REV. A
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.
a
LOGDAC
CMOS Logarithmic D/A Converter
AD7118*
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 617/329-4700 Fax: 617/326-8703
GENERAL DESCRIPTION
The LOGDAC® AD7118 is a CMOS multiplying D/A converter which attenuates an analog input signal over the range
0 to –85.5 dB in 1.5 dB steps. The analog output is determined
by a six-bit attenuation code applied to the digital inputs.
Operating frequency range of the device is from dc to several
hundred kHz.
The device is manufactured using an advanced monolithic
silicon gate thin-film on CMOS process and is packaged in a
14-pin dual-in-line package.
ORDERING INFORMATION
Specified
Temperature Accuracy Package
Model Range Range Option
1
AD7118KN 0°C to +70°C 0 to 42 dB N-16
AD7118LN 0°C to +70°C 0 to 48 dB N-16
AD7118BQ –25°C to +85°C 0 to 42 dB Q-16
AD7118CQ –25°C to +85°C 0 to 48 dB Q-16
AD7118TQ
2
–55°C to +125°C 0 to 42 dB Q-16
AD7118UQ2–55°C to +125°C 0 to 48 dB Q-16
NOTES
1
N = Plastic DIP; Q = Cerdip.
2
To order MIL-STD-883, Class B processed parts, add /883B to part number.
*Protected by U.S. Patent No. 4521,764.
LOGDAC is a registered trademark of Analog Devices, Inc.
FEATURES
Dynamic Range 85.5 dB
Resolution 1.5 dB
Full 625 V Input Range Multiplying DAC
Full Military Temperature Range –558C to +1258C
Low Distortion
Low Power Consumption
Latch Proof Operation (Schottky Diodes Not Required)
Single 5 V to 15 V Supply
APPLICATIONS
Digitally Controlled AGC Systems
Audio Attenuators
Wide Dynamic Range A/D Converters
Sonar Systems
Function Generators
FUNCTIONAL DIAGRAM
PIN CONFIGURATION
AD7118–SPECIFICA TIONS
(VDD = +5 V or +15 V, VIN = –10 V dc, I
OUT
= AGND = DGND = 0 V, output amplifier
AD544 except where noted)
Accuracy Specification for L/C/U Grade Devices at TA = +25°C
REV. A
–2–
TA = +258CT
A
= T
MIN
, T
MAX
Parameter VDD = +5 V VDD = +15 V VDD = +5 V VDD = +15 V Units Test Conditions/Comments
NOMINAL RESOLUTION 1.5 1.5 1.5 1.5 dB
ACCURACY RELATIVE TO V
IN
AD7118L/C/U
0 dB to –30 dB ±0.35 ±0.35 ±0.4 ±0.4 dB max Accuracy is measured using
–31.5 dB to –42 dB ±0.7 ±0.5 ±0.8 ± 0.7 dB max circuit of Figure 1 and includes
–43.5 dB to –48 dB ±1.0 ±0.7 ±1.3 ± 1.0 dB max any effects due to mismatch
AD7118K/B/T between RFB and the R-2R
0 dB to –30 dB ±0.5 ±0.5 ±0.5 ±0.5 dB max ladder circuit.
–31.5 dB to –42 dB ±0.75 ±0.75 ±1.0 ±0.8 dB max
MONOTONIC RANGE
Nominal 1.5 dB Steps L/C/U Grade Monotonic Over Full 0 to –72 0 to –72 dB Digital Inputs 000000 to 110000
K/B/T Grade Code Range 0 to –66 0 to –66 dB Digital Inputs 000000 to 101100
Nominal 3 dB Steps All Grades Monotonic Over Full Code Range
VIN INPUT RESISTANCE All Grades 9 9 9 9 kΩ min
(PIN 12) L/C/U Grade 17 17 17 17 kΩ max
K/B/T Grade 21 21 21 21 kΩ max
RFB INPUT RESISTANCE All Grades 9.45 9.45 9.45 9.45 kΩ min
(PIN 13) L/C/U Grade 18 18 18 18 kΩ max
K/B/T Grade 22 22 22 22 kΩ max
DIGITAL INPUTS
Input High Voltage Requirements V
IH
3.0 13.5 3.0 13.5 V min
Input Low Voltage Requirements V
IL
0.8 1.5 0.8 1.5 V max
Input Leakage Current ±1 ±1 ±10 ±10 µA max Digital Inputs = V
DD
POWER SUPPLY
V
DD
for Specified Accuracy 5 – 5 – V min
– 15 – 15 V max
I
DD
0.5 1 1 2 mA max Digital Inputs = 0 V or V
DD
(See Figure 7)
Specifications subject to change without notice.
