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AD7111 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer AD7111
Beschreibung LC2MOS LOGDAC Logarithmic D/A Converter
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 8 Seiten
AD7111 Datasheet, Funktion
a
FEATURES
Dynamic Range: 88.5 dB
Resolution: 0.375 dB
On-Chip Data Latches
+5 V Operation
AD7111A Pin Compatible with AD7524
Low Power
APPLICATIONS
Audio Attenuators
Sonar Systems
Function Generators
Digitally Controlled AGC System
LC2MOS LOGDAC
Logarithmic D/A Converter
AD7111/AD7111A
FUNCTIONAL BLOCK DIAGRAMS
VDD
VIN
AD7111
17-BIT DAC
RFB
I OUT
AGND
17-BIT LATCH
DECODE LOGIC
GENERAL DESCRIPTION
The LOGDAC® AD7111/AD7111A are monolithic multiplying
D/A converters featuring wide dynamic range in a small pack-
age. Both DACs can attenuate an analog input signal over the
range 0 dB to 88.5 dB in 0.375 dB steps. They are available in
16-pin DIPs and SOIC packages. The AD7111 is also available
in a 20-terminal LCCC package.
The degree of attenuation across the DAC is determined by an
8-bit word applied to the onboard decode logic. This 8-bit word
is decoded into a 17-bit word which is then applied to a 17-bit
R-2R ladder. The very fine step resolution, which is available
over the entire dynamic range, is due to the use of this 17-bit
DAC.
The AD7111/AD7111A are easily interfaced to a standard 8-bit
MPU bus via an 8-bit data port and standard microprocessor
control lines. The AD7111 WR input is edge triggered and re-
quires a rising edge to load new data to the DAC. The AD7111A
WR is level triggered to allow transparent operation of the
latches, if required. It should also be noted that the AD7111A is
exactly pin and function-compatible with the AD7524, an in-
dustry standard 8-bit multiplying DAC. This allows an easy up-
grading of existing AD7524 designs which would benefit both
from the wider dynamic range and the finer step resolution of-
fered by the AD7111A.
The AD7111/AD7111A are fabricated in Linear Compatible
CMOS (LC2MOS), an advanced, mixed technology process that
combines precision bipolar circuits with low power CMOS logic.
LOGDAC is a registered trademark of Analog Devices, Inc.
CONTROL LOGIC
8-BIT BUFFER
CS WR
VDD
VIN
D0 – D7
AD7111A
17-BIT DAC
DGND
RFB
I OUT
AGND
17-BIT LATCH
DECODE LOGIC
CONTROL LOGIC
8-BIT BUFFER
CS WR
D0 – D7
DGND
PRODUCT HIGHLIGHTS
1. Wide Dynamic Range: 0 dB to 88.5 dB attenuation range in
0.375 dB steps.
2. Small Package: The AD7111/AD7111A are available in
16-pin DIPs and SOIC packages.
3. Transparent Latch Operation: By tying the CS and WR in-
puts low, the DAC latches in the AD7111A can be made
transparent.
4. Fast Microprocessor Interface: Data setup times of 25 ns and
write pulse width of 57 ns make the AD7111A compatible
with modern microprocessors.
REV. 0
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700
Fax: 617/326-8703






AD7111 Datasheet, Funktion
AD7111/AD7111A
For example, the AD7111L is guaranteed monotonic in 0.375 dB
steps from 0 dB to –54 dB inclusive and in 0.75 dB steps from
0 dB to –72 dB inclusive. To achieve monotonic operation over
the entire 88.5 dB range it is necessary to select input codes so
that the attenuation step size at any point is consistent with the
step size guaranteed for monotonic operation at that point.
EQUIVALENT CIRCUIT ANALYSIS
Figure 2 shows a simplified circuit of the D/A converter section
of the AD7111/AD7111A, and Figure 3 gives an approximate
equivalent circuit.
