Output Current And Referenbe Resistance - NEC mPD7755 User Manual

3.2 D/A converter
The built-in D/A conv e rter is a unipolar, current output
type with a 9-bit resolution. The digital input to the
converter is converted to a 9-bit offset binary code.
This code has a range of 0 to IfFH with a midrange value
of lOOH.
The schematic drawing of the D/A converter is shown in
Fig. 3-1. As can be seen, a constant-current source is
connected in parallel for each bit of the digital input.
pPD7755/ 7756
D/A
con-
verter
AVO
AVO
I AVO : Output current
Rr,:
Load resistance
.va:
Potential across RL
[
~r-----'-'
_____
[ 1 r - - - - - - - r - - - - - < ,
[i
0 AVO
'- I
9-bit digital input
Fig. 3-1 Schematic Diagram of D/A Converter
3 . 2.1 Output Current and Reference Resistance
The output current of the D/A converter can be controlled
by the voltage ap p lied tq the REF pin. This is realized
by the resistor connected across the VDD and REF pins as
shown in Fig. 3-2.
The relations between the voltage to the REF pin, V REF ,
and input reference current IREF or D/A converter output
current IVAO are shown in Figs. 3-3 and 3-4.
If the impedance of an amplifier to be connected to the
AVO pin is as high as several kn or more, refer to fig.
3-3 to determine the value of I REF • In Fig. 3-3, the
3-2