AC PERFORMANCE CHARACTERISTICS
These characteristics are included for design guidance only and are not subject to test.
TA = +258CT
A
= T
MIN
, T
MAX
Parameter VDD = +5 V VDD = +15 V VDD = +5 V VDD = +15 V Units
DC Supply Rejection, ∆Gain/∆V
DD
0.01 0.005 0.01 0.005 dB per % max ∆VDD = ±10%,
Input code = 100000
Propagation Delay 1.8 0.4 2.2 0.5 µs max Full-Scale Change
Digital-to-Analog Glitch Impulse 225 1200 – – nV secs typ Measured with ADLH0032CG
as output amplifier for input
code transition 100000 to 000000.
C1 of Figure 1 is 0 pF.
Output Capacitance (Pin 14) 100 100 100 100 pF max
Input Capacitance Pin 12 and Pin 13 7 7 7 7 pF max
Feedthrough at 1 kHz L/C/U Grade –86 –86 –68 –68 dB max Feedthrough is also deter-
K/B/T Grade –80 –80 –63 –63 dB max mined by circuit layout
Total Harmonic Distortion –85 –85 –85 –85 dB typ VIN = 6 V rms
Intermodulation Distortion –79 –79 –79 –79 dB typ per DIN 45403 Blatt 4
Output Noise Voltage Density 70 70 70 70 nV/√Hz max Includes AD544 amplifier noise
Digital Input Capacitance 7 7 7 7 pF max
Specifications subject to change without notice.
(VDD = +5 V or +15 V, VIN = –10 V except where stated, I
OUT
= AGND = DGND =
0 V, output amplifier AD544 except where noted)
Accuracy Specification for K/B/T Grade Devices at TA = +25°C
AD7118
REV. A
–3–
ABSOLUTE MAXIMUM RATINGS*
(TA = +25°C unless otherwise noted)
VDD (to DGND) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +17 V
V
IN
(to AGND) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ±35 V
Digital Input Voltage to DGND . . . . . –0.3 V to V
DD
+ 0.3 V
I
OUT
to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3 V to V
DD
AGND to DGND . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to V
DD
DGND to AGND . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to V
DD
Power Dissipation (Any Package)
To +75°C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 450 mW
Derates Above +75°C by . . . . . . . . . . . . . . . . . . . 6 mW/°C
Operating Temperature Range
Commercial (K, L Versions) . . . . . . . . . . . . .0°C to +70°C
Industrial (B, C Versions) . . . . . . . . . . . . . –25°C to +85°C
Extended (T, U Versions) . . . . . . . . . . . . –55°C to +125°C
Storage Temperature . . . . . . . . . . . . . . . . . . –65°C to +150°C
Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . +300°C
*Stresses above those listed under “Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only and functional operation of
the device at these or any other conditions above those indicated in the operational
sections of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
TERMINOLOGY
RESOLUTION: Nominal change in attenuation when moving
between two adjacent binary codes.
MONOTONICITY: The device is monotonic if the analog out-
put decreases (or remains constant) as the digital code increases.
FEEDTHROUGH ERROR: That portion of the input signal
which reaches the output when all digital inputs are high. See
section on Applications.
OUTPUT LEAKAGE CURRENT: Current which appears on
the I
OUT
terminal with all digital inputs high.
TOTAL HARMONIC DISTORTION: Is a measure of the
harmonics introduced by the circuit when a pure sinusoid is
applied to the input. It is expressed as the harmonic energy
divided by the fundamental energy at the output.
ACCURACY: Is the difference (measured in dB) between the
ideal transfer function as listed in Table I and the actual transfer
function as measured with the device.
OUTPUT CAPACITANCE: Capacitance from I
OUT
to
ground.
DIGITAL-TO-ANALOG GLITCH IMPULSE: The amount
of charge injected from the digital inputs to the analog output
when the inputs change state. This is normally specified as the
area of the glitch in either pA-secs or nV-secs depending upon
whether the glitch is measured as a current or voltage signal.
Digital charge injection is measured with V
IN
= AGND.
PROPAGATION DELAY: This is a measure of the internal
delays of the circuit and is defined as the time from a digital input change to the analog output current reaching 90% of its
final value.
INTERMODULATION DISTORTION: Is a measure of the
interaction which takes place within the circuit between two
sinusoids applied simultaneously to the input.
The reader is referred to Hewlett Packard Application Note 192
for further information.
WARNING!
ESD SENSITIVE DEVICE
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the AD7118 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
Applications Information–
AD7118
REV. A
–4–
CIRCUIT DESCRIPTION
GENERAL CIRCUIT INFORMATION
The AD7118 consists of a 17-bit R-2R CMOS multiplying D/A
converter with extensive digital input logic. The logic translates
the 6-bit binary input into a 17-bit word which is used to drive
the D/A converter. Table I gives the nominal output voltages
(and levels relative to 0 dB = 10 V) for all possible input codes.
