AD8842AR-REEL Analog Devices Inc, AD8842AR-REEL Datasheet - Page 9

• Manufacturer Part Number: AD8842AR-REEL
• Description: IC DAC MULT 8BIT OCTAL 24-SOIC
• Manufacturer: Analog Devices Inc
• Series: TrimDAC®r

Specifications of AD8842AR-REEL

• Rohs Status: RoHS non-compliant
• Settling Time: 2.9µs
• Number Of Bits: 8
• Data Interface: Serial
• Number Of Converters: 8
• Voltage Supply Source: Dual ±
• Power Dissipation (max): 135mW
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 24-SOIC (7.5mm Width)

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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
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.
Figure 27. Functional Equivalent Circuit to the AD8842
Results in a 4-Quadrant Multiplying Channel
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
plifier outputs to zero volts. This asynchronous input pin PR
V
–2
–4
IN
4
0
2
AD8842 INPUT-OUTPUT VOLTAGE RANGE
V
V
V
OUT
DAC
OUT
–4
= 2 (D/256)
= (D/128 – 1)
= V
= D/256
= 2
IN
H
(D/128 – 1), WHERE D = 0 TO 255
V
–2
DAC
which in turn forces all the buffer am-
R
V
V
– V
IN
IN
V
IN
IN
– Volts
V
0
IN
IN
– V
times the digital input control
IN
V
R
DAC
2
D = FFH
D = C0H
D = 80H
D = 40H
D = 00H
4
V
OUT
–9–
can be activated at any time to force the DAC registers to the
half-scale code 80
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.
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.
Using the equations given in Figure 29 one can calculate the
final output equation as follows:
Figure 28. One Traditional Technique to Achieve Four-
Quadrant Multiplying with a Complementary Current
Output DAC
V
IN
V
V
IN
O
R
REF
Figure 29. Current Conveyor Amplifier
– 1 –
REF
GND
H
CURRENT OUT
256
. This is generally the most convenient place
D
256
(1- D)
256
128
256
256
D
2D
D
D
DAC
V
REF
R
V
– 1 V
– 1 V
– 1 V
REF
R
V
R
IN
I
I
2
1
IN
IN
IN
I
I
1
2
R –
CURRENT CONVEYOR
256
D
AMPLIFIER
256
–D
V
R
IN
R/2
R/2
A1
R
A2
AD8842
V
R
IN
V
= V
OUT
IN
V
X
V
O
R
OUT
(D/128–1)