NEC uPD78P078 - Manual (Page 314)

314
CHAPTER 16 D/A CONVERTER
16.5 D/A Converter Cautions
(1) Output impedance of D/A converter
Because the output impedance of the D/A converter is high, use of current flowing from the ANOn pins (n
= 0,1) is prohibited. If the input impedance of the load for the converter is low, insert a buffer amplifier between
the load and the ANOn pins. In addition, wiring from the ANOn pins to the buffer amplifier or the load should
be as short as possible (because of high output impedance). If the wiring may be long, design the ground
pattern so as to be close to those lines or use some other expedient to achieve shorter wiring.
Figure 16-3. Use Example of Buffer Amplifier
(a) Inverting amplifier
(b) Voltage-follower
(2) Output voltage of D/A converter
Because the output voltage of the converter changes in steps, use the D/A converter output signals in general
by connecting a low-pass filter.
(3) AV REF1 pin
When only either one of the D/A converter channels is used with AV REF1 < V DD , the other pins that are not
used as analog outputs must be set as follows:
• Set PM13x bit of the port mode register 13 (PM13) to 1 (input mode) and connect the pin to V
SS
.
• Set PM13x bit of the port mode register 13 (PM13) to 0 (output mode) and the output latch to 0, to output
low level from the pin.
µPD78078, 78078Y
ANOn
R
1
C
R 2
• The input impedance of the buffer amplifier is R
1
.
µPD78078, 78078Y
ANOn
R
R 1 C
• The input impedance of the buffer amplifier is R 1 .
• If R 1 is not connected, the output becomes
undefined when RESET is low.