The transfer function for the circuit of Figure 1 is given by:
VO=−VIN10exp−
1.5N
20
or
V
O
V
IN
dB
=−1.5N
where N is the binary input for values 0 to 57. For 60 ≤ N ≤ 63
the output is zero. See note 3 at bottom of Table I.
Figure 1. Typical Circuit Configuration
EQUIVALENT CIRCUIT ANALYSIS
Figure 2 shows a simplified circuit of the D/A converter section
of the AD7118 and Figure 3 gives an approximate equivalent
circuit.
The current source I
LEAKAGE
is composed of surface and junction leakages and as with most semiconductor devices, roughly
doubles every 10°C–see Figure 10. The resistor R
O
as shown in
Figure 3 is the equivalent output resistance of the device which
varies with input code (excluding all 0’s code) from 0.8R to
2R. R is typically 12 kΩ. C
OUT
is the capacitance due to the
N-channel switches and varies from about 50 pF to 80 pF depending upon the digital input. For further information on
CMOS multiplying D/A converters refer to “Application Guide
to CMOS Multiplying D/A Converters” which is available from
Analog Devices, Publication Number G479–15–8/78.
Figure 2. Simplified D/A Circuit of AD7118
Figure 3. Equivalent Analog Output Circuit of AD7118
Digital Input Attenuation
ND5D0 dB V
OUT
1
N Digital Input Attenuation V
OUT
1
00 00 00 00 0 0.0 10.00 31 01 11 11 46.5 0.0473
01 00 00 01 01.5 8.414 32 10 00 00 48.0 0.0398
02 00 00 10 03.0 7.079 33 10 00 01 49.5 0.0335
03 00 00 11 04.5 5.957 34 10 00 10 51.0 0.0282
04 00 01 00 06.0 5.012 35 10 00 11 52.5 0.0237
05 00 01 01 07.5 4.217 36 10 01 00 54.0 0.0200
06 00 01 10 09.0 3.548 37 10 01 01 55.5 0.0168
07 00 01 11 10.5 2.985 38 10 01 10 57.0 0.0141
08 00 10 00 12.0 2.512 39 10 01 11 58.5 0.0119
09 00 10 01 13.5 2.113 40 10 10 00 60.0 0.0100
10 00 10 10 15.0 1.778 41 10 10 01 61.5 0.00841
11 00 10 11 16.5 1.496 42 10 10 10 63.0 0.00708
12 00 11 00 18.0 1.259 43 10 10 11 64.5 0.00596
13 00 11 01 19.5 1.059 44 10 11 00 66.0 0.00501
14 00 11 10 21.0 0.891 45 10 11 01 67.5 0.00422
15 00 11 11 22.5 0.750 46 10 11 10 69.0 0.00355
16 01 00 00 24.0 0.631 47 10 11 11 70.5 0.00299
17 01 00 01 25.5 0.531 48 11 00 00 72.0 0.00251
18 01 00 10 27.0 0.447 49 11 00 01 73.5 0.00211
19 01 00 11 28.5 0.376 50 11 00 10 75.0 0.00178
20 01 01 00 30.0 0.316 51 11 00 11 76.5 0.00150
21 01 01 01 31.5 0.266 52 11 01 00 78.0 0.00126
22 01 01 10 33.0 0.224 53 11 01 01 79.5 0.00106
23 01 01 11 34.5 0.188 54 11 01 10 81.0 0.000891
24 01 10 00 36.0 0.158 55 11 01 11 82.5 0.000750
2
5 01 10 01 37.5 0.133 56 11 10 00 84.0 0.000631
26 01 10 10 39.0 0.112 57 11 10 01 85.5 0.000531
27 01 10 11 40.5 0.0944 58 11 10 10 87.0 0.000447
28 01 11 00 42.0 0.0794 59 11 10 11 88.5 0.000376
29 01 11 01 43.5 0.0668 60 11 11 XX
2
∞
30 01 11 10 45.0 0.0562
NOTES
1
VIN = –10 V dc
2
X = 1 or 0. Output is fully muted for N ≥ 60
3
Monotonic operation is not guaranteed for N = 58, 59
Table I. Ideal Attenuation vs. Input Code
AD7118
REV. A
–5–
DYNAMIC PERFORMANCE
The dynamic performance of the AD7118 will depend upon the
gain and phase characteristics of the output amplifier, together
with the optimum choice of PC board layout and decoupling
components. Figure 4 shows a printed circuit layout which
minimizes feedthrough from V
IN
to the output in multiplying
applications. Circuit layout is most important if the optimum
performance of the AD7118 is to be achieved. Most application
problems stem from either poor layout, grounding errors, or inappropriate choice of amplifier.
