AD7111A Analog Devices, AD7111A Datasheet - Page 6

• Manufacturer Part Number: AD7111A
• Description: LC2MOS LOGDAC Logarithmic D/A Converter
• Manufacturer: Analog Devices

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 I
tion leakages. The resistor R
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 . C
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.
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.
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.
Figure 2. Simplified D/A Circuit of AD7111/AD7111A
OUT
is the capacitance due to the N channel switches
LEAKAGE
0
is composed of surface and junc-
as shown in Figure 3 is the
–6–
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.
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.
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 R
bias current of less than 10 nA be used (e.g., AD711) to mini-
mize this offset.
Figure 5. Response of AD7111/AD7111A with 1/2 OP275
Figure 4. Response of AD7111/AD7111A with AD711
V
V
V
V
OUT
OUT
OUT
OUT
FB
100
90
10
0%
100
90
10
0%
. It is recommended that an amplifier with an input
5V
A1
5V
A1
–0.2V
0.4V
5V
5V
DATA CHANGE FROM 80H TO 00H.
200ns
DATA CHANGE FROM 80H TO 00H.
200ns
DATA CHANGE
MSB
C1 = 0pF
C1 = 15pF
DATA CHANGE
MSB
C1 = 0pF
C1 = 15pF
REV. 0