AD8842 Ver la hoja de datos (PDF) - Analog Devices
Número de pieza
componentes Descripción
Fabricante
AD8842 Datasheet PDF : 13 Pages
| |||
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 VIN 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
R
VIN
VDAC
VOUT
AD8842
can be activated at any time to force the DAC registers to the
half-scale code 80H. 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
I1
REF
VIN
CURRENT OUT
DAC
GND
I2
A1
R/2
R
A2
VO
VDAC = D/256 × VIN
VOUT = 2 × VDAC – VIN
= 2 (D/256) × VIN – VIN
= (D/128 – 1) × VIN
AD8842 INPUT-OUTPUT VOLTAGE RANGE
4
D = FFH
2
D = C0H
D = 80H
0
D = 40H
–2
D = 00H
–4
–4
–2
0
2
4
VIN – Volts
VOUT = VIN (D/128 – 1), WHERE D = 0 TO 255
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 80H which in turn forces all the buffer am-
plifier outputs to zero volts. This asynchronous input pin PR
Figure 28. One Traditional Technique to Achieve Four-
Quadrant Multiplying with a Complementary Current
Output DAC
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.
VIN
REF
R
D VREF
256 R
I1
(1- D) VREF I2
256 R
CURRENT CONVEYOR
AMPLIFIER
VOUT X
VOUT
= VIN (D/128–1)
R
Figure 29. Current Conveyor Amplifier
Using the equations given in Figure 29 one can calculate the
final output equation as follows:
VO
=
– 1 –
D
256
× V IN
R
×
R
–
–D
256
× V IN
R
×
R
D
256
– 1 V IN
+
D
256
×V IN
=
2D
256
–
1
V IN
=
D
128
– 1 V IN
REV. A
–9–
Share Link: 