
when ADBSY = 0, and gated to the mux output pin. The input
path to the A/D converter should also be enabled. This
allows the input channel to be selected and settled before
starting a conversion. The sequence to perform conversions
using the Mux Out feature is a multistep process and is listed
below.
1. Select the desired channel, excluding the temperature
sensor channel, and operating modes and load them
into ENAD without setting ADBSY.
2. Wait the appropriate time until the analog input has
settled. This will depend on the application and the
response of the external circuit.
3.
Select the same desired channel and operating modes
used in step 1 and load them into ENAD and also set
ADBSY. This will start the conversion.
4. After conversion is completed, obtain the results from
the result registers.
20006356
FIGURE 26. A/D with Single Ended Mux Output Feature
Enabled
20006357
FIGURE 27. A/D with Differential Mux Output Feature
Enabled
www.national.com
50
NSC [ NATIONAL SEMICONDUCTOR ]