The current source ILEAKAGE is composed of surface and junc-
tion leakages. The resistor R0 as shown in Figure 3 is the
equivalent output resistance of the device which varies with in-
put code (excluding all 0s code) from 0.8R to 2R. R is typically
12 k. COUT is the capacitance due to the N channel switches
and varies from about 20 pF to 50 pF depending upon the digi-
tal input. For further information on CMOS multiplying D/A
converters, refer to “CMOS DAC Application Guide” which is
available from Analog Devices, Publication Number
G872b–8–1/89.
A1
100
90
–0.2V
MSB
DATA CHANGE
C1 = 0pF
V OUT
V OUT
10
0%
5V
5V
C1 = 15pF
200ns
DATA CHANGE FROM 80H TO 00H.
Figure 4. Response of AD7111/AD7111A with AD711
A1
100
90
0.4V
V OUT
V OUT
10
0%
5V
5V
MSB
DATA CHANGE
C1 = 0pF
C1 = 15pF
200ns
DATA CHANGE FROM 80H TO 00H.
Figure 5. Response of AD7111/AD7111A with 1/2 OP275
Figure 2. Simplified D/A Circuit of AD7111/AD7111A
In conventional CMOS D/A converter design, parasitic capaci-
tance in N-channel D/A converter switches can give rise to
glitches on the D/A converter output. These glitches result from
digital feedthrough. The AD7111/AD7111A has been designed
to minimize these glitches as much as possible.
For operation beyond 250 kHz, capacitor C1 may be reduced in
value. This gives an increase in bandwidth at the expense of a
poorer transient response as shown in Figures 5 and 11. In cir-
cuits where C1 is not included, the high frequency roll-off point
is primarily determined by the characteristics of the output am-
plifier and not the AD7111/AD7111A.
Figure 3. Equivalent Analog Output Circuit of
AD7111/AD7111A
DYNAMIC PERFORMANCE
The dynamic performance of the AD7111/AD7111A will depend
upon the gain and phase characteristics of the output amplifier,
together with the optimum choice of PC board layout and
decoupling components. Circuit layout is most important if the
optimum performance of the AD7111/AD7111A is to be
achieved. Most application problems stem from either poor lay-
out, grounding errors, or inappropriate choice of amplifier.
Feedthrough and absolute accuracy are sensitive to output leak-
age current effects. For this reason it is recommended that the
operating temperature of the AD7111/AD7111A be kept as
close to 25°C as is practically possible, particularly where the
device’s performance at high attenuation levels is important. A typi-
cal plot of leakage current vs. temperature is shown in Figure 10.
Some solder fluxes and cleaning materials can form slightly con-
ductive films which cause leakage effects between analog
input and output. The user is cautioned to ensure that the
manufacturing process for circuits using thc AD7111/AD7111A
does not allow such films to form. Otherwise the feedthrough,
accuracy and maximum usable range will be affected.
It is recommended that when using thc AD7111/AD7111A with
a high speed amplifier, a capacitor (C1) he connected in the
feedback path as shown in Figure 1. This capacitor, which
should be between 10 pF and 30 pF, compensates for the phase
lag introduced by the output capacitance of the D/A converter.
Figures 4 and 5 show the performance of the AD7111/AD7111A
using the AD711, a high speed, low cost BiFET amplifier, and
the OP275, a dual, bipolar/JFET, audio amplifier. The perfor-
mance without C1 is shown in the middle trace and the re-
sponse with C1 in circuit shown in the bottom trace.
STATIC ACCURACY PERFORMANCE
The D/A converter section of the AD7111/AD7111A consists of
a 17-bit R-2R type converter. To obtain optimum static perfor-
mance at this level of resolution it is necessary to pay great
attention to amplifier selection, circuit grounding, etc.
Amplifier input has current results in a dc offset at the output of
the amplifier due to the current flowing through the feedback
resistor RFB. It is recommended that an amplifier with an input
bias current of less than 10 nA be used (e.g., AD711) to mini-
mize this offset.
–6– REV. 0

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