AD8842
CIRCUIT OPERATION
The AD8842 is a general purpose 8-channel ac or dc signal-
level adjustment device designed to replace potentiometers used
in the three-terminal connection mode. Eight independent
channels of programmable signal level control are available in
this 24-pin package device. The outputs are completely buffered
providing up to 3 mA of output drive-current to drive external
loads. The functional equivalent DAC and amplifier combina-
tion shown in Figure 27 produces four-quadrant multiplication
of the signal inputs applied to V
IN
times the digital input control
word. In addition the AD8842 provides a 50 kHz full power
bandwidth in each four-quadrant multiplying channel. Operat-
ing from plus and minus 5 V power supplies, analog inputs and
outputs of
±
3 V are easily accommodated.
R
V
IN
R
can be activated at any time to force the DAC registers to the
half-scale code 80
H
. This is generally the most convenient place
to start general purpose adjustment procedures.
Achieving 4-Quadrant Multiplying with a Current Conveyor
Amplifier
The traditional current output CMOS digital-to-analog con-
verter requires two amplifiers to perform the current-to-voltage
translation and the half-scale offset to achieve four-quadrant
multiplying capability. The circuit shown in Figure 28 shows
one such traditional connection.
R/2
A1
I
1
REF
V
IN
CURRENT OUT
DAC
R
R/2
V
OUT
GND
V
DAC
I
2
A2
V
O
V
DAC
= D/256
×
V
IN
V
OUT
= 2
×
V
DAC
– V
IN
= 2 (D/256)
×
V
IN
– V
IN
= (D/128 – 1)
×
V
IN
Figure 28. One Traditional Technique to Achieve Four-
Quadrant Multiplying with a Complementary Current
Output DAC
AD8842 INPUT-OUTPUT VOLTAGE RANGE
4
D = FFH
2
V
OUT
– Volts
D = C0H
D = 80H
D = 40H
D = 00H
A single new current conveyor amplifier design emulates ampli-
fiers A1 and A2 shown in Figure 28. Figure 29 shows the con-
nection and equations that define this new circuit that achieves
four-quadrant multiplication with only one amplifier.
0
V
IN
REF
D
256
R
V
REF
R
I
1
I
2
CURRENT CONVEYOR
AMPLIFIER
V
OUT
X
V
OUT
(D/128–1)
–2
(1- D) V
REF
256
R
–4
R
= V
IN
–4
–2
0
V
IN
– Volts
2
4
V
OUT
= V
IN
(D/128 – 1), WHERE D = 0 TO 255
Figure 27. Functional Equivalent Circuit to the AD8842
Results in a 4-Quadrant Multiplying Channel
Figure 29. Current Conveyor Amplifier
In order to simplify use with a controlling microprocessor a
PCB space saving three-wire serial data interface was chosen.
This interface can be easily adapted to almost all microcom-
puter and microprocessor systems. A clock (CLK), serial data
input (SDI) and a load (LD) strobe pins make up the three-wire
interface. The 12-bit input data word used to change the value
of the internal DAC registers contains a 4-bit address and 8-bits
of data. Using this word combination any DAC register can be
changed at a given time without disturbing the other channels.
A serial data output SDO pin simplifies cascading multiple
AD8842s without adding address decoder chips to the system.
During system power up a logic low on the preset
PR
pin forces
all DAC registers to 80
H
which in turn forces all the buffer am-
plifier outputs to zero volts. This asynchronous input pin
PR
Using the equations given in Figure 29 one can calculate the
final output equation as follows:
–D
V
IN
D
V
IN
V
O
=
–
1 –
×
×
R
–
256
×
R
×
R
256
R
D
D
– 1
V
IN
+
×V
IN
256
256
2D
=
– 1
V
IN
256
D
=
– 1
V
IN
128
REV. 0
–9–
AD [ ANALOG DEVICES ]