Figure 4. Suggested Layout for AD7118 and Op Amp
It is recommended that when using the AD7118 with a high
speed amplifier, a capacitor C1 be connected in the feedback
path as shown in Figure 1. This capacitor, which should be
between 30 pF and 50 pF, compensates for the phase lag introduced by the output capacitance of the D/A converter. Figures 5
and 6 show the performance of the AD7118 using the AD517, a
fully compensated high gain superbeta amplifier, and the
AD544, a fast FET input amplifier. The performance without
C1 is shown in the middle trace and the response with C1 in
circuit is shown in the bottom trace.
Figure 5. Response of AD7118 with AD517L
Figure 6. Response of AD7118 with AD544S
In conventional CMOS D/A converter design parasitic
capacitance in the N-channel D/A converter switches can give
rise to glitches on the D/A converter output. These glitches result from digital feedthrough. The AD7118 has been designed
to minimize these glitches as much as possible. It is recommended that for minimum glitch energy the AD7118 be operated with V
DD
= 5 V. This will reduce the available energy for
coupling across the parasitic capacitance. It should be noted
that the accuracy of the AD7118 improves as VDD is increased
(see Figure 8) but the device maintains monotonic behavior to
at least –66 dB in the range 5 ≤ V
DD
≤ 15 volts.
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 6 and 11. In circuits where C1 is not included the high frequency roll-off point
is primarily determined by the characteristics of the output amplifier and not the AD7118.
Feedthrough and absolute accuracy for attenuation levels beyond 42 dB are sensitive to output leakage current effects. For
this reason it is recommended that the operating temperature of
the AD7118 be kept as close to 25°C as is practically possible,
particularly where the device’s performance at high attenuation
levels is important. A typical plot of leakage current vs. temperature is shown in Figure 10.
Some solder fluxes and cleaning materials can form slightly conductive films which cause leakage effects between analog input
and output. The user is cautioned to ensure that the manufacturing process for circuits using the AD7118 does not allow
such films to form. Otherwise the feedthrough, accuracy and
maximum usable range will be affected.
STATIC ACCURACY PERFORMANCE
The D/A converter section of the AD7118 consists of a 17-bit
R-2R type converter. To obtain optimum static performance at
this level of resolution it is necessary to pay great attention to
amplifier selection, circuit grounding, etc.
Amplifier input bias current results in a dc offset at the output
of the amplifier due to the current flowing through the feedback
resistor R
FB
. It is recommended that an amplifier with an input
bias current of less than 10 nA be used (e.g., AD517 or AD544)
to minimize this offset.
Another error arises from the output amplifier’s input offset
voltage. The amplifier is operated with a fixed feedback resistance, but the equivalent source impedance (the AD7118 output impedance) varies as a function of attenuation level. This
has the effect of varying the “noise” gain of the amplifier, thus
creating a varying error due to amplifier offset voltage. To
achieve an output offset error less than one half the smallest step
size, it is recommended that an amplifier with less than 50 µV of
input offset be used (such as the AD517 or AD OP07).
If dc accuracy is not critical in the application, it should be
noted that amplifiers with offset voltage up to approximately 2
millivolts can be used. Amplifiers with higher offset voltage may
cause audible “thumps” due to dc output changes.
The AD7118 accuracy is specified and tested using only the
internal feedback resistor. It is not recommended that “gain”
trim resistors be used with the AD7118 because the internal
logic of the circuit executes a proprietary algorithm which approximates a logarithmic curve with a binary D/A converter: as a
result no single point on the attenuator transfer function can be
guaranteed to lie exactly on the theoretical curve. Any “gainerror” (i.e., mismatch of R
FB
to the R-2R ladder) that may exist
in the AD7118 D/A converter circuit results in a constant
attenuation error over the whole range. Since the gain error of
CMOS multiplying D/A converters is normally less than 1%,
the accuracy error contribution due to “gain error” effects is
normally less than 0.09 dB.
Applications Information–
AD7118
REV. A
–6–
Figure 7. Digital Threshold & Power Supply Current vs.
Power Supply
Figure 8. DC Attenuation Error vs. Attenuation & V
DD
Figure 9. DC Attenuation Error vs. Attenuation &
Temperature
Figure 10. Output Leakage Current as Temperature at
V
DD
= 5, 10 and 15 Volts
Figure 11. Frequency Response with AD544 and
AD517 Amplifiers
Figure 12. Distortion vs. Frequency Using AD544
Amplifier
–Typical Performance Characteristics
C628a–10–3/83
PRINTED IN U.S.A.
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